Abdellatif Elghali

Mostafa Benzaazoua

Julien Couvidat

Yassine Taha

Carmen Mihaela Neculita

Vincent Chatain

Stabilization/solidification of contaminated sediments: challenges and novelties

Keywords: Contaminated sediments, stabilization/solidification, cement, physical trapping, chemical fixation

Introduction

The release of organic and inorganic contaminants into aquatic eco-systems increased is a challenging issue to the environment (human, fauna, flora and water resources). Contaminants may result from human activities (e.g. mining industries) and/or natural rocks weathering [START_REF] Ota | Geochemical distribution of heavy metal elements and potential ecological risk assessment of Matsushima Bay sediments during 2012-2016[END_REF]. In aquatic ecosystems (e.g. river, lake); sediments are the host of various contaminants depending on their mineralogical properties. Heavy metals release into aquatic ecosystems presents a serious risk to human health through food chain. Heavy metals are characterized by their long term persistence and bioaccumulation causing harmful effects at pints for from their sources [START_REF] Karaouzas | Heavy metal contamination status in Greek surface waters: A review with application and evaluation of pollution indices[END_REF]. In aquatic eco-systems, the reactivity of certain minerals, such as sulfides, is limited due to low concentrations of soluble oxygen which is around 8 mg/L. However, contaminants may be released due to dissolution mechanisms. Due to wind and waves energy the sediments, especially their fine fractions, are resuspended. Consequently, the contaminants are released in water. For this these reasons, contaminated sediments need to be properly managed to eliminate risk of contamination. Sediments dredging before their stabilization and/or revalorization is widely used for contaminated sediments management [START_REF] Wang | Recycling dredged sediment into fill materials, partition blocks, and paving blocks: Technical and economic assessment[END_REF][START_REF] Zhang | Dredged marine sediments stabilized/solidified with cement and GGBS: Factors affecting mechanical behaviour and leachability[END_REF][START_REF] Couvidat | Characterization of how contaminants arise in a dredged marine sediment and analysis of the effect of natural weathering[END_REF][START_REF] De Gisi | Recycling contaminated marine sediments as filling materials by pilot scale stabilization/solidification with lime, organoclay and activated carbon[END_REF][START_REF] Gang | Environmental assessment of contaminated marine sediments treated with solidification agents: Directions for improving environmental assessment guidelines[END_REF]. However, sediments dredging changes their environmental conditions in terms of temperature, humidity, redox potential, grain accessibility by oxidizing agents (e.g. oxygen, micro-organisms) and stability of organic matter. Therefore, the potential of contaminants release and remobilization is enhanced. In oxidizing conditions, sulfide oxidation by water and oxygen takes place to generate acidic leachates and enhances metal(oid)s mobility [START_REF] Nordstrom | Hydrogeochemistry and microbiology of mine drainage: An update[END_REF]. Then, the kinetic of sulfide oxidation is accelerated by bacterial activity (e.g. Acidithiobacillus ferrooxidans) especially in acidic conditions [START_REF] Silverman | Mechanism of bacterial pyrite oxidation[END_REF][START_REF] Mcneill | Microbial community structure within a weathered waste-rock pile overlain by a monolayer soil cover[END_REF]. It is known that sulfide (e.g. pyrite) oxidation and carbonate/silicates dissolution can be a source of various metal(oid)s such as As, Cd, Cu, Zn, Pb, Ni and Bi [START_REF] Dmitrijeva | Multivariate Statistical Analysis of Trace Elements in Pyrite: Prediction, Bias and Artefacts in Defining Mineral Signatures[END_REF]. In general, the contaminants can present as soluble ions within porewater and/or associated to solid matrix as oxides or organic/inorganic complexes or associated to structural composition of minerals. Consequently, if the sediments were left without and management scenario the contamination will be stopped by the complete depletion of contaminants-bearing minerals or complexes. At the long-term scale, the contamination is provided by the reactivity of solid phases because soluble form can be easily washed by water. The contamination is generated by the interaction between solid matrix (contaminants-bearing minerals) and oxidized agent (e.g. water, oxygen). Previously, ocean dumping was a major scenario for disposal of contaminated dredged sediments, but this method was abandoned since 1972 [START_REF] Gang | Environmental assessment of contaminated marine sediments treated with solidification agents: Directions for improving environmental assessment guidelines[END_REF][START_REF] Iannelli | Electrokinetic remediation of metal-polluted marine sediments: experimental investigation for plant design[END_REF]. Recently, solidification/stabilization (S/S) using cementitious materials was widely used as a viable remediation technique for contaminated sediments. USEPA [START_REF] Usepa | Treatment technologies for site cleanup: Annual status report[END_REF] stated that, between 2002 and 2005, around 24% of contaminated materials were treated using S/S (Ex-situ and in situ) in the United States. The S/S technology aims to modify geo-mechanical, hydrogeological and geochemical properties of contaminated materials (e.g. sediments, soils) in order to reduce the release of organic and inorganic. This chapter will discuss: i) the main characteristics of sediments, ii) the main materials used for S/S of contaminated sediments, iii) the main physical and chemical mechanisms responsible of contaminants fixation during S/S as well as the main tests used for chemical effectiveness of S/S and iv) how contaminated sediments can be integrated in a circular economy (e.g. valorization).

Sediments genesis and their main characteristics 2.1.

Sediments genesis

From a general point of view, a sediment is a deposit of mineral or organic clastic materials. Clastic particles may have an allochthonous origin (from the catchment basin) or an autochthonous origin (from immediate distance of the sedimentation place) [START_REF] Ramade | Dictionnaire encyclopédique de l'écologie et des sciences de l'environnement[END_REF].

Geologically, the breakdown of earth's rocks generates the mineral particles which, carried by wind and/or water, will accumulate on the streambed or seabed. This process, calls weathering, may act along two pathways: one physical, by cracking caused by expansion-contraction of the parent material, or erosion through the action of water, ice or wind, and one biogeochemical, through the action of chemical processes (hydration, hydrolysis, dissolution, and oxidationreduction) and biological processes. Diagenetic processes will transform sediment particles into sedimentary rocks. Four different sources may generate sediment particles (Insert Figure 1Figure 1):

-A terrigenous source, which particles originate from the erosion of emerged surfaces, and may be carried by runoff, aerial way, or fluvial water stream; -An endogenous source, where particles are produced by the environment where the sedimentation takes place, such as organic or inorganic debris of the autochthonous flora and fauna; -Neoformed particles, either by alteration, transfer, or precipitation in the sedimentary basin, or inside the sediment; -An anthropic source concerning industrial, urban or agricultural discharge, rich of organic matter (OM), suspended matter, micropollutants, metals and metalloids.

A sediment composition is complex and usually heavily depends on the catchment basin, indigenous contributions and local hydrodynamics able to mix or transport particles from a close basin. Mineral part of autochthonous sources is mainly composed of carbonates and evaporates.

Main sediments characteristics

For simplification, two categories may divide dredged sediments:

-Marine sediments, usually dredged into harbors or estuaries; -Continental (freshwater) sediments dredged into rivers, waterways, channels, reservoirs, lakes or dams.

In this chapter, continental sediments also include urban sediments issued from stormwater management structures/facilities (retention/detention or infiltration ponds) in urban areas or alongside roads and highways. Despite a growing interest, the volumes of dredged urban sediments are intermittent and disparate and data in literature upon their use of stabilization/solidification processes (S/SP) for treatment are scarce, and the present chapter will not address them. Most water structures require regular maintenance to ensure their proper functioning. Usually, restoring the water level is the main consideration of dredging operations, however others may apply, especially environmental considerations. The diversity of sedimentation beds induces a wide variety of sediment's characteristics, in line with both the influence of the catchment basin, and local physicochemical and environmental conditions. Among the main physicochemical characteristics to describe sediments, particle-size distribution (PSD) is essential as it may influence S/SP and further affect the characteristics of considered reuse product. Sediments' constituents may be sorted according to their size: in descending order, block, stone, pebbles, gravel, sand, silt and clay, added to colloidal molecules able to flocculate reversibly [START_REF] Ramade | Dictionnaire encyclopédique de l'écologie et des sciences de l'environnement[END_REF]. PSD is highly dependent of generation and transport processes of sediments, mainly erosion, hydrodynamics conditions, transportation and deposition (local diagenetic processes, aggregation, etc.) [START_REF] Laceby | The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: a review[END_REF][START_REF] Mclaren | The effects of sediment transport on grain-size distributions[END_REF]. Continental and marine sediments show some slight differences (Insert Table 1Table 1). Continental sediments have a higher clay content (particles <2 µm) with a median value of 11.9 against 5.71 wt.% for marine sediments, but a lower silt content (52.3 vs 63.0 wt.%). Sand content is approximately close. From a given site's perspective, local hydrodynamics also may influence PSD of accumulated sediments, as in a reservoir where the inlet is enriched in coarse particles and the area near the dam is enriched with fine particles [START_REF] Faure | Beneficial reuse of dam fine sediments as clinker raw material[END_REF][START_REF] Faure | Ability of two dam fine-grained sediments to be used in cement industry as raw material for clinker production and as pozzolanic additional constituent of Portland-composite cement[END_REF]. From Insert Table 1Table 1, we observe that continental sediments tend to a higher total organic carbon (TOC) content, with a mean content of 4.21 wt.% versus 2.60 wt.% for marine sediments, and a median of 3.40 vs 2.50 wt.%. Organic matter content might be deduced from TOC, considering that OM (wt.%) = 2 × TOC (wt.%). We can deduct from TOC values that fluvial sediments have a median organic matter content of about 6.8 wt.% versus 5 wt.% for marine sediments. However, dispersion of the results shows a high variability of this parameter depending on many physical and geochemical processes. Organic matter, either natural or anthropic, is of great importance in sediments both from a biogeochemical perspective as a carbon source, electron donor, adsorption surfaces, chelating agent, or particles textural component; and from geotechnical perspective where OM hampers with hydration processes of hydraulic and pozzolanic binders [START_REF] Beddaa | Effect of potassium humate as humic substances from river sediments on the rheology, the hydration and the strength development of a cement paste[END_REF][START_REF] Couvidat | Feasibility of the reuse of total and processed contaminated marine sediments as fine aggregates in cemented mortars[END_REF][START_REF] Pollard | Organic compounds in the cement-based stabilisation/solidification of hazardous mixed wastes-Mechanistic and process considerations[END_REF][START_REF] Tremblay | Influence of the nature of organic compounds on fine soil stabilization with cement[END_REF]. OM is mainly composed of humic acids, fulvic acids, proteins, polysaccharides, and lipids [START_REF] Baldock | Cycling and composition of organic matter in terrestrial and marine ecosystems[END_REF]. Hydrogen bonds and van der Walls forces allow OM to form supramolecules, and to interact with trace metals and metalloids and mineral particles [START_REF] Breault | Copper speciation and binding by organic matter in coppercontaminated streamwater[END_REF][START_REF] Chen | FTIR and synchronous fluorescence heterospectral two-dimensional correlation analyses on the binding characteristics of copper onto dissolved organic matter[END_REF][START_REF] Hirose | Metal-organic matter interaction: ecological roles of ligands in oceanic DOM[END_REF][START_REF] Lin | The relationship between adsorption of heavy metal and organic matter in river sediments[END_REF] Dissolved ionic species represent an additional distinction between freshwater and marine sediments. More concentrated in seawater than in freshwater, dissolved ionic speciesand chlorides in particularare present in marine sediments porewater, or as precipitates when the sediments dry. This difference induces a higher ionic strength for seawater, a slightly higher pH and lower dissolved oxygen, which will in return influence sedimentation processes, or contaminants solubility. Salinity is also a key parameter of numerous valorization processes, in particular in S/SP where chlorides interfere with cements components or steel bars in the case or reinforced concrete [START_REF] Angst | Critical chloride content in reinforced concrete-A review[END_REF][START_REF] Florea | Chloride binding related to hydration products: Part I: Ordinary Portland Cement[END_REF]. More generally, other physicochemical parameters will vary drastically even in a given type of sediments, often intercorrelated to each other: pH, Eh, cation-exchange capacity (CEC), carbonates content, C/N ratio, available nutrients, major and trace metals and metalloids speciation… Water content for example may vary in a wide range (20-90%) and depends on the fineness of the texture and content in organic matter.

