Pascal Lafon
email: pascal.lafon@univ-troyes.fr

Laurence Giraud-Moreau

Coupling of evolution strategy and lagrangian augmented algorithm for optimal design of mechanical system

This article presents a new optimization algorithm for nonlinear optimization problems with mixed variables. This algorithm is based on a coupling of deterministic and evolutionnary methods of optimization. We first point out the general context of optimal design problems and the characteristics of the these nonlinear problems with mixed variables. We briefly recall the general principles of the evolutionnary and deterministic methods. Then the principle of coupling between these two approaches is presented. Finally we discuss the results obtained on a test problem of the literature and a problem of dimensioning of a gear pair. We show in both cases the profits compared to evolutionnary methods in terms of reliability and speed of convergence.

Résumé

: Cet article présente un nouvel algorithme d'optimisation destiné aux problèmes d'optimisation non linéaires en variables mixtes. Cet algorithme repose sur un couplage d'une méthode déterministe et évolutionnaire d'optimisation. Nous rappelons tout d'abord le contexte général des problèmes de conception optimale et les particularités de ces problèmes non linéaires en variables mixtes. Les principes généraux des méthodes évolutionnaires et déterministes sont ensuite exposés. Puis le principe du couplage entre ces deux approches est présenté. Enfin nous exposons les résultats obtenus sur un problème test de la littérature et ensuite sur un problème de dimensionnement d'un train engrenage. Nous montrons dans les deux cas les gains obtenus par rapport à une méthode évolutionnaire seule en termes de précision et de vitesse de convergence.

Introduction : In today's Computer Aided

Design software, a mechanical system is essentially represented by the volumes of its parts. Even if concepts of feature and complex assembly structure have been added to this representation, it is not well suited for an easy design of the mechanical system. Dimensioning mechanical component is quite difficult with this kind of software. Our aim is to propose design-making tools, which can be introduced in Computer Aided Design software. Thank to the analysis of the design process of a known mechanical system, we can formulate the mechanical design problem as an optimization problem, called problem of optimal design. The latter contains nonlinear equations, inequality constraints and mixed variables that are continuous and discrete. There are also interdependent discrete parameters whose values may be taken in normalized tables and which directly depend on the choice of one of the discrete variables. These problems of optimal design have been solved with genetic algorithm [START_REF] Daidié | Réalisation d'un logiciel de conception automatique appliqué à la détermination des engrenages cylindriques : interfaçage avec un logiciel industriel de CAO[END_REF] and evolution strategy [START_REF] Giraud | Evolution strategies for optimal design of mechanical systems[END_REF]. The obtained results have shown the efficiency of these algorithms, but many evaluations of function are necessary to reach a good optimal solution. In this paper, a hybrid evolution strategy is presented to solve this optimal design problems with a low number of evaluations.

2.

The optimal design problem : The general mixed discrete continuous problem is defined as: Minimize f x ( ) Subject to :

g x j m g x j m m l u x v u v x R R R j j n n n C D ( ) , . . ( ) , .. , , , ≤ =
= = + + ≤ ≤ ∈ = ∪ 0 1 0 1
where f is the objective function and g j are nonlinear inequality constraint functions. The components of the mixed variable vector ( )

x x x C D T = , are divided into n C continous variables, expressed as x R C n C ∈ and n D continous variables, expressed as x E R D n n D D ∈ ⊂ . Here E R n n D D
⊂ is the space of discrete values, acceptable by the problem and it can be randomly distributed. In this class of optimal design problem, we consider that the first order derivatives of the objective function and constraints with respect the continuous variables exist, but the first derivatives of the functions with respect to the discrete variables are supposed unknown. Moreover, this type of problems are strongly nonlinear and typically contain about ten variables.

