Elodie Hochscheid
email: elodie.h@crai.archi.fr

Gilles Halin

A framework for studying the factors that influence the BIM adoption process

Keywords: BIM adoption, framework, influencing factors, barrier, driver

Building Information Modeling/Management (BIM) is an emerging technological and procedural innovation within the Architectural, Engineering, Construction and Operation (AECO) industry. Adoption of BIM has increased significantly over the last few years and BIM adoption is an active research area that aims to construct a better understanding of the spread of BIM and factors that may explain the speed of BIM adoption and its diffusion. Unfortunately, in this literature, the factors that influence the adoption process are unclear: those who influence the decision to adopt (Decision Factors or DFs) are confused with those that influence the success of the implementation (Implementation Factors or IFs). IFs are very rarely studied, although they could make it possible to produce recommendations to help firms to implement BIM. This paper aims to identify the elements that can influence the BIM adoption process. The main findings of this paper is a classification model of innovation adoption influencing factors, a critical view of methods used to study BIM adoption factors, and an overview of factors that can influence the adoption of BIM, including DFs and IFs.

Introduction

Building Information Modeling/Management (BIM) is an emerging technological and procedural innovation within the Architectural, Engineering, Construction and Operation (AECO) industry. Adoption of BIM has increased significantly over the last few years and is now an active research area. It aims to construct a better understanding of the spread of BIM and factors that may explain the speed of BIM adoption and its diffusion. Our understanding of the BIM adoption process has recently evolved, through the Diffusion of innovation theory. It is described in a five-stages process that opens up new (as yet untapped) perspectives for the study of BIM Adoption Influencing Factors (BIM-AIFs). It reveals that some of BIM-AIFs are not taken into account in the BIM adoption literature.

In a first section, the model of the adoption process is briefly introduced. It brings out the difference between two types of BIM-AIFs: those who intervene before and after the decision of adoption. The value of this distinction and reasons of its absence in BIM-specific literature are explored.

In a second section, literature on innovation is investigated, taking into account the distinction made in the first section. Factors that may be involved in innovation adoption are identified and classified to facilitate the study of specific innovation (as BIM). As a methodology, we started from the generic literature to avoid omissions that may appear in the BIM-specific literature.

In a third section, BIM-specific literature that explore BIM-AIFs is investigated to identify methodology strengths and unexplored opportunities for the study of BIM-AIFs.

As main findings, this paper proposes a framework that can be exploited for the study of BIM-AIFs. This framework takes into account non-BIM-specific literature, recent distinctions made in the BIM adoption area and a critical analysis of the methods used today to study BIM-AIFs.

The BIM adoption process and what impacts it

In this section, the model of the adoption process is briefly introduced. Possibilities that emerge from this model and reasons why they have not yet been studied are explored.

The BIM adoption process

As BIM is an innovation, its spread follows the generic models of the diffusion, adoption and implementation of innovations. BIM adoption is here described in a five-stage process (Hochscheid & Halin, 2019a) (fig. 1), as in the Diffusion of Innovation Theory [START_REF] Rogers | Diffusion of Innovations[END_REF]. This process begins when possible adopters become aware of the existence of BIM (fig. 1, stage 1), evaluate the possibility of using it (stage 2), and take a decision: to adopt or to reject it (stage 3). If decision of adoption is made, adopters undertake a set of activities to deploy BIM processes, tools and methods (stage 4), and need time to anchor these new uses and practices in their habits (stage 5).

Key moments milestone this process: Decision of Adoption (DoA) is the moment when stakeholders make a commitment to start implementing BIM, Effective Implementation (EI) is when stakeholders have tested and used BIM on a part of their production, Confirmation of Adoption (CoA) refers to the moment when stakeholders have anchored BIM in their practice, reached a certain level of mastery of it and have indicated their willingness to continue.

This process can however stop at different moments (Adoption failures), including during implementation [START_REF] Klein | The challenge of innovation implementation[END_REF]. Factors affect the continuation, speed, and cessation of the whole adoption process, here called "BIM adoption influencing factors" (BIM-AIFs) are considered differently depending on whether they occur before or after the DoA.

Figure 1: Model of the BIM adoption process, adapted from (Hochscheid & Halin, 2019a) 2.2 BIM-AIFs: before and after DoA

The DoA (fig. 1) appears to be a turning point in the adoption process. Potential adopters move from a period in which they will need elements to position themselves to make a decision (before DoA) to a period in which they need to achieve an objective they have set for themselves: acquire some mastery of BIM (after DoA). Factors that intervene in the adoption process before the DoA (Decision Factors / DFs) therefore influence the decision-making period whereas factors that intervene after the DoA (Implementation Factors / IFs) will influence the success of implementation and the anchoring of new practices. Both DFs and IFs intervene during the adoption process (and are therefore BIM-AIFs), but they do not concern the same phenomena, and must be differentiated. However, it seems that the difference between DFs and IFs is not explicitly made in BIM adoption literature. The study of the factors that influence the adoption of BIM has rarely been completed with a clear differentiation between the stages of the adoption process, except for very recent work (A. L. [START_REF] Ahmed | A unified BIM adoption taxonomy: Conceptual development, empirical validation and application[END_REF]. Thus, the study of factors that influence BIM adoption have often been confined to the study of DFs. Some hypotheses are put forward here to explain this.

First, it should be noted that Rogers' five-stages model [START_REF] Rogers | Diffusion of Innovations[END_REF] concerns the innovationdecision process. This model focuses on the decision to adopt: even the confirmation stage refers to confirmation of decision. Although this is a seminal work, many research have criticized the Diffusion of Innovation theory, in particular on the vagueness left in the definition of the term "adoption" [START_REF] Bayer | A critique of diffusion theory as a managerial framework for understanding adoption of software engineering innovations[END_REF]. The concept of adoption has several and contradicting definitions (A. L. [START_REF] Ahmed | A unified BIM adoption taxonomy: Conceptual development, empirical validation and application[END_REF]Hochscheid & Halin, 2019a). It is often considered that adoption is synonymous with a decision to adopt [START_REF] Mohammad | Overview of Building Information Modelling (BIM) adoption factors for construction organisations[END_REF], but recent work considers that adoption is a process that includes implementation (A. [START_REF] Ahmed | A Conceptual Model for Investigating BIM Adoption by Organisations[END_REF]. Recently, a study has aggregated BIM-AIFs by positioning them on the adoption process. The authors indicated, by addressing only the first three phases of the process, that they focused on DFs (A. L. [START_REF] Ahmed | A unified BIM adoption taxonomy: Conceptual development, empirical validation and application[END_REF]. It is a step towards understanding the adoption process and the demarcation between DFs and IFs.

Theories of technology adoption were exploited very early on to study BIM-AIFs. These theories describe and explain how individuals choose a technology. After being investigated in a few research, BIM-AIFs extracted from these theories were aggregated in literature reviews and synthesis on the factors that influence the adoption of BIM (A. [START_REF] Ahmed | A Conceptual Model for Investigating BIM Adoption by Organisations[END_REF][START_REF] Ahuja | Factors influencing BIM adoption in emerging markets-the case of India[END_REF]. These syntheses have led to a consensus on the BIM-AIFs that do not include IFs.

It is also possible the difficulty that companies have in implementing BIM has been underestimated. It would seem that we considered that making the decision to adopt would necessarily lead to successful implementation, hence the fact that DFs have been more studied than IFs. But the adoption process can stop between the DoA and CoA, hence the interest of studying IFs.

As a conclusion for this section, we note that there is a significant difference between DFs and IFs. However, this difference has only rarely been exploited in the BIM literature where BIM-AIFs have often been summarized to DFs. This omission may be explained in different ways, but probably finds its source in the confusion about the definition of the word adoption, which has long been considered synonymous with decision to adopt.