For mineralogical composition, freshwater and marine sediments share commonly found minerals detailed in Erreur ! Source du renvoi introuvable.Table 2, mainly silicates and aluminosilicates, carbonates, metallic oxides and hydroxides, phosphates, sulfides, evaporites and clays [START_REF] Couvidat | Characterization of how contaminants arise in a dredged marine sediment and analysis of the effect of natural weathering[END_REF][START_REF] Chatain | Mineralogical study and leaching behavior of a stabilized harbor sediment with hydraulic binder[END_REF][START_REF] Martin-Garin | Aqueous cadmium uptake by calcite: A stirred flow-through reactor study[END_REF]. We will address specific issues and problematics of several classes of minerals, or particular minerals. Clays exhibit a specific affinity for trace metals and metalloids and interact with their mobility, as well as with organic matter to form clay-humic complex [START_REF] Alloway | Heavy metals in soils: trace metals and metalloids in soils and their bioavailability[END_REF][START_REF] Couvidat | An innovative coupling between column leaching and oxygen consumption tests to assess behavior of contaminated marine dredged sediments[END_REF][START_REF] Yariv | Organo-clay complexes and interactions[END_REF] . Carbonates regulate buffering power of sediments, influencing the local pH and ionic strength, as well as the mobility of trace metals and metalloids through various mechanisms (precipitation, co-precipitation, adsorption, absorption and substitution) [START_REF] Martin-Garin | Aqueous cadmium uptake by calcite: A stirred flow-through reactor study[END_REF][START_REF] Couvidat | An innovative coupling between column leaching and oxygen consumption tests to assess behavior of contaminated marine dredged sediments[END_REF][START_REF] Rimstidt | Distribution of trace elements between carbonate minerals and aqueous solutions[END_REF]. Metallic (oxi)hydroxidesmainly of Fe and Mnalso interact with trace metals and metalloids through adsorption and co-precipitation [START_REF] Alloway | Heavy metals in soils: trace metals and metalloids in soils and their bioavailability[END_REF]Dijkstra et al., 2004;Lions et al., 2010), as well as phosphates minerals used to immobilize trace metals and metalloids by adsorption or precipitation [START_REF] Cao | Phosphate-induced metal immobilization in a contaminated site[END_REF][START_REF] Cotter-Howells | Remediation of contaminated land by formation of heavy metal phosphates[END_REF][START_REF] Lions | Zinc and cadmium mobility in a 5-year-old dredged sediment deposit: experiments and modelling[END_REF]. Finally, sulfides are a well-known mineral class with close interaction to trace metals and metalloids, such as Cd, Cu, Ni, Pb, or Zn. Metallic sulfides are insoluble in water and readily precipitate when formed from water column [START_REF] Alloway | Heavy metals in soils: trace metals and metalloids in soils and their bioavailability[END_REF][START_REF] Huerta-Diaz | Geochemistry of trace metals associated with reduced sulfur in freshwater sediments[END_REF]. As reduced species of sulfur, metallic sulfides are highly sensitive to oxidation, susceptible to release associated trace metals and metalloids [START_REF] Chatain | Mineralogical study and leaching behavior of a stabilized harbor sediment with hydraulic binder[END_REF][START_REF] Couvidat | An innovative coupling between column leaching and oxygen consumption tests to assess behavior of contaminated marine dredged sediments[END_REF][START_REF] Schippers | Biogeochemistry of metal sulfide oxidation in mining environments, sediments, and soils[END_REF]. Sulfides oxidation and sulfates reduction highly depends on environmental geochemical conditions and bacterial activity [START_REF] Couvidat | Evaluation of biogeochemical reactivity of fresh and weathered contaminated dredged sediments[END_REF] Insert Figure 1.PNG here: Conceptual model illustrating sediments genesis Insert Table 1 here Insert Table 2 here

Stabilization/solidification technics

Definition of the technology

Solidification/stabilization of a contaminated material implies the addition of cementitious additives which improves the physical and chemical properties of the contaminated material to produce an environmentally acceptable mixture. Solidification and stabilization have different objectives: solidification which refers to the hardening of matrix which makes it easy to handle and ii) stabilization which refers to the immobilization of one or several chemical species of potential concern (e.g. organic contaminants, heavy metals). Within S/S technologies, the contaminants are not removed but transformer from leachable form to stable and low-solubility form. This technology can be used in-situ and ex-situ. Moreover, S/S technology is considered is time and cost-efficient [START_REF] Wang | Green remediation of As and Pb contaminated soil using cement-free clay-based stabilization/solidification[END_REF]. Solidification of contaminated sediments improves their workability and may create an opportunity for their valorization [START_REF] Amar | From dredged sediment to supplementary cementitious material: characterization, treatment, and reuse[END_REF]. However, there is no 'magical formulation' that can be used successfully for all types of contaminants; S/S technology is site-specific [START_REF] Council | Development of Performance Specifications for Solidification/Stabilization[END_REF]. The choice of cementing materials must consider the characteristics of contaminated sediments and the compatibility of cement and sediments [START_REF] Spence | Stabilization and solidification of hazardous, radioactive, and mixed wastes[END_REF].

Most used materials for S/S technology

Ordinary Portland cement is the most used material for S/S of contaminated sediments due to its demonstrated effectiveness [START_REF] Nehdi | Stabilization of sulphidic mine tailings for prevention of metal release and acid drainage using cementitious materials: a review[END_REF][START_REF] Elghali | In Situ Effectiveness of Alkaline and Cementitious Amendments to Stabilize Oxidized Acid-Generating Tailings[END_REF]. Cement-based stabilization/solidification was used successfully to immobilize heavy metals and metaloidsmetalloids (Insert Table 3Table 1). Recently, several researches were conducted to substitute ,atsubstitute, at least, partially cement in S/S technology [START_REF] Elghali | In Situ Effectiveness of Alkaline and Cementitious Amendments to Stabilize Oxidized Acid-Generating Tailings[END_REF][START_REF] Kumar | Mechanical activation of granulated blast furnace slag and its effect on the properties and structure of portland slag cement[END_REF]. The recent trends aim to reduce cost related to cement and valorize by-products of other industries, which participate in the principal of circular economy. Moreover, in some cases cement substitution by industrial by-products (e.g. cement kiln dust, fly ashes) improves the mechanical properties of solidified materials and increases contaminants fixation. Insert Table 3Table 1 presents a summary the most used materials for S/S of contaminated materials. The main conclusion is that the choice of the materials and their dosages is case-specific. In general, cement is substituted by materials providing additional acid neutralization capacity or pozzolanic materials. Park (200) [49] substituted 20 wt.% of ordinary Portland cement by CKD to stabilize hazardous wastes containing heavy metals. CKD is a by-product of Portland cement manufacture and characterized by an uniform particle size distribution. CKD contains high CaO and SiO2 contents (around 40 and 10 wt.%, respectively). The use of CKD provided additional alkalinity which accelerated cement setting and hydration. Additionally, the formulation containing CKD showed the highest compressive strength and consequently the lowest metals release. CKD can be combined with other industrial by-products such as lime dust. Eisa et al (2020) [START_REF] Mohammad Eisa | Evaluation of solidification/stabilization in arsenic-contaminated soils using lime dust and cement kiln dust[END_REF] showed that the use of lime dust improves arsenic stabilization within contaminated soils. However, if the levels of alkaliesalkalis within CKD are low, contaminants fixation and compressive strength of stabilized material are negatively affected [START_REF] Eker | Immobilization of heavy metals in waste phosphate coating sludge using kiln dust as Portland cement substitute[END_REF]. Metal leaching can be significantly reduced when CKD is combined with red mud bauxite (RMB) which is characterized by a high affinity to metals. Indeed, Doye and Duschene (2003) [START_REF] Doye | Neutralisation of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests[END_REF] stabilized high reactivity materials (sulfides) using formulations based on CKD and RMB (10 wt.%).