Evolutionary strategies :

Evolutionary algorithms have started to receive significant attention during the last decade [START_REF] Bäck | Evolutionary algorithms in theory and practice[END_REF][START_REF] Bäck | Evolutionnary computation : comments on the history and current state[END_REF][START_REF] Goldberg | Genetic algorithms in search, optimization and machine learning[END_REF][START_REF] Holland | Adaptation in natural and artificial systems[END_REF]. Part of their success is due to the large domain of application of those methods, their robustness and their gain of flexibility. Those methods are specially well suited for solving difficult optimization problems [START_REF] Le Riche | Optimisation de structures composites par algorithmes génétiques[END_REF][START_REF] Powell | Using genetic algorithms in engineering design optimization with non linear constraints[END_REF][START_REF] Wu | Genetic Algorithms for solving mixed-discrete optimization problems[END_REF][START_REF] Wu | Steady-state genetic algorithms for discrete optimization of trusses[END_REF]. However, their computational cost is generally very high. A large number of evaluations must generally be performed for a satisfying result to be found. Evolution strategies are based on the principle of evolution, i.e. survival of the fittest. Unlike classical methods, they do not use a single search point but a population of points called individuals. Each individual represents a potential solution to the problem. At each iteration, called a generation, a new population of χ offspring individuals is created by means of recombination and mutation, starting from the old population of µ parents (χ > µ). This new population is then evaluated with an evaluation function. This evaluation function is equal to the function to be minimized when the problem does not contain any constraint. It may be a quadratic penalty function for handling constraints of the optimization problem [START_REF] Michalewicz | Genetic Algorithms + Data Structures = Evolution Programs[END_REF]. The µ best individuals out of the population of parents and offspring are selected to reproduce and replace the old population of parents. By means of these randomized processes, the population evolves toward better and better regions of the search space.

Deterministic methods :

On the other hand, deterministic methods, which exploit local information like gradient information are rapid methods when the gradients of the functions can be calculated. But

d k such as : x x d C k C k k k + = + 1 α .
. The calculation of the step α k and the direction d k depend on the chosen deterministic method. They are generally based on the gradients of the functions.

5.

Coupling evolutionary and deterministic methods : In order to combine the respective advantages of the deterministic method and evolution strategy we have proposed to couple the two methods. We have chosen to use a dual formulation of the problem with the augmented lagrangian function. This function is used to evaluate each individual in the evolution strategy. The algorithm is based on a maximization of the dual augmented function with respect to the dual variables. This dual augmented function is obtained by a minimization of the augmented lagrangian function with respect the primal variables, i.e the mixedcontinuous variables of the optimization problem. In this paper we are handling a population of µ parents, where each individual is represented by a 3 vectors The step α along this direction is calculated by using the monotonicity analysis [START_REF] Papalambros | Regional monotonicity in optimum design[END_REF] when the constraints and objective function of the optimal design problem are monotonic. Here ∇f(x k ) and ∇g j (x k ) are the gradients of the objective and constraint functions with respect to the continuous variables, the discrete variables being kept fixed. The method of monotonicity analysis is based on the identification of active or inactive constraints.

df d f x d k T α α = = ∇ 0 ( ) (1)
and

dg d g x d j j k T α α = = ∇ 0 ( ) , j=1,.., m (2)
For motion in the descent direction d, we identify constraints for which the derivatives in equations ( 1) and ( 2) have opposite signs.

Only constraints showing an opposite derivative sign to df/dα are able to be active. We use a linear approximation of the constraints to compute the motion α j corresponding to each of these candidate active constraints.

α j j k j T k g x g x d = -∇ ( ) ( ). , (3)
The constraint showing the smallest value of α j is considered dominant and defines the step size along the direction d.

When the objective and constraints function are not monotonic, the step α along the descent direction d is calculated with a golden section search coupled with a polynomial interpolation. This basic iteration is repeat until the minimum of the augmented lagrangian function is reached. The dual variables, i.e the lagrange multipliers are then updated and the overall procedure is repeated until the maximum of the dual function is reached.

Application and results :

We have applied this hybrid algorithm to several problems of optimal design. We present here the detail of the results obtained on two of these problems.