Classification of BIM Adoption Influencing Factors (BIM-AIFs)

This section is a non-BIM-related literature review that gives an overview of factors that may influence adoption of an innovation. Studies focusing more on the elements that influence the decision of adopters (individuals and organizations) have been treated separately from those that attempt to analyze the success factors of implementing an innovation in an organization. The elements proposed here are a follow-up to previous work (Hochscheid & Halin, 2019a), but they are here supplemented and structured. The identified factors have been classified into different fields and categories, according to the model presented in fig. 2.

Why do people choose to use an innovation?

A consensus is beginning to emerge on the generic factors that influence the decision to adopt a technology. Presented in different ways in the literature on innovation [START_REF] Waarts | The dynamics of factors affecting the adoption of innovations[END_REF], they can be classified in three factor fields: characteristics of the innovation, internal context of the firm and external context of the firm.

Characteristics of the innovation that play an important role in the choice of this innovation has been described in seminal work of Rogers, in the first edition of Diffusion of Innovation, in 1962. Five attributes of innovation are considered: (1) relative advantage, "the degree to which an innovation is perceived as being better than the idea it supersedes"; (2) compatibility, "the degree to which an innovation is perceived as consistent with the existing values"; (3) complexity "the degree to which an innovation is perceived as relatively difficult to understand and use"; (4) trialability, "the degree to which an innovation may be experimented with on a limited basis"; (5) observability, "the degree to which the results of an innovation are visible to others". Around the 90's and 2000's, research in sociological psychology area focused on user adoption and acceptance of technology with several models called Technology adoption models [START_REF] Collan | Lazy User Model: solution selection and discussion about switching costs[END_REF]. These theories evoke (among other things) characteristics of the innovation that can be involved in technology selection: (6) perceived usefulness, (7) perceived ease of use), (8) technical and (9) economical aspects of the technology. Among the factors mentioned here, some concern the perception that potential users have of innovation (1,2,3,6,7), others concern the intrinsic characteristics of the innovation, identical for all potential users (4,5,8,9), see fig. 3. Internal context of the firm refers to all elements that constitute the firm from the point of view of human relations and the material context of work. The constituent elements of a firm presented here are the result of previous literature review synthesis (Hochscheid & Halin, 2019a). The influence of the internal context of organizations for technology selection has been studied in several disciplinary fields and theories. The abovementioned Adoption Theories also focused on (1) individuals and personal characteristics that can influence their willingness to use a technology. Population ecology theory [START_REF] Hannan | The population ecology of organizations[END_REF] indicates that previous choices made in the firm for (2) systems and staff created internal inertia that impacts future technological choices. In extreme cases, these previous choices may totally prevent an individual or a firm from changing technology [START_REF] Arthur | Competing technologies, increasing returns, and lock-in by historical events[END_REF][START_REF] Liebowitz | Path dependence, lock-in, and history[END_REF]. Individuals react differently to innovations, depending on their experience, seniority, career stage, and skills [START_REF] Mintzberg | Cycles of organizational change[END_REF][START_REF] Sainsaulieu | Vers une théorie sociologique de l'entreprise[END_REF]. Internal political constraints and history of the firm (firm's (3) culture), and (4) interactions within the firm, participate to this inertia and influences the perceived ease of implementation and decision to adopt. all the above-mentioned elements are summarized in fig. 4.

External context of the firm refers to social, economic, political and competitive environment of the firm. Organizations need to be pushed by external forces to change [START_REF] Hannan | The population ecology of organizations[END_REF], as legal barriers, information channels, legitimacy constraints and collective problems. Institutional theory [START_REF] Dimaggio | The iron cage revisited institutional isomorphism and collective rationality in organizational fields[END_REF] classifies these forces in three types : (1) coercive isomorphism (formal and informal pressures from government and other organizations) , (2) mimetic processes (when organizations model themselves on other organizations), and (3) normative pressures (sharing and normalization of conditions and methods of work). They are here referred to as "forces". It is possible to differentiate between those that apply directly from the external context (1 and 3), and another one that the actors apply to themselves (2), see fig. 5.

What influences the success of innovation implementation in a firm?

Much work in the field of management has focused on how an organization can implement change [START_REF] Hannan | Structural inertia and organizational change[END_REF] identified factors that may affect mortality of firms due to change. According to them, characteristics of a company (i.e., size, age-specialized, or generalist), external environment (i.e., stable, uncertain), and the implementation method for change (i.e., type, speed) are involved. [START_REF] Pettigrew | Context and action in the transformation of the firm: A reprise[END_REF], developed the "receptive" and "non-receptive" concept for change context in a firm, and pointed out that "environmental pressure, a supportive organizational culture, the quality and coherence of policy, key people leading change, the change agenda and its locale, the quality of managerial clinical relations, simplicity and clarity of change goals, and co-operative interorganizational relationships" are eight signs that seem to be associated with a faster pace of change. These work are in line with the conclusions of a previous qualitative study of the BIM implementation [START_REF] Hochscheid | BIM implementation in architecture firms : Interviews,case studies and action research used to build a method that facilitates implementation of BIM processes and tools[END_REF]) that identified IFs : implementation time management, change agents chosen, previous habits, availability of BIM-educated professionals, type of projects made by the firm, training process, the firm's culture, project teams, and external partners. In this section, a non-BIMspecific literature is therefore explored to identify and classify possible IFs.

On the basis of the elements presented above, it is here proposed to examine the following fields : characteristics of the innovation, internal context of the firm, external context of the firm, and characteristics of change. [START_REF] Pettigrew | Context and action in the transformation of the firm[END_REF] also evokes the role of chance and surprise in this process. Let us keep this in mind to remind ourselves that change within a firm is a very complex process that certainly cannot be predicted or fully modeled.

Characteristics of the innovation that influence decision to adopt described in the previous section (fig. 3) may also influence implementation and confirmation. The subjective part of these characteristics (if the innovation is perceived useless or too complex for example) will influence individual's motivation for implementation and can cause high resistance to change during implementation [START_REF] Kotter | Choosing strategies for change[END_REF]. Intrinsic properties of innovation can influence individual's perception of it [START_REF] Klein | The challenge of innovation implementation[END_REF] and can be directly involved during implementation, for example when individuals are confronted with interoperability problems (technical aspects), lack of help and tutorials (observability, availability), or an insufficient return on investment (economical aspects) [START_REF] Hochscheid | BIM implementation in architecture firms : Interviews,case studies and action research used to build a method that facilitates implementation of BIM processes and tools[END_REF].

Internal context of the firm's impact on successful change (and therefore during the implementation) has been highlighted in several research [START_REF] Franklin | Characteristics of successful and unsuccessful organization development[END_REF][START_REF] Johnson | Managing strategic change-strategy, culture and action[END_REF][START_REF] Kim | The link between individual and organizational learning[END_REF][START_REF] Pettigrew | Context and action in the transformation of the firm[END_REF][START_REF] Pettigrew | Context and action in the transformation of the firm: A reprise[END_REF]. [START_REF] Mintzberg | Cycles of organizational change[END_REF] identified different levels of change in an organization: culture, structure, systems, people, vision, positions programs and facilities. The firm's culture can, for example, integrate a general openness to innovation and an active and permanent desire to improve effectiveness, which facilitates implementation. Demographic characteristics of the firm (number of employees, projects size, number of hierarchical levels), interactions within the firm (between top-management and employees) or relations with partners can boost or lower implementation easiness [START_REF] Laforet | Organizational innovation outcomes in SMEs: Effects of age, size, and sector[END_REF]. As indicated in previous section, the nature and functioning of previous systems and characteristics of individuals (i.e. tolerance to change) also play a role in the ease of implementation. All the levels of the company presented above (fig. 4) are therefore involved in the decision to adopt as well as in the implementation process.