Recently, the use of low carbon cements for the stabilization/solidification of sediments is progressively needed due to their advantageous economic and environmental spinoffs. Unlike conventional cements, low carbon cements allow to reduce the GHGs emissions and production costs either via the decrease of clinkerization temperature or via the substitution of clinker by supplementary cementitious materials (SCMs). Sulfoaluminate and reactive MgO cements have been used as green alternatives to stabilize hazardous wastes due to their capacity to immobilize toxic contaminants in the hydration products; ettringite in Sulfoaluminate cements [START_REF] Kiventerä | Solidification/stabilization of gold mine tailings using calcium sulfoaluminate-belite cement[END_REF] and hydrated brucite (Mg(OH)2) in reactive MgO cements [START_REF] Wang | Novel synergy of Si-rich minerals and reactive MgO for stabilisation/solidification of contaminated sediment[END_REF]. Otherwise, many alternative SCMs were tested worldwide going from waste streams (i.e. silica fume, fly ashes, blast furnace slag, etc.) to thermally activated clays (i.e. metakaolin, calcined clays, etc.). Recently, researchers have started to evaluate the performance of limestone calcined clay cement (LC3) to immobilize pollutants [START_REF] Reddy | New ternary blend limestone calcined clay cement for solidification/stabilization of zinc contaminated soil[END_REF]. This kind of binders is mainly composed of calcined clays at low temperatures mixed with limestone and clinker. It allows the development of materials with high durability, low water absorption and high chloride resistance [START_REF] Wang | Green remediation of As and Pb contaminated soil using cement-free clay-based stabilization/solidification[END_REF]. Also, alkali activated cements are promising binders for the S/S of sediments. They allow to obtain stabilized matrices with low permeability, better acid and sulfate resistance, better heavy metals stabilization [START_REF] El-Eswed | Chemical evaluation of immobilization of wastes containing Pb, Cd, Cu and Zn in alkali-activated materials: a critical review[END_REF].

Insert Table 3 3.3.

Discussion of the main chemical and physical mechanisms responsible for contaminant stabilization during S/S technology

Cementitious additives alter the physical and hydrogeochemical properties of contaminated materials. The main objective of S/S technology is to produce a stabilized material environmentally acceptable by decreasing its reactivity and chemical species release. In general, fixation and immobilization of contaminants is ensured by mechanisms related to i) cement hydration, ii) precipitation of secondary minerals at neutral to alkaline environment and iii) hardening of contaminated materials. Two main categories of mechanisms are responsible of contaminants immobilization: i) chemical and physical mechanisms [START_REF] Spence | Stabilization and solidification of hazardous, radioactive, and mixed wastes[END_REF]. However, quantifying the share of each of these two categories is difficult because of their interaction. The major mechanisms responsible for contaminants stabilization are summarized in Insert Figure 2Figure 2.

Physical mechanisms

Basically, solidification of contaminated sediments alters their mechanical properties. Cement hydration products formation is translated by the hardening of the matrix such as the increase of its mechanical resistance. Solidification of contaminated sediments by cement hydration alters its microstructure. Indeed, the porosity of the stabilized contaminated sediments is considerably reduced [START_REF] Nehdi | Stabilization of sulphidic mine tailings for prevention of metal release and acid drainage using cementitious materials: a review[END_REF]. Basically, reduction of contaminants release by stabilized sediments is ensured by reduction of liberation degree (i.e exposure rate) of reactive minerals. It is demonstrated that reactivity of minerals is correlated to their liberation degree [START_REF] Elghali | Determination of the available acid-generating potential of waste rock, part I: Mineralogical approach[END_REF][START_REF] Elghali | Determination of the available acid-generating potential of waste rock, part II: Waste management involvement[END_REF][START_REF] Lorenzen | The interrelationship between mineral liberation and leaching behaviour[END_REF]. Indeed, during sediments hardening by cement hydration, several mechanisms are responsible of the decrease of the available surface area of reactive minerals such as:

i)
Precipitation of cement products at the surface of grains. Indeed, reaction of hydration of cement compounds such as alite and belite leads to the formation of CSH which is responsible of cementing action of Portland cement [START_REF] Paria | Solidification-stabilization of organic and inorganic contaminants using portland cement: a literature review[END_REF]. In general, cement hydration products are precipitated at the porosity of sediments and at the surface of grains. Additionally, cement hydration removes the free sediments pore water which is generally highly charged in soluble ions. ii) Reduction of contact surface and time contact between oxidizing agent and reactive grains. Indeed, sediments cementation improves their geo-mechanical performances (e.g. UCS). Consequently, the porosity and saturated hydraulic conductivity are decreased, which affect the hydrogeological behavior stabilized sediments. There are few papers that studied the effect of cement on the water balance within stabilized materials. Elghali et al (2020) [START_REF] Elghali | In Situ Effectiveness of Alkaline and Cementitious Amendments to Stabilize Oxidized Acid-Generating Tailings[END_REF] studied the effect of cementitious additives on water content and infiltration within contaminated tailings using volume water probes. The authors demonstrated that contaminated materials cementation decreases water vertical infiltration rate and favors surface and sub-surface runoff, which is explained by reduction of hydraulic conductivity. Spence and Shi [START_REF] Spence | Stabilization and solidification of hazardous, radioactive, and mixed wastes[END_REF] stated that hydraulic conductivity of mature Portland cement can reach 10 -13 m/s. When water vertical infiltration is limited, both contact time and surface contact between reactive grains and oxidizing agents are limited, which reduces the overall reactivity of contaminated sediments. It is known that the chemical species release from reactive grains depends on their accessibility by oxidizing agent (e.g. water, oxygen) [START_REF] Safari | A shrinking particle-shrinking core model for leaching of a zinc ore containing silica[END_REF].

Chemical mechanisms

Cementitious additives allow fixation of contaminants by several chemical mechanisms including: i) precipitation of metal(loid)s as oxy-hydroxides, ii) adsorption of contaminants on the surface of cement hydration products (e.g. CSH) and iii) fixation of contaminants by secondary oxy-hydroxides.

i)

Cements are characterized by their high acid neutralization capacity. This means that pH of sediments pore water is increased to reach circumneutral and alkaline values. In these environments, the metal(loid)s are characterized by low solubility, which means that soluble ions will precipitate as hydroxides [START_REF] Cravotta | Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 2: Geochemical controls on constituent concentrations[END_REF]. Benzaazoua et al ( 2002) [START_REF] Benzaazoua | Chemical factors that influence the performance of mine sulphidic paste backfill[END_REF] demonstrated that arsenic mobility is controlled by calcium content within binding agent. Indeed, in lime rich mixtures, arsenic is immobilized by portlandite dissolution. The arsenic is precipitated as CaAs(OH)4. Other authors demonstrated the same conclusions regarding other heavy metals such as Cd [START_REF] Amar | From dredged sediment to supplementary cementitious material: characterization, treatment, and reuse[END_REF]. Moreover, depending on iron activity in the pore water secondary iron oxy-hydroxides precipitate in circumneutral/alkaline conditions (Figure 2). These secondary phases are known for their affinity to several chemical species (e.g. Cd, As). The precipitation of iron-oxyhydroxides is often associated with fixation of contaminants [START_REF] Appelo | Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic[END_REF][START_REF] Appleton | Adsorptive removal of trace elements from coal fly-ash wastewaters onto iron oxyhydroxide[END_REF][START_REF] Bowell | Sorption of arsenic by iron oxides and oxyhydroxides in soils[END_REF]. The mechanisms by which the contaminants are fixed by secondary oxyhydroxides are commonly called 'sorption' which includes adsorption on the surface, complexes formation, physical adsorption and eventually substitution [START_REF] Manceau | Sorption and speciation of heavy metals on hydrous Fe and Mn oxides. From microscopic to macroscopic[END_REF]. Fixation of contaminants by secondary phases may be reversible depending on the fixation mechanism and the crystallinity of the phase. Indeed, the release of a substituted contaminated from well crystallized is negligible compared to a contaminant adsorbed in an amorphous phase [START_REF] Davranche | Release of Metals from Iron Oxyhydroxides under Reductive Conditions: Effect of Metal/Solid Interactions[END_REF]. The durability of chemical fixation of contaminants by secondary phases can be altered if the geochemical conditions change (e.ge.g. pH, redox potential). ii)

During cement hydration results in the formation of several products including calcium hydroxide, calcium sufoaluminate and calcium silicate hydrate (CSH) [START_REF] Benzaazoua | Chemical factors that influence the performance of mine sulphidic paste backfill[END_REF]. Formation of CSH gel-like, which is characterized by high specific surface area, is responsible of the adsorption of several contaminants. Contessi et al (2020) [START_REF] Contessi | Stabilization of lead contaminated soil with traditional and alternative binders[END_REF] highlighted the formation layers of CHS containing high concentrations of Pb during the stabilization of a Pb-contaminated materials stabilization. Fixation of contaminants by CSH phase is pH dependent. If the pH of stabilized material is suitable for CSH phase stability, the contaminants release will be negligible. Similarly, at acidic conditions fixation of contaminants by CSH is reversible. Moreover, Yakubi et al (2018) [START_REF] Yakubu | Effects of pH dynamics on solidification/stabilization of municipal solid waste incineration fly ash[END_REF] stated that stabilization of metals (Cu, Pb, Zn, Cr, Cd, As and Mn) is possible within cementitious materials at a pH values ranging between 5 and 11 except Zn which is was found at its soluble form at all pH ranges. Additionally, As the chemical composition of CSH is variable depending on cement composition, its capacity to immobilize metals is variable. Indeed, retention of metals by CSH depends on its Ca/Si ratio

Biological mechanisms

Biological processes are not directly involved in S/S classical technology. However, unlike organic contaminants, which are bio-chemically degradable, heavy metals are not, but they can change speciation, solubility and toxicity depending on environmental conditions (e.g. pH, redox potential, presence of organic material, microbial activity). As a result, in metal contaminated solids, remediation strategies can either enhance metal mobilization (increased solubility via biohydrometallurgy for example) or immobilization (decreased bioavailability). Biological-based processes (e.g. biosorption, bioaccumulation) are therefore promising for the treatment of contaminated sediments [START_REF] Akcil | A review of approaches and techniques used in aquatic contaminated sediments: metal removal and stabilization by chemical and biotechnological processes[END_REF]. The S/S popularity is based on the rapid, versatile (in situ or ex situ), broad range of contaminants immobilized and low-cost achievement of treatment objectives. However, stabilization of contaminants without matrix solidification could also sometimes prove sustainable, especially when coupled to a controlled release of reactants/microbes for enhanced remediation in the long term [START_REF] Shen | Solidification/stabilization for soil remediation: an old technology with new vitality[END_REF] or to green/alternative binders for diverting hazardous wastes from disposal following an life-cycle assessment [START_REF] Hossain | Evaluating the environmental impacts of stabilization and solidification technologies for managing hazardous wastes through life cycle assessment: A case study of Hong Kong[END_REF].