Pressure vessel :

The first one is the design of pressure vessel [START_REF] Zhang | Mixed discrete nonlinear optimization with simulated annealing[END_REF]. The objective is to minimize the total manufacturing cost for the pressure vessel. The design variables are the dimensions T s (the shell thickness), T h (the spherical head thickness), R (the radius of the cylindrical shell), and L (the length of the shell). Minimize the objective function :

f(x) = 0,6224.T s .R.L + 1,7781.T h .R 2 + 3,1611.T s 2 L + 19,84.T s 2 .R
Subject to the non linear inequality constraints :

g 1 (x) = 0,0193.R -T s ≤ 0 g 2 (x) = 0,00954.R -T h ≤ 0 g 3 (x) = 752 x 1728 -π.R 2 .L 2 -(4π.R 2 )/3 ≤ 0 g 4 (x) = 1,1 -T s ≤ 0 g 5 (x) = 0,6 -T h ≤ 0 Variables : x = { T s , T h, R, L } T Bounds on variables : 1,1 ≤ T s ≤ 1,9375 0,5625 ≤ T h ≤ 1,5 50 ≤ R ≤ 70 30 ≤ L ≤ 50
This problem belong to a set test problems for which we know the theoretical optimal solution. Here , this solution is : The determination of the teethes is a complex operation. There are however simplified and standardized methods usually used in the engineering and design departments. The formulation of the problem that we present is based on the simplified "C" method recommended by standard [START_REF] Nf E | Engrenages : détermination de la capacité de de charge des engrenages cylindriques extérieurs de mécanique générale[END_REF].

x theo = {
From kinematics imposed conditions (a given speed ratio reduction), the normal module m n0 , the helix angle β 0 , the number of teeth of the pinion Z 1 , the facewidth b and finally the addendum modification of the pinion x 1 and the wheel x 2 , are determined. The standard recommends to first determine the parameters m n0 , β 0 , Z 1 , b checking of the conditions of strength on surface pressure and of rupture on the level of teeth. The addendum modifications x 1 and x 2 are then calculated from conditions on the balancing of the specific slips and fatigue strength Although these parameters are all interdependent, the calculation complexity does not allow to tackle the problem differently.

The formulation of this problem like a problem of optimal design was already undertaken [START_REF] Daidié | Réalisation d'un logiciel de conception automatique appliqué à la détermination des engrenages cylindriques : interfaçage avec un logiciel industriel de CAO[END_REF]. However because of the intrinsic limitations of the used optimization methods , the authors had to solve this problems in three steps. At the time of the first step, the values of b and d 10 are determined. This allows to obtain an optimization problem with continuous variables. Then the values of m n0 , β 0 , Z 1 , are obtained by an optimization method based on the graphs of variations [START_REF] Lafon | Conception optimale de systèmes mécaniques : optimisation en variables mixtes[END_REF], and finally the addendum modification of teeth are determined, thanks to a particular algorithm.

We propose to formulate this problem of optimal design overall, in order to simultaneously determine the values of the variables of design:

m n0 , β 0 , Z 1 , Z 2 , x 1 , x 2 , b.
The objective function is formulated to minimize the mass of the gears and the relative variation of the specific slips of the wheel and the pinion. The importance of each criterion is given by the values of α 1 and α 2 .

The functions g 1 (x) and g 2 (x) are conditions on the acceptable maximum powers with the surface pressure and the rupture of teeth. The functions g 3 (x) and g 4 (x) are conditions on the transverse contact ratio ε α and the overlap ratio ε β . The function g 5 (x) expresses a condition on the linear velocity of teeth. The functions g 6 (x) and g 7 (x) limit the values of the addendum modification. The functions g 8 (x) and g 9 (x) express conditions on meshing interference. The functions g 10 (x) and g 11 (x) are conditions on the minimal and maximum values of the diameter of the pinion. The function g 12 (x) imposes a maximum error on the speed reduction ratio u. Finally the functions g 13 (x) and g 14 (x) limit the minimal value of the facewidth compared to the diameters of the pinion and the wheel. The expression of this problem thus requires 14 nonlinear constraint functions, and 7 variables including 4 continuous and 3 discrete. It is thus a particularly complex problem of optimal design, the more so as the calculation of the factor C B4 requires to solve, for each evaluation of the function g 2 (x), the equation :

inv(α) = 0 (inv(α) = α -tan(α)).
The optimization problem formulation is :