External context of the firm can impact change implementation at different levels [START_REF] Franklin | Characteristics of successful and unsuccessful organization development[END_REF][START_REF] Pettigrew | Context and action in the transformation of the firm[END_REF]. Conditions of demands, sharing and exploration of innovation with partners and clients (normative forces, coercive forces) can encourage or discourage people for implementation [START_REF] Pettigrew | Context and action in the transformation of the firm: A reprise[END_REF]. The availability of implementation protocols (normative forces) or possibility for firms to identify good practices through other firms that have already taken the plunge (mimetic forces) impacts ease of implementation. These three factors have been put in place to explain decision to adopt BIM (fig 5), but can be translated into IFs as well.

Change characteristics refers to the way in which change is implemented in the company. Change management in the company is essential to the success of the implementation. Main components of change that play a role in the success of implementation are here extracted of change management literature, and classified in three main categories, according to [START_REF] Hochscheid | Micro BIM adoption in design firms: Guidelines for doing a BIM implementation plan[END_REF]) (fig. 6). [START_REF] Kotter | The heart of change: Real-life stories of how people change their organizations[END_REF], (b) the extent of change within the company [START_REF] Pettigrew | Context and action in the transformation of the firm[END_REF], (c) the distance between previous practices and new practices, which refers to the amount of change to be made (depth).

(2) Interest and involvement of individuals (top management and employees) in change matches three aspects: (a) attitude of individuals towards change and climate in the firm that can lead to resistance or openness to change [START_REF] Franklin | Characteristics of successful and unsuccessful organization development[END_REF][START_REF] Johnson | Managing strategic change-strategy, culture and action[END_REF][START_REF] Klein | The challenge of innovation implementation[END_REF]), (b) motives and commitment that led to implementation effectiveness [START_REF] Greenwood | Organizational design types, tracks and the dynamics of strategic change[END_REF][START_REF] Klein | The challenge of innovation implementation[END_REF], (c) base, which refers internal dynamics related to the change (top-down or bottom-up).

(3) The practical solutions that can be deployed within the firm to manage implementation of an innovation were the subject of further study in [START_REF] Hochscheid | Micro BIM adoption in design firms: Guidelines for doing a BIM implementation plan[END_REF]. These are here summarized in six categories : (a) change management, refers to the way change is organized (needs assessment, transition planning, evolution of the firm's strategy, staff and team management), (b) characteristics of the change agent [START_REF] Franklin | Characteristics of successful and unsuccessful organization development[END_REF], which is the person or group that leads implementation in the firm and who can from inside and outside of the firm (both are needed [START_REF] Johnson | Managing strategic change-strategy, culture and action[END_REF], (c) the first project on which tests are made (pilot project), (d) the training progress, topics, and assessment of internal standards, (e) Metrics (key performance indicators and maturity metrics), and (f) risk management for the firm towards implementation.

Synthesis

In this section, factors that intervene throughout the adoption process (DFs and IFs) have been identified and classified, on the basis of literature area not related to a specific innovation. The respective literatures associated with these two types of factors are different and bring out factors that are expressed differently. It appears that the main areas involved in the adoption process are: the internal context of the firm, the external context of the firm, the innovation characteristics, and the change characteristics. Change characteristics only concern the implementation and confirmation phases of the adoption process (IFs), whereas the three other factor fields concern both DFs and IFs. The classification model of factors proposed here is very similar to the taxonomy proposed by (A. L. [START_REF] Ahmed | A unified BIM adoption taxonomy: Conceptual development, empirical validation and application[END_REF], which covers the first three phases of the adoption process (and therefore only the DFs). However, in this paper, an additional level of classification is proposed and the entire adoption process is covered. This overview includes a wide variety of factors involved in innovation adoption process and is intended to be used for the study of innovation adoption process.

BIM Adoption influencing factors

The previous section provides an overview and structuration of the factors involved during a generic innovation adoption process. In this section, the way this topic is handled in BIM-specific literature is explored; with an emphasis on the methodologies used rather than the results obtained.

A critical view of the methods used for the study of BIM-AIFs

Several theories and fields that make it possible to study the adoption of an innovation have been presented previously. BIM studies on adoption influencing factors are generally mono-oriented: it focus on one theory or one field but does not offer an overview. Technology adoption theories are for instance investigated in [START_REF] Lee | BIM Acceptance Model in Construction Organizations[END_REF][START_REF] Son | What drives the adoption of building information modeling in design organizations? An empirical investigation of the antecedents affecting architects' behavioral intentions[END_REF], institutional theory are covered in [START_REF] Dongping | Impacts of Isomorphic Pressures on BIM Adoption in Construction Projects[END_REF]. However, some recent studies undertake a classification of factors and propose an overview to get a research framework, notably technological, environmental, and organizational fields of influencing factors (A. [START_REF] Ahmed | A Conceptual Model for Investigating BIM Adoption by Organisations[END_REF][START_REF] Ahuja | Factors influencing BIM adoption in emerging markets-the case of India[END_REF]. Change characteristics don't seem to appear in a structured way in BIM-specific literature, even if some change-related factors are mentioned [START_REF] Aibinu | Status of BIM Adoption and the BIM Experience of Cost Consultants in Australia[END_REF].

BIM-AIF are seldom positioned on the adoption process. However, it has been shown that the significance of the impact of a factor depends on the stage of the adoption process concerned (A. L. [START_REF] Ahmed | A unified BIM adoption taxonomy: Conceptual development, empirical validation and application[END_REF]. This impact also depends on the diffusion process [START_REF] Waarts | The dynamics of factors affecting the adoption of innovations[END_REF]. For instance, the adopters who decided to adopt BIM 5 years ago were not driven by the same reasons (factors) as those that are doing it today. But taking into account all these parameters (factor field, adoption stage, positioning in relation to the diffusion process) makes the study very complex.

Methodologies used in BIM-specific literature to study these factors are varied. Most studies carry out a literature review to identify factors that influence the adoption of an innovation, and include at least one theory or field mentioned above. To identify the factors that influence the adoption of BIM specifically, some studies use qualitative methods (i.e. interviews, action research [START_REF] Bin Zakaria | Exploring the adoption of Building Information Modelling (BIM) in the Malaysian construction industry: A qualitative approach[END_REF]), but most of them exploit quantitative methods. Studies that combine qualitative and quantitative approaches are rare [START_REF] Aibinu | Status of BIM Adoption and the BIM Experience of Cost Consultants in Australia[END_REF][START_REF] Dongping | Impacts of Isomorphic Pressures on BIM Adoption in Construction Projects[END_REF]. The number of respondents for the questionnaire surveys (quantitative) is generally relatively small (137 for [START_REF] Dongping | Impacts of Isomorphic Pressures on BIM Adoption in Construction Projects[END_REF], 125 in [START_REF] Acquah | Acceptance of Building Information Modelling: a survey of professionals in the construction industry in Ghana[END_REF], 184 in [START_REF] Ahuja | Factors influencing BIM adoption in emerging markets-the case of India[END_REF], 102 [START_REF] Arunkumar | A feasibility study on the implementation of building information modeling (BIM): from the architects' & engineers' perspective[END_REF]. Quantitative research generally provide in-depth statistical analysis to study relations (correlations) that can exist between the different factors (correlation matrix, Pearson's chi-squared test, Cronbach coefficient, Spearman's rho coefficient, or the Kendall's Tau coefficient).