In this sense, the literature shows a slow but steady evolution towards environmentally sustainable solutions: from high impact (thermal treatment and cementification) to low impact (chemical and biological) processes [START_REF] Akcil | A review of approaches and techniques used in aquatic contaminated sediments: metal removal and stabilization by chemical and biotechnological processes[END_REF]. Moreover, in some countries (Denmark, South Korea) the S/S technology is prohibited because removal of contaminants from soils is required over their stabilization [START_REF] Shen | Solidification/stabilization for soil remediation: an old technology with new vitality[END_REF].

Insert Figure 2.JPG here: Conceptual model explaining the main mechanisms responsible for contaminants immobilization during S/S technology

Main tests used for assessment of the effectiveness of S/S technology

The main objective of S/S technology is to reduce the risk of organic and inorganic contamination of environment by the reactivity of the contaminated material. Despite hydrogeotechnical properties of stabilized matrix, the reactivity of stabilized matrix is crucial to evaluate the effectiveness of the used S/S technology. After stabilization/solidification of contaminated sediments, interaction with oxidizing agents (e.g. water, oxygen, microorganism) took place [START_REF] Conner | A Critical Review of Stabilization/Solidification Technology[END_REF]. The chemical quality analysis of the leachates can indicate the effectiveness of S/S technology to immobilize contaminants. During last decades, several procedures were developed to evaluate chemical species release by solid materials (e.g. stabilized matrix). The most used tests for the leaching of stabilized materials are tank leaching tests [START_REF] Nen | Leaching characteristics of moulded or monolithic building and waste materials. Determination of leaching of inorganic components with the diffusion test[END_REF], toxicity characteristics leaching procedure (TCLP), 1313, 1315 and 1316 methods, column tests and field cells. All these tests consist of submitting a monolith or crushed sample to weathering by deionized water or an acid with different L/S ratio and time duration. Within each test, the leachates are collected and analyzed for their physio-chemical characteristics (e.g. pH, electrical conductivity, chemical composition). The leachates analysis allows quantifying chemical species release from the stabilized matrix. This way, the chemical effectiveness of S/S technology to immobilize contaminants is studied. Thank leaching tests consist of submerging a monolith sample with a known external surface (cylinder, parallelepiped) for 64 days. the ration L/S (cm 3 /cm²) is about 10/1. The deionized water is renewed after 0.25, 1, 2.25, 4, 9, 16, 36 and 64 days. This test is primarily designed to determine the leaching of inorganic components from monolithic material. It is used to quantify diffusion and effective diffusion coefficient. This test cannot be used for soluble materials [START_REF] Nen | Leaching characteristics of moulded or monolithic building and waste materials. Determination of leaching of inorganic components with the diffusion test[END_REF]. Toxicity characteristics leaching procedure (TCLP) was designed to determine leaching of organic and inorganic components from industrial wastes. During TCLP tests, around 20 g of crushed sample is allowed to react for 18h with acetic acid. The method 1313 allows representation of liquid solid partitioning in function of pH for organic, inorganic and semivolatile constituents in a solid material (e.g. stabilized sediments). This method is realized in a liquid/solid ration of 10 ml/1 g of dry sample [START_REF] Epa | Method 1313. Leaching Test (Liquid-Solid Partitioning as a Function of Extract pH) of Inorganic Species in Solid Materials Using a Parallel Batch Extraction Test[END_REF]. It consists of nine parallel extractions of crushed sample in dilute acid or base and water (more details can be found in [START_REF] Epa | Method 1313. Leaching Test (Liquid-Solid Partitioning as a Function of Extract pH) of Inorganic Species in Solid Materials Using a Parallel Batch Extraction Test[END_REF]).additionally, the method 1315 can performed on monolithic sample, like TLT, to evaluate release rates of chemical species in saturated conditions. The sample is allowed to react with reagent water in a liquid/surface ratio of about 9±1 mL/cm² at nine predetermined intervals (for more details about experiments, see leaching environmental assessment framework). The method 1316, is relatively like 1313 method, is designed to provide intrinsic parameters controlling the leaching of inorganic chemical species under equilibrium conditions. It provides the liquid/soldi partitioning at neutral pH [START_REF] Garrabrants | Background information for the leaching environmental assessment framework (LEAF) test methods[END_REF]. Operationally, the sample is allowed to react with reagent water for a solid/liquid ration varying between 0.5 and 10 depending on the sample's particle size [START_REF] Garrabrants | Background information for the leaching environmental assessment framework (LEAF) test methods[END_REF]. The tests described above are all performed on small sample mass for relatively a short time duration which makes prediction of the long-term leachability of tested material difficult. Nowadays, column tests which are used in the field of environmental geochemistry are used successfully for the leachability of stabilized materials. Column tests consist of submitting stabilized materials to natural weathering by deionized water [START_REF] Ichrak | Effect of cementitious amendment on the hydrogeological behavior of a surface paste tailings' disposal[END_REF][START_REF] Yilmaz | Influence of disposal configurations on hydrogeological behaviour of sulphidic paste tailings: A field experimental study[END_REF]. There are no limitations related to disposal configuration, sample mass, liquid/solid ration, and time duration. Several configurations can be performed depending on the objective of the study. Column tests can be run in saturated or unsaturated conditions. In general, laboratory tests can provide several information related to stabilized material reactivity and chemical species release. So, they can be considered as complementary tests except those that have the same operational procedure (e.g. TLT and 1315 method). The leachates chemical composition is compared to environmental regulation which allow to evaluate the effectiveness of S/S technology [START_REF] Nehdi | Stabilization of sulphidic mine tailings for prevention of metal release and acid drainage using cementitious materials: a review[END_REF]. However, laboratory tests can not represent field conditions. There is no unique mathematical model than can extrapolate laboratory results to field conditions [START_REF] Plante | Lab to field scale effects on contaminated neutral drainage prediction from the Tio mine waste rocks[END_REF]. This can be explained by several factors: i) in field conditions hydrogeological factors may favor surface runoff and consequently contact time between stabilized matrix and reagent is reduced, ii) leachable surface is greater within field conditions than that with laboratory conditions, iii) sample dimensions are generally larger for field conditions, vi) liquid/solid ratio, v) flushing frequency which is dictated by climatic conditions within field conditions and predefined for laboratory conditions, and vi) one of the most influencing factor is that the presence of heavy metals results in retard of cementitious materials' hydration to set and harden [START_REF] Park | Hydration and solidification of hazardous wastes containing heavy metals using modified cementitious materials[END_REF]. Consequently, the best practice to evaluate contaminants immobilization within a S/S technology is to implement field cells despite their elevated cost [START_REF] Elghali | In Situ Effectiveness of Alkaline and Cementitious Amendments to Stabilize Oxidized Acid-Generating Tailings[END_REF][START_REF] Wang | The performance of blended conventional and novel binders in the insitu stabilisation/solidification of a contaminated site soil[END_REF].

Integration of contaminated sediments in circular economy

Worldwide dredging operations are behind the production of high volumes of sediments frequently contaminated. The prevention of this kind of wastes is most difficult as it is the result of many natural processes added to anthropogenic sources. The most suitable solution in the hierarchy of the integrated waste management diagram is to reuse/recycle these sediments as alternative materials. This solution could help to avoid the consumption of new finite primary resources (gravel, sand, clays, etc.) and to reduce the degradation of natural ecosystems. Various applications could use contaminated sediments due to their significant volume, and their chemical, mineralogical and geotechnical characteristics. Mineralogically, dredged sediments are mainly composed of silicates, carbonates, clays, sulfides, (oxi)hydroxides and organic matter. From a particle size point of view, classification of sediments can fall into different categories: gravel, sand, silt and clays. Making use of wastes is always welcome when it is economically viable, technically feasible, environmentally sustainable and socially acceptable. The direct reuse of sediments in their initial state is often difficult. Therefore, many parameters should be controlled and mastered: the initial water content and salinity, organic matter, heavy metals content and speciation, etc.