Minimize the objective function :

= ) (x f α β 1 0 1 2 2 0 1 2 b m Z b d n ( ) (cos ) maxi maxi + α 2 1 1 1 2 2 2 2 2 2 Z YU Z Y U Z Y U -      
Subject to the non linear inequality constraints :

( )= x g 1 0 1 mini 6 5 4 3 2 1 ≤ ⋅ ⋅ ⋅ ⋅ ⋅ - P K C C C C C C p B = ) ( 2 x g 0 1 mini 6 5 4 3 2 1 ≤ ⋅ ⋅ ⋅ ⋅ ⋅ - P K C C C C C C R B B B B B B B ( )= x g 3 α ε - 3 . 1 ≤ 0 ( )= x g 4 β ε - 8 . 0 ≤ 0 ( )= x g 5 10 100 2 1 2 2 2 2 1 - + Z Z Z V Z ≤ 0 ( )= x g 6 2 1 x x - - ≤ 0 ( )= x g 7 1 2 1 - + x x ≤ 0 ( )= x g 8 ' 1 1 1 tan 2 t Z U Y α π - ≤ 0 ( )= x g 9 ' 2 2 2 tan 2 t Z U Y α π - ≤ 0 ( )= x g 10 0 1mini 1 0 cos 1 β d Z m n - ≤ 0 ( )= x g 11 1 cos 0 1maxi 1 0 - β d Z m n ≤ 0 ( )= x g 12 u Z Z u 01 . 0 1 2 - - ≤ 0 ( )= x g 13 0 1 0 cos 1 β b Z m n - ≤ 0 ( )= x g 14 1 cos 1 . 0 0 2 0 - β b Z m n ≤ 0 Bounds on variables : maxi mini b b b ≤ ≤ 0maxi 0 0mini β β β ≤ ≤ maxi 0 0 mini 0 n n n m m m ≤ ≤ maxi 1 1 1mini Z Z Z ≤ ≤ 2maxi 2 2mini Z Z Z ≤ ≤ 1maxi 1 1mini x x x ≤ ≤ 2maxi 2 2mini x x x ≤ ≤ Variables : x = { m n0 , Z 1 , Z 2 , β 0 , x 1 , x 2 , b } T
Data associated with bounds on variables :

mini b =10 mm, maxi b =500 mm, mini 0 β =10°, maxi 0 β =20°, mini 0 n m =0,5 mm, maxi 0 n m =50 mm, 1mini Z =15, 1maxi Z =45, mini 2 Z =40, maxi 2 Z =150, mini 1 x = mini 2 x =-1 mm, maxi 1 x = 2maxi x =1 mm d Max d d u m m 1mini =      1 2 , ; d Max d d u V N M M 1maxi =      1 2 1 60 , , max π
Data on the materials : 2 : Results for the gear pair optimization Detail expressions for coefficient C j and C Bj for j=1..5 can be found in [START_REF] Nf E | Engrenages : détermination de la capacité de de charge des engrenages cylindriques extérieurs de mécanique générale[END_REF].