The vocabulary used for describing BIM-AIFs is wide and varies from one paper to another: they can be referred to as factors, determinants, drivers, or barriers. It is sometimes considered that factors can take two forms: drivers (if they positively influence adoption) or barriers (when they hinder adoption) [START_REF] Ahuja | Factors influencing BIM adoption in emerging markets-the case of India[END_REF]. Some research focus exclusively on barriers [START_REF] Hosseini | Roadmap to Mature BIM Use in Australian SMEs: Competitive Dynamics Perspective[END_REF][START_REF] Olawumi | Barriers to the integration of BIM and sustainability practices in construction projects: A Delphi survey of international experts[END_REF] or drivers. There is research that differentiates between the two and addresses both [START_REF] Abubakar | Contractors' perception of the factors affecting building information modelling (BIM) adoption in the Nigerian construction industry[END_REF][START_REF] Arunkumar | A feasibility study on the implementation of building information modeling (BIM): from the architects' & engineers' perspective[END_REF]. The wording given to the factors makes it possible to know whether the impact studied is a priori rather negative (i.e." absent harmonization between standards") positive (i.e. "efficient interoperability"), or neutral (i.e. "interoperability"). However, this can have a strong impact in a questionnaire survey because respondents can be influenced by the question's wording.

Synthesis

Studying BIM-AIFs is difficult because it depends on many different elements such as the stage of the adoption process and the dissemination process. Many studies carry out literature review to find factors that may influence adoption, but provide only a partial view of these factors. This underlines the need of a framework for the study of BIM-AIFs. The explored approaches (qualitative, quantitative) are varied but rarely combined in a single study, while a variety of methods might produce interesting new results. Also, if we note that a lot of literature review work has already been done on DFs, it seems that IFs are very little conceptualized. Lastly, the way the factor is worded is important and can convey the kind of impact the factor may have. We therefore propose to extend the classification model (fig. 2) to include the factor's expression and its properties (fig. 7) (positive formulation: driver, neutral formulation: determinant, negative formulation : barrier).

Conclusion

Adoption is here presented in a five-stages process from which we can deduce two main parts: the decision part and the implementation part. These two parts do not refer to the same phenomenon among adopters and the factors that influence them must therefore be differentiated. BIM adoption influencing factors (BIM-AIFs) may address users' decision to start using BIM (Decision Factors, DFs), or the success of the implementation (Implementation Factors, IFs). This distinction is fundamental because DFs cannot by themselves explain the speed of diffusion, while BIM-specific literature, though, seems to focus exclusively on DFs. Adoption has, indeed, often been considered synonymous with decision-making. This paper provides an overview of DFs and IFs, by investigating non-BIM-specific literature. The latter have been classified in four fields of influence: internal context of the firm, external context of the firm, characteristics of the innovation and characteristics of change. There is overlap between the two types of factors within these fields, but depending on whether they are DFs or IFs, they do not seem to operate in the same way. Research on BIM-AIFs mainly provide partial view of these factors, by confining oneself to the study of a single field, or only to the decision part of the adoption process. This underlines the need of a framework for the study of BIM-AIFs. Also, the distinction between factors that act negatively (barriers) or positively (drivers) is sometimes not made although it can play a predominant role, especially in questionnaire surveys. This research

Figure 2 :

 2 Figure 2: classification model of innovation adoption influencing factors

Figure 3 (

 3 Figure 3 (left): classification of adoption influencing factors for "innovation characteristics" field Figure 4 (right): classification of adoption influencing factors for "firm's internal context" field

Figure 5 :

 5 Figure 5: classification of adoption influencing factors for "firm's external context" field

Figure 6 :

 6 Figure 6: classification of adoption influencing factors for "change characteristics" field

Figure 7 :

 7 Figure 7: classification model of innovation adoption influencing factors (extended)

therefore proposes a complete framework for the study of BIM-AIFs. The authors have already used this framework to create a questionnaire, in which both DFs and IFs are addressed, and distinction between barriers and drivers is made.


Donglai Gao

Guanbin Chen

Wenli Chen

Yewei Huang

Hui Li

Effects of steady wake-jets on subcritical cylinder flow

Keywords: Active jet control, cylinder flow, jet momentum coefficient, aerodynamic forces; wake vortex evolution

We investigate the effects of active wake-jets (characterized by a dimensionless jet momentum coefficient C µ ) on the suppression of aerodynamic forces and the manipulation of wake flow topology behind a cylindrical model through wind tunnel tests. The active jets are positioned at the rear stagnation points of the cylindrical test model. The experimental campaign is conducted at a subcritical Reynolds number of Re= 3.33 × 10 4 .

The surface pressure distributions around the bare and controlled cylinders are obtained by using a pressure measurement system. Apart from pressure measurements, we also obtain the streamwise and spanwise flow structures around the circular cylinder with different C µ (including C µ = 0) by employing the particle image velocimetry (PIV) technique. Pressure measurement results demonstrate that the lift force acting on the cylindrical test model is greatly reduced and drag decreased with the implementation of active wake-jets.

Besides, it is found that a higher C µ contributes to a better control effectiveness in unsteady lift forces but not necessarily a better drag reduction. PIV measurement results indicate that the mechanism of the active jet control scheme is to impose steady and symmetric perturbations into the unsteady and asymmetric flows in the cylinder wake. Owing to the dynamic competition of the wake-jet flow and shear layer flows, the vortex shedding pattern behind the controlled cylinder is greatly modified, vortex formation length significantly elongated and the fluctuations of aerodynamic forces conceivably suppressed.

Introduction

Structures with a cylindrical section are prevalent in engineering applications, examples include long stay cables of cable-stayed bridges, transmission lines, cooling towers, boat masts, chimney stacks and some modern high-rise buildings. When these cylindrical structures are exposed to wind, the surrounding airflow experiences a flow separation and leads to a periodic vortex street downstream in the near wake. This alternating process of vortex shedding generates highly unsteady aerodynamic forces acting onto the cylinder [START_REF] Brika | Vortex-induced vibrations of a long flexible circular cylinder[END_REF]). If the frequency of the periodic vortex shedding matches some-order modal frequency of the structure, a frequency-resonance will develop. In consequence, a fluid-structure synchronization called lock-in may lead to vortex induced vibrations (VIVs) of the structures. One typical case is the frequently observed VIVs of stay cables of cable-stayed bridges in the subcritical Reynolds number range (see [START_REF] Zuo | Interpretation of field observations of wind-and rain-wind-induced stay cable vibrations[END_REF] for instance).