Water content is particularly a common impediment to valorization, usually handled by water removal readily after dredging. In case of noteworthy contamination level, a pretreatment should apply to the sediment before suitable use for removal or immobilization of the contaminants. In most cases, the nature of the sediment and the final application will prescribe the useful pretreatment(s) [START_REF] Amar | From dredged sediment to supplementary cementitious material: characterization, treatment, and reuse[END_REF]: Civil engineering and public infrastructures sectors efficiently use alternative materials as stabilized sediments. For instance, massive infrastructures worldwide used a substantial amount of sediments such as the Palm island in Dubai, the Hong Kong airport, or the National theatre of London [START_REF] Limeira | Mechanical and durability properties of concrete made with dredged marine sand[END_REF]. They were also used in large-scale road projects in Dunkirk Harbor in France while respecting economic and environmental constraints [START_REF] Achour | Valorization of unauthorized sea disposal dredged sediments as a road foundation material[END_REF][START_REF] Wang | Durability analysis of fly ash/cement-solidified dredged materials[END_REF][START_REF] Siham | Marine dredged sediments as new materials resource for road construction[END_REF]. However, these sediments have been stabilized before their use by hydraulic binders (cement, lime, fly ashes and slag) and/or mixed with other mineral granular correctors (sand, slags, etc.) to satisfy the mechanical performances and environmental requirements. More generally, civil engineering frequently valorizes dredged sediments because construction field is an extensive consumer of natural resources to produce aggregates (with sand) or hydraulic binder as Portland cement (with clay and limestone). Partial or total substitution of natural sand as aggregate in mortars, concretes or in unfired bricks is a common valorization of dredged sediments [START_REF] De Gisi | Recycling contaminated marine sediments as filling materials by pilot scale stabilization/solidification with lime, organoclay and activated carbon[END_REF][START_REF] Wang | Recycling contaminated sediment into eco-friendly paving blocks by a combination of binary cement and carbon dioxide curing[END_REF][START_REF] Maierdan | Recycling of waste river sludge into unfired green bricks stabilized by a combination of phosphogypsum, slag, and cement[END_REF][START_REF] Mymrin | Utilization of sediments dredged from marine ports as a principal component of composite material[END_REF][START_REF] Junakova | Reservoir sediment as a secondary raw material in concrete production[END_REF], with actual production of effective harbors' accropodes [START_REF] Achour | Valorization of unauthorized sea disposal dredged sediments as a road foundation material[END_REF], pedestrian and vehicles pavers [START_REF] Wang | The roles of biochar as green admixture for sediment-based construction products[END_REF], synthetic cobblestones [START_REF] Xue | Non-sintered dredged sediments artificial cobblestones: Preparation, structure and properties[END_REF] or eco-friendly foamed concrete [START_REF] Yang | Sustainable conversion of contaminated dredged river sediment into ecofriendly foamed concrete[END_REF]. In some cases, a synthetic polymer replaces Portland cement to produce polymer mortars with epoxy resin [START_REF] Maherzi | Study of the polymer mortar based on dredged sediments and epoxy resin: Effect of the sediments on the behavior of the polymer mortar[END_REF][START_REF] Zouch | An eco-friendly epoxy polymer binder for the treatment of Tunisian harbor sediments: Laboratory investigations for beneficial reuse[END_REF], or floor-covering concrete tiles with polyester resin [START_REF] Ennahal | Eco-friendly polymers mortar for floor covering based on dredged sediments of the north of France[END_REF]. Silt content and soluble organic matter, two intrinsically linked parameters, may impair mechanical and durability properties of produced mortars and concretes (hydration, setting time, compressive strength, shrinkage strain). Dredged sediments are thermally or chemically pretreated [START_REF] Wang | Mixture design and treatment methods for recycling contaminated sediment[END_REF], or sieved for coarser fractions [START_REF] Couvidat | Feasibility of the reuse of total and processed contaminated marine sediments as fine aggregates in cemented mortars[END_REF] to overcome these issues. A hightemperature thermal treatment (>1100°C) produces synthetic lightweight aggregates to make structural lightweight concretes [START_REF] Hamer | Light weight aggregates made from dredged harbour sediments[END_REF][100][101][102]. Overall, sand substitution is usually between 10 and 20 wt.% and can rise to 100 wt.% (up to 95 wt.% of the total of the mix) for structures with low loading.

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Recently, a second approach emerged with the valorization of sediments without thermal treatment through substitution of cement for production of mortars and concretes (until 40 wt.%) [START_REF] Junakova | Reservoir sediment as a secondary raw material in concrete production[END_REF]. However, most studies focus on thermally pretreated sediment able to remove organic matter and enhance pozzolanic activity

Durability of stabilized sediments

As discussed above, S/S technology allows stabilization of contaminated sediments by physical and chemical processes. The main objective of a S/S technology is to produce a safe stabilized matrix at the long-term scale. However, exposure of stabilized sediments to climatic conditions may alter their physical properties and consequently the immobilization of contaminants. [START_REF] Sun | The rainfall effect onto solidification and stabilization of heavy metalpolluted sediments[END_REF] [117] studied the effect of rainfall on the effectiveness of S/S of heavy metalspolluted sediments. The authors concluded that deterioration of mechanical resistance of stabilized sediments depends on curing time. The deterioration of mechanical resistance is negatively correlated to curing time; higher is the curing lowest is the deterioration of mechanical resistance. After 28-days of curing, the deterioration of mechanical resistance was about 8%, while after 7-days of curing time the deterioration was around 34%. Consequently, at filed conditions the effect of rainfall can be reduced if the stabilized matrix had enough time to set. The Moreover, the deterioration of mechanical resistance didn't affect the stabilization of heavy metals. However, at the long term-scale the stabilization of heavy metals can be affected by increasing the available surface area of reactive grains and contact time between oxidizing agents and reactive grains (Insert Figure 3Figure 3). Cracks formation and development may also increase the contact time and surface contact between oxidizing agent and reactive grains may result in remobilization of contaminants. Cracks can be formed due to freezing-unfreezing cycles and precipitation of expansive minerals because of sulfate attack [118]. However, the stabilized matrix can be protected by adding protection layer that can limit water vertical infiltration and limit evaporation [119].

Insert Figure 3.JPG here: Effect of climatic conditions on the effectiveness of S/S technology

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  Physical methods (flotation, hydro-cycling, vacuum extraction, etc.), Physicochemical methods (phosphating, acidification, dichlorination, leaching, etc.), Chemical treatments (solidification-stabilization using a hydraulic binder such as cement, lime, pozzolans, clay, or other binders, such as polymer resins), Biological treatments (bioremediation, phytoremediation, etc.), Heat treatment (thermal desorption, incineration, vitrification, pyrolysis).

  [103], to produce blended or composite sediment-based cement (usually 8 to 15 wt.% in the raw mix, until 40 wt.%)[104]. Efficient and durable results are obtained with a replacement rate of 10 wt.%. Dredged sediments may also enter the composition of self-consolidating concrete[105]. For more information on the use of dredged sediments as supplementary cementitious materials (SCM), the curious reader can refer to a recently published review[START_REF] Amar | From dredged sediment to supplementary cementitious material: characterization, treatment, and reuse[END_REF]. Finally, Portland cement production may use dredged sediments to replace a part of raw materials in the clinker mix, predominantly with freshwater sediments (dam, fluvial) for their low chloride contents, but not only[106]. Nonetheless, heterogeneity of sediments deposits might be an obstacle for the upscaling to industrial level, requiring proper characterization before each considered application. Sediments were also good alternative materials to substitute the use of clays and sand in building bricks and tiles manufacturing. Many applications in the laboratory scale were investigated in the literature [107, 108]. Full-scale production examples were reported in France [109], Germany[110] and China[111]. For all these applications, sediments undergo a common Novosol® pretreatment aiming to immobilize heavy metals and metalloids and remove the organic matter by a chemical treatment (phosphatation) followed by calcination (> 650°C)[112]. Even though the use of the sediments in brick making presents several advantages from a technical, economic and environmental point of views, many challenges are faced when coming to consumption at a large scale and introduction to the market. Sensitization and communication with final customers were recommended[113]. Another valorization way exists, such as coastal nourishment [114], as a growing medium for lettuce production [115], or production of ceramic glaze[116].

Mis en fo


Fagnant Laura

Olivier Duchene

David Christophe

Beckers Bodson Bernard

Beckers Yves

Bindelle Jérôme

Dumont Benjamin

Influence of nitrogen fertilisation and autumn mowing on dual use of thinopyrum intermedium

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INFLUENCE OF NITROGEN FERTILISATION AND AUTUMN MOWING ON DUAL USE OF THINOPYRUM INTERMEDIUM.

Fagnant Laura 1 , Duchêne Olivier 2 , Roiseux Olivier 3 , David Christophe 2 , Bodson Bernard 1 , Beckers Yves 1 , Bindelle Jérôme 1 , Dumont Benjamin 1 .

1 Liege University -Gembloux Agro-Bio Tech, TERRA Res. and Teach. center, Plant Sciences, Crop Sciences, Gembloux, Belgium. Laura.fagnant@uliege.be 2 ISARA, Agroecology and Environment Research Unit, Lyon, France.

3 Walagri, Sombreffe, Belgium.

The double production grain-forage of Thinopyrum intermedium (Host) Barkworth & D.R. Dewey for several years could be of major interest for crop diversification and environmental improvement of the highly productive agroecosystems in Belgium. Indeed, the perenniality of the species could be a way to combine sustainability and agricultural productions. Grain production of Th. intermedium is limited compared to annual cereals, but can be offset by its above-ground biomass production used as forage. As the crop has never been implanted in our regions, characterizing its potential is crucial. The aim of this research is to optimize the dual use of the crop by studying different agronomic levers such as nitrogen fertilisation and autumn mowing. Our results show that the nitrogen fertilisation is very useful in the establishment year, but does not seem significant the subsequent years. During first year, N fertilization has increased grain yield from 0,8 to 1,5 tons/ha and aerial biomass production from 4,3 to 10,6 tons/ha of DM with a fertilisation of 100 kgN/ha applied before stem elongation (BBCH 30). The aestival grain productions can furthermore be combined with post-harvest biomass regrowth that can be mowed in autumn. This regrowth represented roughly 1,3tons/ha of DM during the first year. Although decreasing the amount of total above-ground biomass the following season, this mowing allows to valorise an aerial biomass of good feed values. This cut could also have a positive effect when the nitrogen fertilisation is high by reducing intraspecific competition. These results must be confirmed in the coming years, but they constitute a first understanding of field management to optimize dual production which must be coordinated with grain production objectives. Keywords : Thinopyrum intermedium, perennial grains, nitrogen, mowing


Olivier Duchene

Dumont Benjamin

Douglas J Cattani

Christophe David

et al.. Modelling phenological development of Thinopyrum Laura Fagnant

Florian Celette

Modelling phenological development of Thinopyrum intermedium reveals a photoperiodic effect, affecting flowering earliness