lim H σ =1500 MPa, lim F σ =460 MPa, m R =3,5 µm, Broue H =560 HB, 1 ν = 2 ν =0,3, 1 E = 2 E =206 GPa Specific data problems : 1 N =263 tr/mm, u =2,75, mini P =2740 KW, max V =25 m/s, m d 1 =20 mm, M d 1 =1500 mm, m d 2 =20 mm, M d 2 =15000 mm, A K =1.5, R K =1, N Z =1, NT Y =1. Data for K Hβ coefficient [18] : 1 A =1,11, 1 B =0,18, 1 C =1,5 10 -4 , 1 D =1 Data for Y relT δ coefficient [18] : 2 A =0,04, 2 B =0,93 Data for Y RrelT coefficient [18] : 3 A =1,674, 3 B =0,329, 3 C =0,1 Data for Y X coefficient [18] : 4 A =1,03, 4 B =0,003 Data for Y FA coefficient [18] : 0 a h =1.25, 0 a ς =0.375 α 1 = 1 α 2 = 0 α 1 = 0 α 2 = 1 α 1 = 1 α 2 =
Table 2 shows the computation results obtained at the end of 20 executions with an initial population of 7 parents and 50 offspring individuals, for several values combinations for α 1 and α 2 . The case α 1 = 1 and α 2 = 0 corresponds to a minimization of the mass of the gears. For α 1 = 0 and α 2 = 1, we only minimize the relative difference between the specific slips of the pinion and the wheel. Finally for α 1 = 1 and α 2 = 1, the two criteria are simultaneously optimized. In table 2 we present the optimal values of the design variables after convergence of the method, as well as the results obtained by Daidié [START_REF] Daidié | Réalisation d'un logiciel de conception automatique appliqué à la détermination des engrenages cylindriques : interfaçage avec un logiciel industriel de CAO[END_REF]. These last results are calculated with the same data values. By comparing the lines "f * " and "f Daidié ", we can note that the solutions identified by the evolution strategy (column "ES") and the hybrid algorithm (column "hybrid") are better than those obtained by [START_REF] Daidié | Réalisation d'un logiciel de conception automatique appliqué à la détermination des engrenages cylindriques : interfaçage avec un logiciel industriel de CAO[END_REF]. This difference is explained by the fact that the sum of the addendum modification is null in the solution calculated by Daidié. These restrictions were made in order to simplify the calculation of the addendum modification. The difference between the number of evaluation of functions with ES and this with the hybrid method is much less significant than in the case of the problem of the pressure vessel. This can be explained by the fact that the monotonous analysis can not be used for this problem to determine the continuous variables of each offspring individual.

Conclusions :

In this paper, we have proposed a hybrid method, based on the coupling of the evolution strategy with deterministic methods. Our aim was to use in parallel the advantages of evolutionary algorithms for the discrete variables and those of deterministic methods for the continuous variables. In the proposed hybrid method, the creation of a new individual is realized at each generation by a step in the discrete search space by mean of recombination and mutation, and a step in the continuous search space by mean of a move in a Lagrangian based quasi-Newton direction. The hybrid method, applied to two strongly constrained mechanical design problems with mixed variables, has given in association with the monotonicity analysis interesting results.

  and discrete variables associated to the individual. p are strategy parameters which control the application of mutation. The basic iteration of the algorithm consists in updating the discrete variable x D by creating χ offspring individuals by means of recombination and mutation of the parent individuals. The continuous variables x C of each individual are updated by a minimization along a descent direction d of the augmented lagrangian function. These updated continuous variables are the new continuous variables of each offspring. This descent direction d is a quasi-Newton direction based on the approximate Hessian parents P(k+1) = best individuals among (P(k) ∪ P e (k)) Creation of a offspring's population P e (k) from P(k). k = k + 1 Evaluation of P e (k) For each offspring individual of P e (k) : Recombination and mutation for discrete variables Deterministic step for continuous variables Evaluation of the population P(k) (calculation of the augmented lagrangian function). Random generation of a population P(k) of µ parents.

Figure 1 :

 1 Figure 1 : Hybrid algorithm

Figure 2 :

 2 Figure 2: Pressure vessel

Figure 3

 3 Figure 3 shows the evolution of the objective function during generations. It makes it possible to highlight the best speed of convergence of the hybrid algorithm.

Figure 3 : 6 . 2 :Figure 4 :

 3624 Figure 3 : Evolution of objective function along with the number of generation

Table 1 :

 1 Results for the pressure vessel problem

		1,125; 0,625; 58,29; 43,69 } T .
		f	theo = 7197,7289
	The results presented in table 1 were
	obtained at the end of 15 executions for a
	population of 7 parents and 50 offsprings.
	The error between the theoretical optimum
	and the best solution identified by the hybrid
	algorithm is indicated in the column "Error".
	The column "Average Error %" specifies the
	average error on 15 executions. The column
	"Eval 10%" indicates the average number of
	evaluations of functions necessary to reach
	the theoretical optimum with 10% of error.
	The column " Eval total" gives the total
	number of evaluations for 15 runs. This table
	allows to compare the hybrid algorithm with
	the evolution strategy alone.
		Error Average	Eval	Eval
			Error	10%	total
	ES	0 %	1,69 %	267	37605
	hybrid 0 %	0,84 %	174	17507