Most of the flow induced vibrations of structures are undesired. For example, the wind induced cable vibrations may lead to the breakdown of protections against corrosion and impair the cable-deck connections [START_REF] Pacheco | Keeping cables calm[END_REF]). Therefore, various counter-measures have been proposed and developed to suppress the wind induced vibrations of structures. Since the alternating vortex shedding is the main cause of wind induced vibrations, a well established philosophy is to alleviate the vortex shedding from cylindrical structures thus to suppress the fluctuating lift forces. A great quantity of simple control schemes are introduced to control the periodic vortex shedding from a circular cylinder so as to decrease the drag and lift forces. Typical examples are splitter plates, guiding vanes, surface protrusions, base-bleed, as reviewed previously in [START_REF] Zdravkovich | Review and classification of various aerodynamic and hydrodynamic means for suppressing vortex shedding[END_REF]. Recently, a passive jet flow control scheme was developed by [START_REF] Chen | Passive jet control of flow around a circular cylinder[END_REF]. Measurement results of flow structures demonstrate that the mechanism of passive jet control is to cause passive jets into the wake of cylinder and convert the vortex shedding mode from antisymmetric to symmetric. As a result, the drag is reduced and lift fluctuations suppressed. [START_REF] Bouak | Passive control of the aerodynamic forces acting on a circular cylinder[END_REF] presented a control by placing a smaller cylinder in the cylinder wake and interacting with the upstream main cylinder. The results show that at a stagger angle of 45 • , the lift fluctuation of the upstream main cylinder is reduced by 41%. [START_REF] Zhou | Control of flow past a dimpled circular cylinder[END_REF] studied the flow around a cylindrical model with dimpled surface. The experiment results indicate that the dimpled surface contributes to a drag reduction of 10% and affects the vortex-shedding strength in the near wake of the cylinder. [START_REF] Oruç | Piv measurements on the passive control of flow past a circular cylinder[END_REF] conducted PIV measurements on flow around a cylindrical test model modified with drop-shaped mesh. They found that the vortex formation length is longer, the turbulence kinetic energy and normalized Reynolds shear stress contours are conspicuously weaker than the bare cylinder without control. [START_REF] Durhasan | Vortex street suppression of a circular cylinder using perforated semi-circular fairing in shallow water[END_REF] used a perforated fairing coating on a circular cylinder to realize vortex street suppression. They observed that the wake formation region of the modified cylindrical model is elongated notably and the strength of shear flows are decreased compared to the natural case.

The schemes discussed above usually fall into a category of passive flow control, since they need no external energy to maintain the control process once implemented. Another kind of control schemes is active flow control. In a case of active control, continuous energy input is required to maintain the control process.

Classical active control schemes are steady suction [START_REF] Chen | Suppression of vortex-induced vibration of a circular cylinder using suction-based flow control[END_REF] and [START_REF] Chen | An experimental study on a suction flow control method to reduce the unsteadiness of the wind loads acting on a circular cylinder[END_REF]), synthetic jets (Feng et al. (2010), Feng and Wang (2010), [START_REF] Feng | Synthetic jet control of separation in the flow over a circular cylinder[END_REF]) and plasma actuators [START_REF] Benard | Response of a circular cylinder wake to a symmetric actuation by non-thermal plasma discharges[END_REF]). A steady suction flow control method was employed by [START_REF] Chen | Suppression of vortex-induced vibration of a circular cylinder using suction-based flow control[END_REF] to alleviate the periodic vortex shedding in the wake of a vibrating cylinder, thereby decrease the amplitude of its vortex induced vibration. The wind tunnel tests reveal that the steady suction flow control shows good control effectiveness by dramatically reducing the vibration amplitudes of the cylinder in the lock-in range. Followed by vibrational tests, the effects of active suction on manipulation of the wake vortex-shedding and suppression of the unsteady lift forces are investigated in detail by [START_REF] Chen | An experimental study on a suction flow control method to reduce the unsteadiness of the wind loads acting on a circular cylinder[END_REF]. In addition, Feng et al. (2010), Feng and Wang (2010) and [START_REF] Feng | Synthetic jet control of separation in the flow over a circular cylinder[END_REF]) performed serial experiments to find that the synthetic jets implemented at the rear stagnation points of a cylindrical model act as external perturbations on the unsteady and periodic wake flows and lead to a notable modification to the alternating vortex-shedding process. [START_REF] Dong | Elimination of vortex streets in bluff-body flows[END_REF] introduced a control concept characterized by combined windward suction and leeward blowing (WSLB). This concept is investigated experimentally by [START_REF] Gao | Active control with windward suction and leeward blowing[END_REF], and the test results show that the periodic vortex shedding in the cylinder wake is attenuated due to the action of WSLB control.

In the current study, we focus on the effects of a crucial control-parameter, i.e. the equivalent jet momentum coefficient C µ , on the active control of cylinder flow, including effects on the aerodynamic forces, wake flow evolutions and spanwise flow structures. This paper is organized as follows. Details of the cylindrical model with active jet control, the experiment set-up and the experimental campaign are given in Sec. 2, the test results are presented and analyzed in Sec. 3 and some concluding remarks are drawn in Sec. 4.

Experimental set-up

The experimental campaign is performed in a low-turbulence wind tunnel (SMC-WT2), the Joint Laboratory of Wind Tunnel and Wave Flume (WTWF), Harbin Institute of Technology, China. This closed-circuit wind tunnel has a test chamber of 800 mm (width) × 1200 mm (height) × 3000 mm (length). Tunnel walls of the test chamber are designed to be transparent in order to facilitate flow diagnostics with optical techniques.

Honeycombs and mesh structures are mounted ahead of a contraction section to produce uniform wind entering the test chamber. By using a Cobra Probe, the turbulence intensity level inside the test chamber we used in the current study is measured to be less than 0.40% .

Cylindrical model with active jets in the wake

The cylindrical test model is made of a Polymethyl methacrylate (PMMA) tube with a smooth surface.

The outer diameter of the cylindrical model (D) is 100 mm, thus the solid blockage in the present experiment is 8.33%. The horizontal test model, as shown in Fig. 1, is fixed to the end plates and perpendicular to the incoming wind. The end plates are firmly locked onto the section walls. The test model has a total spanwise length (L) of 800 mm to adapt the chamber width. Four jet nozzles, with an inner diameter of 10 mm, are manufactured at the leeward stagnations of the cylinder so the active jet flows will be evacuated to the cylinder wake along the same direction of the incoming airflow. The spanwise spacing between two neighboring jet nozzles, L 0 , is L/8, as shown in Fig. 1. The jet nozzles are made with 3D printing technique to minimize the mechanical errors during manufacturing process. For every test case, the wind speed entering into the test chamber U 0 is fixed at 5.0 m•s -1 , resulting in a Reynolds number, given the model diameter, C µ in the present study. This dimensionless parameter is defined as the ratio of the jet momentum flux to the incoming momentum flux of freestream, i.e. C µ = ( Ūj /U 0 ) 2 (S j /DL 0 ). Here, Ūj is the mean jet velocity from the nozzles, S j is the area of the jet nozzle at each jet section, and L 0 =L/8 is the interval between two neighboring jet nozzles. The jet-flow rate, jet velocity from the nozzles and its resultant jet momentum coefficient C µ for the controlled cases are calculated and collected in Table . 1.

Measurement of surface pressure

In the mid-span of the cylinder, a sectional pressure measurement ring is placed to measure the distributions of wind pressure on the cylinder surface, as can be noted in the left panel Fig. 1. The pressure measurement ring is also made by a 3D printer and its constitutive material is photosensitive resin. 36 pressure taps are arranged axisymmetrically on the measurement plane, as sketched in the right subplot of Fig. 1.

A surface pressure measurement system that collects three models (DSA3217, range: 10 inH 2 0; accuracy: ±0.20% of full scale) is connected to the pressure taps in order to record the instantaneous surface-pressure distributions on the mid-span surface of the cylinder. The taps in the mid-span section and pressure scanivalves are connected via 36 Polyvinyl chloride (PVC) tubes. These PVC tubes have an uniform length of 500 mm and an inner diameter of 0.9 mm. In the present study, the acquisition and recording for instantaneous surface pressure measurement last for 32 s for each test case and the sampling frequency is fixed at 312.5 Hz, as in Gao et al. (2017b).