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The wild wheat-relative intermediate wheatgrass (Thinopyrum intermedium (Host) Barkworth & D.R. Dewey) is a winter-hardy cool-season perennial grass, which has recently undergone development as a perennial grain by numerous breeding programs. Perennial grains may be an interesting option to support the transition to agroecological systems thanks to its perenniality which permits production a forage-grain dual income crop for several years at minimum soil and environmental costs. However, the introduction of IWG in grain systems is currently limited by its reduced grain yields. While breeding efforts have mostly been dedicated to improve performance and heritabilities of seed size, mass and number per head, little attention has been paid to IWG phenological milestones that unlock its reproductive growth cycle. Most perennial grasses have a dual induction requirement for reproductive development. It refers to theprimary inductionexposure to winter conditions, including low temperature and short daylength, followed by asecondary inductionperiod of transition to longer days and higher temperatures initiating inflorescence development. Current worldwide endeavors for IWG production has called for deeper investigations of environmental requirements associated with flowering and yielding capacity. In our study, IWG growth stages were measured at the field level, representing the reproductive growth stage and utilizing the BBCH scale. Fields locations in four countries (France, Belgium, US and Canada), representing 127 field observations, obtained from 2012 to 2019. The IWG phenology model was adapted from the STICS soil crop model. After optimization, IWG phenology model was compliant (RRMSE= ~0.03) with experimental results. Our results indicate that IWG vernalization requirements (primary induction) are fulfilled within a range of temperatures from -5 degree to +11 degrees, with an optimum at +3 degree. The total number of equivalent vernalization days required was about 50 days. Then, plant growth and flowering (secondary induction) are associated to both heat unit accumulation and photoperiodic response, leading to potentially significant differences in heat unit accumulation requirements for flowering depending on latitude. Reproductive growth was found to be possible from 10h to 17h daylengths. Overall, modelling IWG phenology enables researchers to virtually explore suitable growth contexts, to optimize field management choices, and to advise further breeding strategies. IWG phenology provides critical information for dealing with sowing choices, weed issues, fertilization or risk of water stress during reproduction. However, phenologic and ontogenic plasticity of such perennial grasses may lead to divergent ecotypes under various agroecosystems with different resource availabilities and management options.

Keywords: perennial grains, Thinopyrum intermedium, phenology, modelling

  Benjamin Dumont 2 , Douglas Cattani 3 , Christophe David 1 , Laura Fagnant 2 , Florian Celette 1 1 ISARA, Agroecology and Environment Research Unit, 23 Rue Jean Baldassini, 69364, Lyon cedex 07, France. olduchene@isara.fr 2 ULiege -Gembloux AgroBio-Tech, Plant Sciences Axis, Crop Science lab., B-5030 Gembloux, Belgium. 3 Department of Plant Science, University of Manitoba, 66 Dafoe Road, Winnipeg, MB, Canada, R3T 2N2


Thomas Jaroslawski
email: thomas.jaroslawski@onera.fr

Maxime Forte

Jean-Marc Moschetta

Erwin Gowree

FP97-2022-jaroslawski BOUNDARY LAYER TRANSITION OVER A LOW REYNOLDS NUMBER ROTOR: EFFECTS OF ROUGHNESS AND FREESTREAM TURBULENCE

Two separate experiments are conducted on a three bladed NACA0012 rotor operating at low Reynolds numbers using phase-locked infrared thermography coupled with simultaneous force and torque measurements. The first, focuses on the effects of freestream turbulence on boundary layer transition over the suction side of the aerofoil of the rotor in an advancing configuration. Freestream turbulence (FST) was generated in an open section wind tunnel using grids and was characterized using Hot-Wire anemometry. In general, when the rotor was subjected to FST, an increase in thrust and efficiency was observed, which could be due to the FST suppressing flow separation or by inducing early transition. The second experiment, consisted of a parametric study on the impact of forcing boundary layer transition using roughness placed on the suction side of the aerofoil, in a hover configuration. The height of the roughness was varied from 52-220µm and was placed at all at at 10% chord, over the entire span of the blade. Force and torque measurements revealed that there could be a optimal roughness height that could lead to a performance increase.

INTRODUCTION

Further knowledge of low Reynolds number flows is becoming increasingly important due to the rise in the demand of micro-aerial vehicles (MAV). This encourages us to rethink the way conventional aerodynamic design is conducted, as these vehicles operate within a Reynolds number (Re = U tip c/ν , where c denotes, ν the kinematic viscosity and for a rotor blade: U tip = RΩ, where R is the blade radius and Ω is the rotational speed.) regime of 10 3 < Re < 10 5 where one can expect that the flow remains laminar to a greater extent but is still suscepti-ble to transition. With MAV systems projected to operate in highly turbulent environments understanding the impact of boundary layer transition becomes critical for the design of low Reynolds number flyers as it could lead to considerable deterioration in performance or in worst case scenarios, massive stall and control loss as they are more prone to separation in the low Reynolds number regime.

Flow separation is an important factor in low Reynolds number aerofoil design philosophy, while trying to extend the amount of laminar flow and delaying transition to turbulence when skin friction drag reduction is necessary. For example, Singh and Ahmed [START_REF] Ronit | Blade design and performance testing of a small wind turbine rotor for low wind speed applications[END_REF] took a lamianr separation bubble (LSB) into consideration when designing a low Reynolds number wind turbine and noted that LSBs over low Reynolds number rotors cause excessive pressure drag, loss in aerodynamic lift and increase the noise produced by the rotor. A recent study, on a rotor blade similar to that of the NASA Mars helicopter, by Argus et al. [START_REF] Finbar | The influence of laminar-turbulent transition on rotor performance at low reynolds numbers[END_REF], used mid-fidelity RANS simulations coupled with a Blade Element Momentum Theory model to find that at Re = 2×10 4 and 8×10 4 increasing the critical N-factor (the amount of perturbation growth required for transition) from 3 to 11 decreased the figure of merit (FM) of the rotor by approximately 40% suggesting that a performance gain is possible. They noted that in their configuration laminar separation without reattachment was the transition mechanism that deteriorated performance.

Contemporary boundary layer transition research on rotors most commonly uses infra-red thermography (IRT), where investigations on laminar to turbulent transition in rotary wing configurations entail more experimental difficulties than in fixed wing configurations. Lang et al. [START_REF] Lang | Boundary-layer transition on a rotor blade measured by temperature-sensitive paint, thermal imaging and image derotation[END_REF] used IRT and PIV to detect the presence of an LSB over a NACA 0015 rotor blade, showing that, the elevated temperature region could be used to deter-mine the location of the bubble(the local Re at which the bubble was measured was 1.4×10 5 ). Recently, Thiessen and Shülein [START_REF] Thiessen | Infrared thermography and dit of quadcopter rotor blades using laser heating[END_REF] conducted transition experiments over a low Reynolds number quadcopter rotor using IRT and oil film topography measurements. They found a region of separated flow, which they claimed to be an LSB that forms over the rotor blade. The laminar separation bubble is a zone of almost stationary or low speed reverse flow, where the shear stress is zero or very low. In the separation region the heat transfer will be minimal and will begin to increase rapidly as transition and turbulent reattachment take place. In general, IR measurements conducted on a model which has a higher surface temperature than the free stream allows for identification of turbulent regions (low temperature, low pixel intensity, I N ), separated regions (high temperature, high values of I N ) and laminar regions (medium temperature regions). In our recent investigations by Jaroslawski et al. [START_REF] Jaroslawski | Characterisation of boundary layer transition over a low reynolds number rotor[END_REF][START_REF] Jaroslawski | Laminar to turbulent transition over a rotor at low reynolds numbers[END_REF] IRT was used coupled with force and torque measurements to characterize the flow topology and performance of a NACA0012 rotor operating at low Reynolds numbers. Boundary layer transition was fostered using large (with respect to the boundary layer thickness) 2D and 3D roughness elements, resulting in a general decrease in performance, which was attributed to increased excrescence drag caused by the roughness elements. Furthermore, it was demonstrated that IRT measurements could provide qualitative quantification of the flow topology, with a possible region of separated flow being present.

Further investigation needs to be conducted on effects of perturbations on the flow development over rotors operating at low Reynolds number, as previous investigators [START_REF] Jaroslawski | Characterisation of boundary layer transition over a low reynolds number rotor[END_REF][START_REF] Koning | Low reynolds number airfoil evaluation for the mars helicopter rotor[END_REF][START_REF] Ronit | Blade design and performance testing of a small wind turbine rotor for low wind speed applications[END_REF] have shown that the state of the boundary layer can have a significant impact on performance. The lack of parametric experimental investigations on boundary transition is addressed in this paper, with the objectives consisting of studies on the effects of different boundary layer forcing techniques on the flow topology and performance, more specifically:

1) What is the impact of freestream turbulence on the performance of a rotor in climb operating at low Reynolds numbers?

2) What is the effect of varying the roughness height of 3D roughness elements on the performance and flow development/topology of the rotor ?

EXPERIMENT AND METHODS

The experiments were conducted at the Institut Supérieur de l'Aéronautique et de l'Espace (ISAE-SUPAERO) with the collaboration of ONERA in Toulouse, France. The rotor had three blades with a NACA0012 profile, an angle of incidence of 10 • and no twist. The radius (R) of the rotor was 0.125 m and the chord length (c) was 0.025 m, resulting the in rotor having an aspect ratio (AR = R/c) of 5. The rotor blades were 3D printed using a reinforced resin, great care was taken to ensure that the blade was smooth. The rotor was placed in a recirculating open test section wind tunnel with an estimated level of freestream turbulence intensity (Tu) of 0.42%. The experimental setup is presented in Fig. 1. Two separate experiments were conducted, the first consisted of the rotor in the climb condition subjected to a freestream flow velocity of U o = 4 m/s and varying levels of freestream turbulence generated by regular square grids (cf. Fig. 2a). Free stream turbulence measurements were conduced using a 5 µm Dantec 55P11 probe, where a 6 mm diameter Dantec 55H24 probe support was used to support the X-wire probes. All test data were acquired using a National Instruments CompactDAQ-9178 with two NI-9239 (built in resolution of 24 bit) modules for voltage measurements. The hotwire was calibrated insitu against a pitot tube connected to a MKS 220DD pressure transducer, using King's law [START_REF] Hans H Bruun | Hot-wire anemometry: principles and signal analysis[END_REF].

The second experiment, consisted of a parametric investigation on the rotor in the hover configuration (U o = 0m/s, ie. wind tunnel shut off) subjected to forcing via 3D roughness (circular cylinders). The roughness elements were placed at x/c = 0.1 on the suction side of each blade, and had a heights (k) varying from 52-220µm.