Fanjuan Meng
email: fanjuan.meng@utt.fr

Carl Labergere

Pascal Lafon

Methodology of the shape optimization of forging dies

Keywords: Forging, Numerical Simulation, Optimization, the number of blows, Energy dissipation

In forging process, the dies shape design is the most important step. Reasonable dies shape can not only reduce raw material cost but also improving product quality. Currently there are no packages for the forging process with both simulation and optimization capabilities. So in our research, the main objective is to use the simulation of forging process coupled with an optimization procedure in order to obtain the optimal parameters of the geometry of the dies.

To develop this forging process optimization system several numerical tools are required: geometric modelling (CATIA V5™), FE analysis (ABAQUS®), and optimization computation. In this paper, the basic methodology of the forging dies shape optimization method is introduced. In addition to that, with the proposed 2D FE model, we simulated the platting and the forging operation of an industrial example with a hammer machine. The total energy for each simulation is the same. But for applying the energy, we use different number of blows. By doing this test, we can evaluate the influence of the number of blows on the energy dissipation and get more knowledge on the formulation of future optimization problem.

INTRODUCTION

Nowadays, in forging process the numerical simulation is used to avoid long and expensive experiments and many research works have been made in this aspect [START_REF] Im | A computer-aided-design system for forming processes[END_REF]. Besides this, the optimization of the geometry of the forging dies is another focus point during the recent years in order to minimize the forging energy, minimize the cost and obtain the desired part with good metallurgical quality.

Figure1: The whole simulation and optimization model

The whole simulation and optimization model is shown in Figure 1. We use the CAO software CATIA V5™ to create the geometry of the dies and define the shape parameters that we want to optimizer. Then, we input the shape and parameters into a software package based on ABAQUS® to simulate the forging process. By changing the values of the parameters, we can get a set of discontinuous results. Then, establishing the response surface model, the advanced optimization algorithms will be performed to reach the optimum for variables.

ESTABLISHMENT OF 2D FE MODEL OF FORGING PROCESS

In the Mechanical System and Concurrent Engineering Laboratory (ICD-LASMIS), an extensive work has been developed since ten years. A finite element package has been developed to solve thermo-visco-elasto-plastic problems with ductile damage in large deformation. This package allows realizing automatically numerical simulation of metal forming processes. And in order to achieve properly the convergence of calculation, meshing and remeshing procedure are also included [START_REF] Labergere | Improving numerical simulation of metal forming processes using adaptive remeshing technique[END_REF][START_REF] Labergere | Numerical design of extrusion process using finite thermo elastoviscoplasticity with damage. Prediction of chevron shaped cracks[END_REF][START_REF] Saanouni | Application to Metal Forming, Computational Damage Mechanics, Numerical and Computational methods[END_REF].

THERMO-VISCO-ELASTO-PLASTIC BEHAVIOUR MODEL

To build the behaviour material model, the simulation of forging operation imposed different multi-physic coupling (temperature, plastic strain, and rate plastic strain). In this work we used an advanced constitutive equations accounting for non linear isotropic and kinematic hardenings strongly coupled with ductile isotropic damage. The coupling between the ductile damage and the elasto-visco-plastic constitutive equations is formulated in the framework of the thermodynamics of irreversible processes together with the Continuum Damage Mechanics (CDM) theory [START_REF] Labergere | Numerical design of extrusion process using finite thermo elastoviscoplasticity with damage. Prediction of chevron shaped cracks[END_REF][START_REF] Saanouni | Application to Metal Forming, Computational Damage Mechanics, Numerical and Computational methods[END_REF][START_REF] Lestriez | Numerical prediction of damage in metal forming process including thermal effects[END_REF].