After pressure measurements, the time-variant aerodynamic drag and lift forces can be estimated by a standard integration process described in [START_REF] Chen | An experimental study on a suction flow control method to reduce the unsteadiness of the wind loads acting on a circular cylinder[END_REF]. The instantaneous coefficients of the drag and lift forces, C d,t and C l,t can be estimated by:

C d,t = 1 2 ∑ C pi ∆φ i • cos(φ i ) C l,t = 1 2 ∑ C pi ∆φ i • sin(φ i ) (1)
where

C pi = 2(p i -p ∞ )/ρU 2 0
is the surface pressure coefficient, p i is the pressure acting on the test model and p ∞ is the static pressure of incoming wind. Both the time-variant p i and p ∞ are obtained by the pressure measurement system. φ i is the azimuthal angle that the i-th pressure tap locates, and ∆φ i = 10 • is the increment between two pressure taps, as shown in Fig. 1. After obtaining the time-histories of the aerodynamic coefficients of the cylinder, counting the mean value of drag coefficients (denoted by C D ) and root mean square (RMS) value of lift coefficients (denoted by C ′ L ) is helpful to assess the control effects of active wake-jet control for different jet momentum coefficients.

PIV measurements

We employ a particle image velocimetry (PIV) system in the current study to measure the flow characteristics around the controlled and bare cylinders. Figure 2 illustrates the experimental set-up for the two-dimensional PIV measurements. Small oil droplets of ∼ 1.0 µm in size are produced to seed the flow.

Illumination is provided by a double-pulsed Nd:YAG laser with a pulse energy of 425 mJ/pulse at a wavelength of 532 nm. The laser sheet is manipulated to be about ∼1.0 mm in thickness to illuminate the target sections. A high-resolution digital camera (PCO1600) is used for PIV image acquisition. The CCD camera and double-pulsed Nd:YAG lasers are communicated to a workstation via a Digital Delay Generator, thus the timing of the laser illumination and the image acquisition can be precisely controlled. In the PIV measurements, the repetition frequency of laser generator and PIV image acquisition is remained at 8 Hz.

Two transverse planes, i.e. the mid-span plane Z = 0 (coordinates are defined in Fig. 1), one jet plane Z/D = 0.5, and one horizontal plane Y = 0 are selected as the target planes of PIV measurements (as sketched in Fig. 2) to get a better understanding of three-dimensional characteristics of the cylinder flow modified with active jets. For every test case, the PIV system records more than 400 pairs of instantaneous images in order to arrive at a good convergence of measurement. In the image processing, a cross-correlation algorithm with an interrogation window size of 32×32 pixels and an effective overlap of 50% of the interrogation windows is applied to derive instantaneous flow vectors. If we take PIV measurements of Z-plane as an example, the velocity (u, v) and spanwise vorticity (Ω z ) can be readily determined by definition. Besides, the time-averaged flow characteristics, such as mean flow velocity (u, v), turbulence kinetic energy (TKE=

0.5 × (u ′ 2 + v ′ 2 )/U 2 0 ) and normalized Reynolds shear stress (τ = -u ′ v ′ /U 2 0 )
can also be obtained. In the current PIV measurements, the uncertainty is estimated to be within 2% for the instantaneous flow vectors, whereas that of the time-averaged flow quantities is estimated to be less than 5% (algorithm according to [START_REF] Park | Uncertainty analysis: particle imaging velocimetry[END_REF]).

Experiment results

Results of aerodynamic forces

Figure 3 presents the mean and fluctuating distributions of the surface-pressure for different jet momentum coefficients C µ . A symmetric region with negative pressure distributions is witnessed to develop at the leeward surface of the cylindrical test model without control. This is due to flow separation and it will lead to a recirculation region in the cylinder wake, as we shall see in section 3.2. With the implementation of active wake-jets, the regions of negative pressure distributions are generally lifted compared to the baseline case.

It is noted that increasing the base pressure on the leeward surface of the cylinder contributes to a drag reduction. Besides, the base pressure distributions experience a gradual enhancement as the jet momentum coefficients increases to 0.0796. This favorable effects will result in a gradually enhanced drag reduction.

When C µ shifts to 0.1146, the pressure distributions experiences no significant change, indicating a relatively steady drag coefficient C D . However, when C µ continues to increase, the pressure distributions on the leeward surface of the cylindrical test model are decreased. In this case, C D will experience a upward tend and the drag reduction effect is weakened. It can also be noticed that the separation points of controlled cylinders are a little bit earlier than the bare cylinder. As will be revealed by PIV measurement results, the active-jet-flow 6 It can be noted that with the increase of C µ , the control effectiveness of the active wake-jets to alleviate the unsteady and periodic vortex shedding is enhanced. These observations from pressure measurement results will be confirmed by PIV measurement results in the section that follows.

On the basis of the time sequences of the dynamic aerodynamic forces, the mean drag (C D ) and RMS lift coefficients (C ′ L ) for different C µ values (including zero C µ value, i.e. the baseline case) are calculated and shown in Fig. 7, to evaluate the active-jet control effectiveness more quantitatively. First, when C µ is zero, C D and C ′ L are calculated to be 0.36 and 1.12 respectively. After that, both C D and C ′ L experience a downward trend as C µ increases. This suggests that the active jet technique can reduce the drag and lift forces effectively. When C µ equals 0.0796, 17.8% drag reduction and 73.6% lift suppression are simultaneously reached. As C µ continues to go high to 0.0963 and 0.1146, C D remains stable. However, when C µ reaches 0.1345 and 0.1560, C D is increasing, indicating a less effective control in drag reduction. On the other hand, when C µ exceeds 0.0963, C ′ L experiences an insignificant reduction and the optimal control effectiveness of fluctuating lift is 81.2% at the maximum C µ of 0.1560. Meanwhile, the decreasing trend is significantly slowed down, because the excitation level is so high that a saturation is obtained. It should be noted that these four jet nozzles are arranged isolatedly at the rear stagnation points, thus the momentum flux they inject into the wake flow is less efficient in flow modification than uniformly distributed jets along the spanwise direction Feng and Wang (2010) for instance). Furthermore, since the pressure taps are arranged in the plane with the longest distance from the jet nozzles, the active control scheme is expected to exhibit the least control effectiveness in the mid-span, i.e. the pressure measurement plane. Accordingly, the evaluation of control effectiveness on the basis of pressure distributions is believed to be on a conservative side and leads to an underestimated control effectiveness of mean-drag reduction and fluctuating-lift suppression of the circular cylinder.

C  =0.1146 C  =0 C  =0.0287

PIV measurement results

In the post processing of flow images obtained by PIV system, a snapshot proper orthogonal decomposition (POD) method [START_REF] Sirovich | Turbulence and the dynamics of coherent structures. i. coherent structures[END_REF], [START_REF] Meyer | A turbulent jet in crossflow analysed with proper orthogonal decomposition[END_REF]) is employed to quantify the flow characteristics of the streamwise and spanwise wakes of the cylinder so as to investigate the control mechanism of active wake-jets.

The energy map of POD modes for different cases is plotted in Fig. 8. It is shown that the several leading modes occupy the most energy for all the test cases, since the first POD modes reflect the large-scale and dominant coherent-structures of the global flow field. In the present study, flow structures are reconstructed by employing the first six POD modes, as in Gao et al. (2017a).

Figure 9 shows the mean flow structures behind the mid-section of the cylinder with different C µ . For the baseline case without active jet control, i.e. C µ = 0, the flow separations take place near the angles of 90 • and 270 • . Because of the flow separation, a recirculation region will be formed in the near wake. 2014)). Consequently, a higher C µ leads to a more stabilized wake and a less fluctuating amplitudes of lift forces, i.e. a smaller C ′ L , as presented in Fig. 7. Another notable observation in Fig. 9 is that the wake formation regions of controlled cylinders are elongated in comparison with the baseline case. This will be addressed in detail associated with spanwise flow structures.