The performance of the rotor was measured with force and torque measurements and the flow development/topology on the suction side of the aerofoil was captured with IRT. The thrust and torque were measured using the same aerodynamic balance as in Desert et al. [START_REF] Desert | Aerodynamic design of a martian micro air vehicle[END_REF], which used load cells to measure the thrust and torque. To measure the thrust, one end of the load cell was fixed to the motor driving the rotor and the other to a fixed support. The torque was obtained by fixing one end of the load cell to a circular plate fixed to the back of the motor (aligned with the axis of rotation) and the other to a fixed frame surrounding the motor. The frame allows for free rotation of the motor along its axis through two ball bearings. Therefore when a torque is applied, it pushes onto the load cell. It must be noted, that the experimental setup measured the sum of the aerodynamic torque exerted by the rotor on the motor and the internal mechanical torque of the motor. The sampling frequency of the force and torque measurements was 500 Hz, 5000 samples were acquired, with a 10 second stabilization period between each speed interval. The force and torque were measured for long enough so that their means and standard deviations converged. The uncertainty of the thrust and torque measurements was calculated using a 95% confidence interval, the total uncertainty for the thrust and torque was approximately 1% and 0.045 % of full scale, respectively. Using a similar experimental setup as in Jaroslawski et al. [START_REF] Jaroslawski | Laminar to turbulent transition over a rotor at low reynolds numbers[END_REF][START_REF] Jaroslawski | Characterisation of boundary layer transition over a low reynolds number rotor[END_REF], phase locked IRT was used to capture the temperature distribution of the rotor blade's surface. A Brüel Kjaer CCLD laser tacho probe was used to synchronize the blade with the IR Camera. A thin reflective film was placed on an opposing blade, so that for each rotation a voltage pulse was sent out by the laser to trigger the camera. In order to increase the temperature difference between the ambient air and the blades surface, a 500W halogen lamp was used to increase the surface temperature of the blade of interest. Before starting the measurement, the blade of interest was systematically heated by the lamp at a distance of 0.15 m for 10 seconds. The room temperature was at 20 • C and the blade was heated to approximately 35-40 • C in each case, however the absolute temperature is not of great importance as the transition detection techniques are based on temperature differences. Energy injected into the flow due to heating of the blade can affect the critical Reynolds number at which boundary layer transition occurs [START_REF] Schäfer | The effect of heat transfer on the stability of laminar boundary layers[END_REF], however this is mainly relevant for modal transition which is bypassed when transition occurs immediately downstream of a 3D roughness. Furthermore, the heating is too low to affect the transition process, especially at the low Reynolds numbers studied, where the boundary layer is much more stable. In previous work, the overheat temperature employed was between 10-20 • C, where [START_REF] Raffel | Differential infrared thermography for unsteady boundarylayer transition measurements[END_REF][START_REF] Miozzi | Skin friction and coherent structures within a laminar separation bubble[END_REF][START_REF] Dallyn | Characterization of laminar separation bubbles using infrared thermography[END_REF] suggested that these levels of overheat do not significantly affect transition position.

Furthermore, The Richardson number can be used to address the extent of the relevance of natural convection to forced convection. The Richardson number is defined in Eq. 1, where θ , represents the volumetric thermal expansion coefficient (105×10 -6 /C • ), the rotational acceleration is defined as a = U 2 tip /r and used instead of the acceleration due to gravity as rotational accelerations would dominate. The surface temperature of the model and the ambient air temperature are denoted by T w and T ∞ , respectively.

Ri = cθ (T w -T ∞ )/r (1)
Natural convection can be considered to be negligible if Ri < 0.1, in the present work Ri ∼ 0.0032 at r/R = 0.1 and decreases as r/R is increased, therefore natural convection effects on the flow can be neglected. During the IR camera measurement the heating lamp was turned off, the motivation of this technique is that in a turbulent flow region there will be more heat transfer between the wall and outer flow than in a laminar flow region. Therefore the temperature (pixel intensity) will vary in a turbulent, laminar or separated region as a function of time. Since a variety of configurations were tested, a pixel intensity normalization technique needed to be implemented for coherent analysis. Each IR image was normalized by the maximum pixel intensity present in the image so that the different test configurations would be compared reliably.

The high speed and high resolution IR camera (Infratec ) was used to characterize the flow topology over the rotor, where the pixel intensity is used to approximate the state of the boundary layer. The camera has a 320 × 256 pixel MCT sensor having a nominal Noise Equivalent Temperature Difference (NETD) < 20mK and was positioned 0.4m away from the rotor blade and at a horizontal angle of 25 • (cf. Fig. 1. The size of the field of view was 1c x 0.8R and the exposure time of the camera was 68µs, allowing for a good compromise between camera noise and motion blur. Since the camera was synchronized with the rotor, effectively, the sampling rate was that of the rotors turning rate (RPM) for the given run. Motion blur is one of the greatest challenges when conducting IR measurements, even with state-of-the-art cameras. The Motion blur can be calculated by multiplying the exposure time of the camera by the local radial velocity. Increasing the rotational speed and radial position results in more blur. For example at the highest speed, 5000 RPM and at the blade tip, the motion blur can be up to 0.18c, however this value is less than 0.1c for a large portion of the blade as the radial velocity decreases moving inboard. When raw images were analyzed the blur was not significant enough to affect the test result potentially due to the reliable synchronisation. Furthermore, at lower rotational speeds, the blur is between 0.01c and 0.1c, depending on speed and radial position.

Freestream turbulence configurations

The free stream turbulence is characterised by its intensity (Tu) and streamwise integral length scale (Λ u ). The integral length scale is the most energetic scale, corresponding to the average size of the average energy containing eddy. Freestream turbulence was generated using a variety of static turbulence generating grids. Different grid soldities (σ ), mesh sizes (M), bar thickness (t) and relative distances between the grid and the leading edge can be used to vary the FST characteristics. In the present work σ was held constant and M was varied to change the levels of turbulence intensity. Turbulence grids which have a small M, reach equilbrium after a shorter stream wise distance, hence, have low streamwise variation of Tu. All grids were placed at least 15M away of from the rotor, ensuring the FST is relatively isotropic and homogeneous. The Tu is defined in Eq. 2.

Tu = u rms U 0 (2)
The Λ u are calculated by integrating the auto correlation of their fluctuating velocity signals and applying Taylor's hypothesis of frozen turbulence, which converts time scales to spatial scales where f (τ) is the auto correlation of the fluctuating velocity signal and τ is the time delay, and is presented in Eq. 3:

Λ u = U 0 ∞ 0 f (τ)dτ (3)
where f (τ) denotes auto correlation function of the signal and τ the time delay. The the grid parameters U 0 [START_REF] Jens Hm Fransson | Transition induced by freestream turbulence[END_REF] Grid mesh, M

0 h L 0 t 0 x 2 x 3 h (b) RG160 L 0 h L 0 t 0 h (d) RG18
the turbulence generating grids. Note that the origin of the ate system is the centre of the grid.

strongest possible turbulence by comparison to smaller features. We further conjectured that, for a meaningful s of grids of different sizes, and even different designs, one s, as this would ensure that the pressure drop behind all e; we note that this is true not only for regular grids, but aizet & Vassilicos 2015). The solidity of all our grids was alues used in previous studies of fractal grids. Finally, to cts, we machined all grids from metallic sheets of the same actually chosen to be the smallest one that would ensure of the grids ("fractal square grid" -FSG) was multiscale, produce extended downstream regions of C " ≠ constant. ls of partially overlapping elements, was made of titanium he smallest elements and was sanded following machining length and the frontal thickness of the largest elements in ted as L 0 and t 0 , and those of subsequent levels as L 1 , t 1 t the thickness ratio, t r , between the largest and smallest nsequently, the ratio between subsequent elements were 1. The spanwise homogeneity of the mean flow and turbulence was verified with hotwire measurements, however for the sake of brevity are not presented here.

Cfg. M(mm) t(mm) x/M Tu(%) Λ u (mm)

G1

Roughness configurations

Due to the experimental difficulty in measuring the boundary layer thickness (δ ) over a small-scale rotating surface, the δ is estimated numerically. The purpose of estimating the boundary layer thickness was to compare it to the height of the roughness, and to estimate U k the undisturbed velocity at the roughness height, to calculate the roughness Reynolds number, Re k . The pressure over the surface of the rotor was calculated using XFOIL [START_REF] Drela | Xfoil: An analysis and design system for low reynolds number airfoils[END_REF] for the local Reynolds number based on local radial speed and the effective aerodynamic angle of attack (α e f f ) for the corresponding radial position and rotation speed was calculated using a Non Linear Vortex Lattice Method (NLVLM) which should take into account some of the 3D effects present, the method is explained in detail in Jo et al. [START_REF] Jo | Prediction of noise from low reynolds number rotors with different number of blades using a non-linear vortex lattice method[END_REF]. Using the pressure obtained XFOIL (calculated with an N-Factor of 7), the δ was calculated using ON-ERA's in house boundary layer code 3C3D, which solves Prandtl's boundary layer equations for three-dimensional boundary layers using a method of characteristics along local streamlines. To set up the boundary layer equations a body fitted coordinate system is used and the momentum equations are discretized along the local streamlines [START_REF] Houdeville | Three-dimensional boundary layer calculation by a characteristic method[END_REF][START_REF] Perraud | Descriptif et mode d'emploi du code 3c3d. ONERA[END_REF].

A frequently used criterion for determining the critical roughness size of three-dimensional roughness was given by Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF] based on a limited body of experimental observations. In Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF], the roughness Reynolds number is introduced as, Re k = U k k/ν, where k is the roughness height and U k . By its nature, such a criterion, does not address the problem of shape and distribution of the roughness elements, however should suffice for the objectives of this current work. From Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF] the critical roughness Reynolds number for turbulent wedge formation (Cf. [START_REF] Jaroslawski | Characterisation of boundary layer transition over a low reynolds number rotor[END_REF][START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF][START_REF] Goldstein | Lateral spreading mechanism of a turbulent spot and a turbulent wedge[END_REF] for more details) is defined as:

Re k,crit ≈ 600(d/k) -2/5 (4)
where d denotes the diameter of the roughness element.