NUMERICAL ASPECT

The fully coupled thermomechanical constitutive equations presented above have been implemented into Abaqus/Explicit FE code using the Vumat user subroutine. The dynamic explicit resolution procedure has been used in order to solve the thermomechanical problem based on both weak forms relative to the equilibrium and thermal equations. Due to the large deformations of the material, the mesh is rapidly distorted. To avoid large mesh distortions, the mesh must be adapted. By using the adaptive remeshing technique in our laboratory, numerical simulations of various metal forming problems have been down and proved its efficiency [START_REF] Labergere | Improving numerical simulation of metal forming processes using adaptive remeshing technique[END_REF][START_REF] Labergere | Numerical design of extrusion process using finite thermo elastoviscoplasticity with damage. Prediction of chevron shaped cracks[END_REF].

ESTABLISHMENT OF THE OPTIMIZATION PROBLEM

A complete simulation design is conducted in an iterative process with different values of parameters. In order to obtain an acceptable time of calculation, simulation are driven in parallel, on several multiprocessor blade calculators [START_REF] Khoury | Towards an optimisation of forging processes using geometric parameters[END_REF]. And after the simulation is finished, the optimization program is called to get the optimum parameters of the forging dies.

According to the industrial requirements, the geometrical parameters of the forging dies (such as fillet radius, draft angle, flash design, machining allowance, etc) and the initial dimension of the cylindrical piece are served as the variables to be optimized. Different objective function can be used to verify different criteria. The one criterion is the minimization of the initial volume of the billet. Because the most important driving force in a manufacturing process is to produce the best product at the lowest cost [START_REF] Vieilledent | Optimisation des Outils en Forgeage à Chaud par Simulation Éléments Finis et Méthode Inverse Applications à des Problèmes Industriels[END_REF]. The second consist in minimizing the strain energy. The strain energy represents the total strain energy used during the forging process in order to deform the billet to obtain the desired form. So from an economic point of view, it is important to minimize it. The third important criterion is to improve the metallurgic quality of forged part.

So the objective functions can be summarized as:

Min 0 2 0 H R V T   Min     2 0 2 1 0 1 2 1 . : . : dt dV dt dV E E E t V c t V c forging platting T forging platting               Min   ref i Nb i T i p p V V ele   1
Where R 0 is the radius; H 0 is the height of the billet; c  is the Cauchy stress matrix;  is the total (elastic and plastic) strain rate; Vi is the volume of element i; p i is the equivalent plastic strain computed at the unique Gauss point of the element i; p ref is the reference equivalent plastic strain. The response surface methodology will be used to model the relationships between the objective functions and the variables. Then, different metamodels based on a polynomial approximation (MLS) or neural network can be used.

APPLICATIONS

Using the proposed 2D FE model, we make a series of simulations to evaluate how the number of blows influences on the total energy during the platting and forging process. A hammer, delivering intermittent blows to the section to be forged, applies the pressure. The mass of the hammer is 6550KG. For each process, we assumed that the total energy is 100KJ. But when the number of blows changed, the total energy is allocated to each blow following Table .1.

PLATTING PROCESS

For the platting process, the initial cylinder billet measures 209mm (height) by 65mm (radius). The material corresponds to 42CrMo4 steel. The initial temperature of the billet is 1300°C and for one blow the initial speed of the upper die is 5526 mm/s. Figure 2 shows the final shape of the billet after platting and some boundary and contract conditions are also been marked.

Figure 3: Speed curves of the different number of blows

Figure 4 shows the relationship between the force and the displacement. It can be seen that the force increase when the upper die contract with the billet and after one blow the force decrease to zero immediately. It also can be seen that the maximum force of each simulation (for different number of blows) is almost the same. So we can conclude that increasing the number of blows can't affect the force of the upper die dramatically.