To get an insight into the mechanism of active wake-jets, it is helpful to pay more attention to the jet section (Z/D=0.5). Figure 10 presents the instantaneous vortical structures behind the wake-jet section for different test cases. As revealed by the instantaneous PIV measurement results, a pair of antisymmetric vortical structures sheds periodically from the bare cylinder (C µ = 0) to form a Karman vortex street in the cylinder wake. This alternating and unsteady process from the solid-wall of cylinder will lead to dramatic pressure fluctuations on the cylinder model, especially on its leeward surface, as can be observed in Fig. 3. Therefore, both lift and drag forces will fluctuate significantly and can possibly excite VIV if the cylinder is free to move. With the implementation of active jet of C µ = 0.0127, a pair of symmetric and counter-rotating jet vortices are evacuated into the cylinder wake to detach the alternating separation flows generated at both sides of the cylinder. As a result of this dynamic competition process, the vortex formation region is elongated and shifted downstream. As C µ increases, the jet vortices are gradually strengthened, and the competitions between the jet flow and the shear flows are gradually enhanced. When C µ reaches a high level (C µ = 0.0796 or higher), two nearly symmetric arrays of vortices can be noticed in the near wake region, suggesting the wake pattern has nearly been converted into a symmetric mode. It is well accepted that the fluctuating lift of a cylinder is closely linked to the alternating vortex shedding. Therefore, a strengthened competition between the jet flow and the shear flows will alleviate the periodicity of the wake flow and result in a smaller C ′ L (as plotted in Fig. 7). In addition, [START_REF] Liu | Suppression of lift fluctuations on a circular cylinder by inducing the symmetric vortex shedding mode[END_REF] also found that symmetric perturbations acting on the cylinder flow lead to a great change of the wake mode from the conventionally periodic one to a nearly symmetric one and could suppress effectively the lift fluctuations acting on the circular cylinder.

Derived by PIV measurement results, Fig. 11 plots the mean flow topology behind the jet section for different natural and controlled cases. The jet flow into the wake interacts with the recirculating flow behind the cylindrical test model. In consequence, the recirculation region is enlarged and the near-wake width increased, in comparison with the bare cylinder (C µ =0). Besides, even when C µ is small, the high-TKE region is shifted away from the cylinder wall, indicating a stabilized mode in the very near wake.

Consequently, the fluctuation of the surface pressure and the resultant dynamic wind loads of the cylinder are decreased. Furthermore, as C µ increases, the distributions of TKE in the cylinder wake are significantly The mean recirculation region of the cylinder flow is characterized by the vortex formation length (L vf ).

C  =0 C  =0.0127 C  =0.0287 C  =0.0509 C  =0.0796 C  =0.1146
C  =0.0287 C  =0.0509 C  =0.0796 C  =0.1146
In the present study, L vf is defined as the length between the cylinder center and the saddle point of timeaveraged streamtraces along the centerline of the test model. It is shown in Fig. 12 that the wake-jet flow control causes L vf to grow considerably longer, i.e. the vortex formation region is elongated to a further location in the wake. Besides, a lager C µ is found to result in a larger recirculation region. This is due to wake-jet flow that prevents the interactions of shear layers from forming the alternating vortex shedding in the near wake. When C µ is sufficiently large, two shear layers from two sides of the cylinder are detached and elongated to be nearly parallel, as shown in Fig. 10.

According to the mean flow structures measured by PIV system, Fig. 13 illustrates the time-averaged profiles of streamwise velocity at variant X/D stations (where X/D=0 denotes the cylinder center) behind the cylindrical test model. In the first subplot of the velocity profiles, i.e. at X/D = 1.0, peaks are readily witnessed near the jet nozzle (Y/D=0), indicating a great modification to the wake flows near the cylinder wall. Generally, a stronger jet flow, denoted by a larger C µ , results in a more noticeable peak. As X/D shifts downstream to 1.5, the peaks of cusp are weakened in comparison with the former case. This is due to the competition of the wake-jet flow and the backward flow in the recirculation region. When X/D moves farther downstream, this competition will decrease the jet flow velocity consistently and the jet cusp disappear gradually. As a result, the mean velocity profiles recover gradually with the increase of X/D. In addition, Fig. 14 presents the distributions of mean Reynolds stress, which is a significant parameter to characterize fluctuations of velocity in the wake. The distributions are nominally antisymmetric about the jet nozzle (Y=0), which is due to the opposite directions of spanwise velocity induced by the counter-rotating vortex pairs. Besides, as X/D shifts from 1.0 to 1.5, the Reynolds stress increases. In this region, jet flow from the nozzle is developed and broadened, indicating that the turbulent velocity fluctuations are enhanced and so are the Reynolds stress distributions. After that, the Reynolds stress distributions experience a gradual decrease, because the wake flow gradually recover towards incoming airflow and its fluctuations both in streamwise and transverse directions are gradually weakened. It can also be noted that, at each X/D station, the Reynolds stress tends to diminish with the increase of C µ , suggesting that the stronger the wake-jet flow is, the more active the momentum exchange between the active jet and the surrounding airflow will be. Therefore, the vortex interaction in the near wake of the controlled cylinder is alleviated and prevented to some extent, as shown in the instantaneous wake flows discussed above.

We note different wake flow patterns behind the mid-span and jet sections (Figs. 9 and 11) due to the three-dimensional nature of the cylinder flow with active jet. The spanwise flow structures measured at the jet plane (Y/D=0) are illustrated in Fig. 17. For the baseline case, there is a velocity region where the backflow obviously forming a recirculation region behind the cylinder, as can also be noted in Fig. 11. For the controlled cylinders, the jet vortex pairs with opposite rotations can be clearly identified for controlled cases. It is found that the instantaneous flow structures at Y/D= 0 are nearly stable and symmetric about the jet nozzle. This observation is different from the wake vortices at the plane of Z/D=0.5. The latter ones are due to the interaction with the shear layers from upper and lower sides of the cylinder. In addition, the active jet flow are observed to affect a farther region along the streamwise direction and the recirculation regions are pushed further downstream with a larger C µ . This is consistent with above discussions.

Discussions

It is noted in Fig. 9 that C D increases while C ′ L trends to be stable as C µ exceeds a turning point about 0.10. The trend in mean drag is similar to some previously reported results. For instance, [START_REF] Apelt | The effects of wake splitter plates on the flow past a circular cylinder in the range 10 4< r4[END_REF] investigated the flow around a circular cylinder modified with a splitter plate and they observed that the mean drag of the cylinder with a short splitter plate (L/D<1) would progressively decrease with the increase of splitter length (L). However, as the splitter length exceeds a critical value (L/D=1), the drag coefficient experiences an upward trend and the drag reduction effect is reduced, as illustrated in Fig. 5 of [START_REF] Apelt | The effects of wake splitter plates on the flow past a circular cylinder in the range 10 4< r4[END_REF]. They suggested that the recirculating regimes near the solid wall acted as a 'virtual surface' to push the streamlines outside. From a similar point of view, the effects of wake jets in the present study are actually to form a 'virtual plate' in the cylinder wake (as shown in Figs. 11 and17), so the aerodynamic behaviors of the controlled cylinder in the present study exhibit similar trends to the cylinder hinged with a solid wake-splitter.

C  =0.0287 C  =0.0509 C  =0.0796 C  =0.
The schematics in Fig. 18 illustrates the saturation effects of 'virtual plate' on vortex formation process. When C µ is small, the virtual plate is short (the upper subplot), and the plate prevents the interaction of separated flows. This prevention process is gradually enhanced with the increase of virtual plate (C µ ), so C ′ L will witness a gradual decrease. The middle subplot of Fig. 18 represents the turning point. In this case, the tails of shear flows touch at the trailing edge of the virtual plate. Furthermore, when C µ continues to grow, the enlarged length of virtual plate will not further create significant change to the vortex formation in the wake (see the lower subplot). As a result, C ′ L remains relatively stable.