Using the boundary layer computations (3C3D), the approximated range of δ /k along span of the rotor blade and Re k,crit for the roughness configurations is pretested in Tab. 2. In addition to the criteria proposed by Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF], Gregory and Walker [START_REF] Gregory | The effect on transition of isolated surface excrescences in the boundary layer[END_REF] conducted experiments using isolated roughness elements at various heights, both smaller and larger than δ . They found that at √ Re k ≈ 25 a turbulent wedge would form immediately downstream the roughness element. Furthermore, they also studied roughness elements which were over 2δ and found that the turbulent wedge would increase in spanwise size near the roughness element. The values of Re k,crit and the boundary layer thickness with respect to the roughness are presented in Where the range in δ /k corresponds to r/R = 0.2 -0.95. All roughness elements are located at x/c = 0.1.

RESULTS AND DISCUSSION

Freestream Turbulence

The characterisation of the freestream turbulence is presented in Fig 3, where the streamwise evolution of the Tu (cf. Fig. 3a) with the exponential decay being present before the leading edge of the rotor and the integral length scales increasing in size further away from the grid (not presented here). The Power Spectral Density (PSD) in Fig . 3b, of the streamwise fluctuating velocity, exhibits a broader inertial sub-range for the configuration with a higher level of Tu, which is coherent with the values of Λ u . The coefficient of thrust (C T ) and efficiency (η), Power Loading (PL) and efficiency are defined in Eqs. 5, 6, and 7, respectively. Where T is the thrust, Q is the torque, ρ is the density of the ambient air, Ω is the rotational frequency, R is the rotor blade radius and U 0 is the incoming velocity.

C T = T ρ(ΩR) 2 πR 2 (5)
PL = T ΩQ (6)
Referring to the C T , η and PL presented in Fig. 4, increasing the level of freestream turbulence in general improves the performance. The effects of FST on the C T are less pronounced at lower rotational speeds where at speeds of 2600 -3200RPM there is a slight decrease in the evolution of C T with no effects of FST on the performance being present. This could be due to the the local angle of incidence not being positive along the entire span of the rotor in this range of Ω, causing " locally negative thrust", which contributes to a slight decrease in the total drag. Once Ω > 3200RPM, the angle of attack is positive along the entire span of the rotor and the effects of FST become more pronounced. The mechanism behind the performance increase cannot be fully explained based on the current measurements. However, based on the low Reynolds number and previous investigations [START_REF] Ronit | Blade design and performance testing of a small wind turbine rotor for low wind speed applications[END_REF][START_REF] Finbar | The influence of laminar-turbulent transition on rotor performance at low reynolds numbers[END_REF] boundary separation is possible in the current configuration, leading to a hypothesis consisting of the FST interacting with the boundary layer developing over the rotor. The increased energy in the boundary layer could excite the flow, which would, in turn, delay or suppress flow separation. The effects of the turbulence on the drag of the rotor and support were also investigated, and it was found that the drag execrated by the freestream flow on both was relatively constant over all turbulence configurations, indicating the FST could be indeed interacting with the boundary layer developing over the surface of the aerofoil. Further IRT measurements should be conducted to investigate the flow topology over the rotor in the presence of FST, however it is unsure whether turbulent streaks (commonly present in high levels of FST [START_REF] Jens Hm Fransson | Transition induced by freestream turbulence[END_REF]) could be measured as the Reynolds number is low, hence the temperature gradient between the low and high momentum zones was too small to measure.

Roughness configurations

The C T , Figure of Merit (FM) and PL are presented in Fig. 5, where the FM is defined, viz,

FM = T 3/2 ΩQ 2ρπR 2 (8)
Upon inspection of the C T , FM and PL, it can be observed that the addition of roughness can have a twofold effect on the performance. Roughness elements with smaller wall-normal distance (52 and 78 µm) show an increase in performance and whereas those with higher wall normal distances (140 and 220 µm) deteriorate the performance. A deterioration in the performance a low Reynolds number rotor with roughness elements was observed by Jaroslawski et al. [START_REF] Jaroslawski | Characterisation of boundary layer transition over a low reynolds number rotor[END_REF] where it was postulated, that very large roughness elements could cause increased levels of excrescence drag. On the other hand, increased levels of rotor performance was observed by Jung and Baeder [START_REF] Su | Simulations for effect of surface roughness on wind turbine aerodynamic performance[END_REF], who conducted numerical simulations on a wind turbine rotor and found that forcing transition with optimal distributed surface roughness affects the performance differently depending on the state of the boundary layer. They found that in the presence of separated flows the addition of upstream roughness could increase the performance up to 23.3% whereas it would result in a decrease of 8.3% for an attached flow. Recent numerical simulations on a rotor similar to the NASA Mars helicopter by Argus et al. [START_REF] Finbar | The influence of laminar-turbulent transition on rotor performance at low reynolds numbers[END_REF] at Re ≤ 1 × 10 5 , representative of the current experiment, showed that early transition caused by a decreased critical amplification factor resulted in a thrust increase through the elimination of laminar separation. At higher Re the benefit of early transition diminished and became detrimental for the thrust produced by the rotor. They showed that the FM could be increased by 40% when the N-factor which is a measure of the growth rate of the perturbation in boundary layers.

IR snapshots of the flow topology over the rotor for each roughness configuration, along with radial evolution of the Roughness Reynolds number (Re k ) at x/c = 0.1, which was calculated using the boundary layer profiles calculated with 3C3D are presented in Fig. 6. IR snapshots show the formation of undeveloped turbulent turbulent streaks and at a critical value ofRe k , breakdown into turbulent wedges. Referring to Fig. 6, the Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF] criterion can provide an approximation for when roughness elements will cause the flow to transition. For example, at 3000 RPM (cf. Fig. 6a) Re k does not reach a critical value for roughness heights of 52 and 78µm resulting in no clear wedge formation behind the roughness elements. Furthermore, increasing Ω to 5000 RPM increases the Re k to critical value for the 78 configuration, resulting in wedge formation at approximately the critical value proposed by Von Doenhoff and Braslow [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF]. Additionally, the performance increase is only present when Re k < Re k,crit , suggesting Re k,crit could be used as a threshold for "overtripping", in terms of performance.

CONCLUSION

Infrared thermography coupled with force and torque measurements on a rotor operating at low Reynolds numbers were conducted. The effects of freestream turbulence and roughness revealed that: 1) Freestream turbulence can increase the performance (C T , η and PL) of a rotor operating at low Reynolds numbers (40000 < Re < 110000). The possible mechanisms behind the performance increase could be a delay/suppression of boundary layer separation via freestream turbulence, due to the energy in the freestream exiting the boundary layer. Further investigation should be conducted in characterising the transition mechanisms of the boundary layer developing over the rotor when subjected to freestream turbulence.

2) 3D roughness placed at the x/c = 0.10c can have a twofold effect on the performance, with the roughness Reynolds number (Re k ) being a critical parameter. Criteria proposed by [START_REF] Doenhoff | The effect of distributed surface roughness on laminar flow[END_REF] work reasonably well in determining when turbulent wakes caused by the 3D roughness elements breakdown into turbulence. Configurations which values of Re k were found to increase performance, suggesting that when Re k ≈ Re k,crit the excrescence drag caused by the roughness can be detrimental. The mechanisms of performance increase could be due to delay/suppression of boundary layer separation as suggested by [START_REF] Finbar | The influence of laminar-turbulent transition on rotor performance at low reynolds numbers[END_REF][START_REF] Su | Simulations for effect of surface roughness on wind turbine aerodynamic performance[END_REF].

The present results could indicate that exciting the boundary developing over the suction side of a NACA0012 rotor operating at low Reynolds numbers can increase the performance. In particular, the FST being efficient in boundary layer forcing as the boundary layer is energized without the addition of excrescence drag as with the use of roughness elements, where a incorrectly designed element can decrease the performance. However, the current results also suggest that it is possible to design an optimal roughness trip which can lead to a performance increase.

Figure 1 :

 1 Figure 1: Experimental Setup

L

0. 355 .

 355 As with previous fractal grid designs (Valente & ernandes et al. 2012; Valente & Vassilicos 2014; Hearst silicos 2015), the ratio between the length of subsequent xample, L 1 = 0.5L 0 . The three other grids (RG160, RG80 are grids, each with elements closely matching the first-, nts of FSG. RG18 also matched the effective mesh size SG.

Figure 2 :

 2 Figure 2: Forcing methods used in the study. (a) 3D printed regular turbulence generating

Figure 3 :

 3 Figure 3: (a) Streamwise evolution of the turbulence intensity Tu and (b) Power Spectral Density 20 mm before the rotor

Figure 4 :

 4 Figure 4: (a) The coefficient of thrust (C T ), (b) Efficiency (η) and (c) Power Loading (PL) for the rotor in the climb configuration. The advance parameter is defined as J = nU o /2R

Figure 5 :

 5 Figure 5: (a) The coefficient of thrust (C T ), (b) Figure of Merit (FM) and (c) Power Loading (PL) for the rotor in the hover configuration.

Figure 6 :

 6 Figure 6: Normalised IR snapshots and the evolution of the radial roughness Reynolds number (Re k ) for (a) 3000 RPM and (b) 5000 RPM. NB the horizontal lines denote the critical roughness Reynolds number for each roughness configuration [21]

Table 1 :

 1 Grid and turbulence characteristics at the leading edge of the rotor. Both grids had values of σ = 0.34.

		12	1	20	4.91	4.24
	G2	18	2	16	6.38	8.01

Table 2

 2

	k	Re k,crit k/δ 3000RPM k/δ 5000RPM
	52µm	12.92	0.01-0.21	0.11-0.27
	78µm	14.02	0.14-0.32	0.17-0.40
	140µm	15.75	0.26-0.57	0.31-0.72
	220µm	17.25	0.41-0.89	0.48-1.13

Table 2 :

 2 Summary of roughness configurations tested.