Conclusion

In the current study, a wind tunnel investigation is conducted at a Reynolds number of Re= 3. The active jet into the wake can suppress effectively the drag and lift forces. On the basis of pressure measurement results, the control effectiveness on fluctuations of lift coefficient meets a saturation when C µ exceeds 0.0963. Compared to the bare cylinder, the RMS value of lift forces is estimated to be decreased up to 81.2% at the C µ of 0.1560. Besides, the best drag reduction (about 16-18%) is achieved in the C µ range of 0.0796-0.1146. As C µ continues to go high, the control effect of drag reduction is slightly decreased.

In addition, with the implementation of the active jet control, the vortex shedding frequency is minimally increased.

The PIV measurement results confirmed the observations from the pressure measurement. Compared to the baseline case, the recirculation regions behind the controlled cylinder are elongated and pushed downstream. Moreover, the TKE distributions in the wake flow behind the controlled cylinders are much smaller than the baseline case, indicating stabilized cylinder wakes. The instantaneous wake vortex structures reveal that these great modifications to the cylinder flow are due to the interaction of the jet flows with the shear flows from both sides of the cylinder. With sufficiently large C µ , the alternating shear layers are switched into two parallel arrays. To conclude, the mechanism of the active wake-jet control is to impose steady and symmetric perturbations into the unsteady and asymmetric flows in the cylinder wake, so as to alleviate the periodic vortex shedding and reduce the fluctuations of aerodynamic forces effectively.

 The effects of active wake-jets on the suppression of aerodynamic forces and the manipulation of wake flow evolution behind a cylinder are investigated at a subcritical Reynolds number.  Pressure measurement results demonstrate that the lift force acting on the test model is greatly reduced and drag decreased with the control of active wake-jets.  A higher jet momentum coefficient contributes to a better control effectiveness in unsteady lift forces but not necessarily a better drag reduction.  the active jet control scheme imposes steady and symmetric perturbations into the unsteady and asymmetric cylinder flows and the vortex shedding pattern behind the controlled cylinder is greatly modified, vortex formation length significantly elongated and the fluctuations of aerodynamic forces conceivably suppressed.

  viscosity of air, is Re= 3.33 × 10 4 . It is noted that O(10 4 ) is a typical Re level for stay cables of cable-stayed bridges undergoing VIVs.

Figure 1 :

 1 Figure 1: Schematics of the cylindrical test model (left) and the pressure measurement plane in the mid-span (right). Origin of the XYZ coordinate system is at the section center of the cylindrical slice in the mid-span.

Figure 2 :

 2 Figure 2: Experimental schematics for PIV measurement set-ups. Two streamwise target-planes of Z =0 and Z/D =0.5 (left) and a spanwise target-plane of Y =0 (right)

  Figure 3: (Color online) Surface pressure distributions around the cylinder model with different Cµ

Figure. 4

 4 Figure. 4 and Fig.5 illustrate the time-variant signals of the drag and lift force coefficients of the cylinder with different C µ . It is shown that both the lift and drag forces are quite unsteady with significant fluctuations for the bare cylinder with zero C µ . The fluctuation are closely associated with the unstable and unsteadywakes of the bare cylinder, as will be revealed by PIV measurement results. When the active control is implemented, the fluctuations of lift forces are substantially decreased. Besides, for the controlled test cases with active jets, the drag coefficients of the controlled cylinders are found to be significantly smaller compared to the baseline case.

Figure 6 Figure 4 :

 64 Figure6shows the non-dimensional frequency (Strouhal number, St = f D/U 0 ) of the time-variant histories of lift coefficients for different cases obtained by using a fast Fourier transform (FFT) analysis.It is observed that the dominant frequency from the bare cylinder is 9.98 Hz and results in a Strouhal number (St 0 ) of 1.996. A dominant frequency identified from the power spectrum of lift suggests a regular and periodic vortex shedding process from the cylindrical test model. Furthermore, when the cylinder is controlled with the active jet flow of C µ =0.0287, the dominant frequency experiences a marginal increase to 10.53 Hz (St 1 = 2.106), indicating an acceleration of vortex shedding. In addition, the energy amplitude that the dominant frequency concentrates is found to be greatly decreased in comparison with the conventional vortex shedding from the bare cylinder. This change reveals a substantial modification and attenuation to

Figure 5 :

 5 Figure 5: Time histories of the instantaneous drag coefficients acting on the cylindrical test model for three different Cµ

Figure 6 :

 6 Figure 6: (Color online) Strouhal numbers for three different Cµ : St 0 = 1.996; St 1 = 2.106; St 2 = 2.11

Figure 8 :

 8 Figure 8: Cumulative energy proportion of the first 100 POD modes

Figure 9 :

 9 Figure 9: (Color online) The mean flow field measured by PIV system in the mid-span section of the cylindrical test model for different Cµ

Figure 10 :

 10 Figure 10: (Color online) The instantaneous flow strucutres measured by PIV system in the jet section of the cylindrical test model for different Cµ

Figure 15

 15 Figure 15 present different profiles of the fluctuating streamwise velocity. The figure shows that two peaks lying at the two sides of the centerline behind the bare cylinder, i.e. a double-cusp pattern. However, the streamwise velocity fluctuation profiles exhibit quadruple-cusp patterns in the near wake for the controlled cylinders. This is associated with the swirling patterns of vortices formed in the recirculation region. When X/D moves farther downstream and gets rid of the backward flow in the recirculation region, the profiles

Figure 11 :Figure 12 :

 1112 Figure 11: (Color online) The mean flow field measured by PIV system in the jet section of the cylindrical test model for different Cµ

Figure 14 Figure 16

 1416 Figure 14: (Color online) Reynolds stress distributions behind the cylindrical test model for different Cµ

  1146

Figure 17 :Figure 18 :

 1718 Figure 17: (Color online) The instantaneous flow structures in the plane of Y=0 for different Cµ

  33 × 10 4 to investigate the effectiveness of active wake-jets (characterized by a dimensionless jet momentum coefficient (C µ )) to manipulate the vortex shedding patterns so as to decrease the fluctuating amplitudes of the aerodynamic coefficients. Both surface pressure and detailed flow measurements (PIV) are performed to study the flow characteristics of the cylindrical model with different C µ . The principal conclusions from this experimental work are as follows.

Table 1 :

 1 Jet flow rates and their corresponding jet momentum coefficients of different test cases in the present study

	Jet flow rate (L• min -1 )	7.5	15	22.5	30	37.5	45	52.5
	Jet flow velocity (m• s -1 )	1.5915 3.1830 4.7745 6.3660 7.9575 9.5490 11.1405
	Jet momentum coefficient (C µ ) 0.0008 0.0032 0.0072 0.0127 0.0199 0.0287	0.0390
	Jet flow rate (L• min -1 )	60	67.5	75	82.5	90	97.5	105
	Jet flow velocity (m• s -1 )	12.732 14.3235 15.915 17.5065 19.098 20.6895 22.281
	Jet momentum coefficient (C µ ) 0.0509 0.0645 0.0796 0.0963 0.1146 0.1345	0.1560

Preprint submitted to Experimental Thermal and Fluid ScienceDecember 26, 2018

Acknowledgments

We wish to acknowledge the supports awarded by the National Nature Science Foundation of China through Grants 51378153, 51578188, and 51722805, and the Fundamental Research Funds for the Central Universities (HITBRETIV. 201803 and HITBRETIII. 201512). D. G also acknowledges the financial support from China Scholarship Council (CSC 201606120189).