Another important point that we can concluded from the figure 4 is that the displacement of the upper die along the axis became larger when the number of blows increases. That is to say, by applying the same total energy, we can get larger deformation of the billet by increasing the number of blows. The relationship between the different energy can be seen form the Figure 5. It shows that by using hammer, for one blow the kinetic energy decreases to zero. And most of the kinetic energy converts to the plastic dissipation (ALLPD) which is used to complete the shape deformation. And a small part of the energy converts to the friction dissipation (ALLFD) which is wasted and maybe leads to the dies over heating. It is obvious that the optimization aims to increase ALLPD and decrease ALLFD.

Figure 6 shows the relationship between the time and the ALLPD; Figure 7 shows the relationship between the time and the ALLFD. It can be seen that the ALLPD almost the same for each simulation. But the ALLFD isn't same and the more the number of blows the lower the ALLFD. And as a general rule, the value of ALLFD is the smaller the better.

ALLPD 0,00E+00 2,00E+07 4,00E+07 6,00E+07 8,00E+07 1,00E+08 1,

FORGING PROCESS

The shape of the forging dies is more complicated than the platting process. The final shape of the forging process is shown in the Figure 8. Seen from the figure 9, the maximum value of the force of the forging process is lager than the platting process, especially, the force around the displacement of 25mm is quite larger. This is because during the forging process, the billet is contracted with the corner of the lower die. So the force grows up suddenly. The history of the plastic dissipation and the friction dissipation of the forging process is shown in Figure 10 and Figure 11. Almost the same results were got with the platting process.

ALLPD 0,00E+00 1,00E+07 2,00E+07 3,00E+07 4,00E+07 5,00E+07 6,00E+07 7,00E+07 8,00E+07 9,

CONCLUSIONS

It is concluded that (1) the methodology of the shape optimization model of forging dies is introduced; (2) an 2D simulation design highlights the influence of the number of blows on the different kinds of energy and all results that we get agree with the forging practice and theories, so, the simulation is proved to be useful and has some reference value and guiding significance; (3) the research results of the energy dissipation during the platting and the forging processes help to design the objective function of the optimization model.

In the future, it is desired that the paralleled calculation of the forging simulation will be carried out. Then the optimization process model can be implemented by using the response surface model and some advanced mathematical methods.

ACKNOWLEDGEMENT

This study was supported by the Mechanical System and Concurrent Engineering Laboratory of University of Technology of Troyes and conseil régional de Champagne Ardenne, FRANCE, which are gratefully acknowledged.

Figure 2 :Figure 3

 23 Figure 2: Final shape of the billet after platting process

FORCEFigure 4 :Figure 5 :

 45 Figure 4: Force curves of the different number of blows

Figure 6 :

 6 Figure 6: History of the Plastic dissipation

Figure 7 :

 7 Figure 7: History of the friction dissipation

Figure 8 :

 8 Figure 8: The final shape after the forging process

FORCEFigure 9 :

 9 Figure 9: Force curves of the different number of blows

Figure 10 :Figure 11 :

 1011 Figure 10: History of the Plastic dissipation

Table 1 :

 1 The distribution of the energy for each blow

		Total energy	Each blow
	One blow	100KJ	100KJ
	Two blows	100KJ	50+50KJ
	Three blows	100KJ	30+30+40KJ

Displaceme nt (mm) Friction Dissipation (Millijoule)

	1,60E+06
	1,40E+06		MAX1_ALLFD=1.46E+06
			MAX2_ALLFD=1.36E+06
	1,20E+06
			MAX3_ALLFD=1.29E+06
	0,00	10,00	20,00	30,00	40,00		50,00	60,00	70,00	80,00	90,00
					1	2	3


Rabia Cherfouh

Universite Mouloud

Mammeri De

Tizi Ouzou

Metal Speciation in Sludges: a Tool to Evaluate Risks of Land Application and to Track Heavy Metals Contamination in Sewage Network

Keywords: urban sewage sludge, agronomic properties, heavy metal speciation, WWTP contamination

Agricultural spreading of dewatered sewage sludge from wastewater treatment plants is economically pro table but care must be taken to ensure that there is neither degradation of the agronomic quality of the soils nor contamination of them in the long term, particularly by accumulation of heavy metals. To evaluate the variability of the sludge in a given geographical area, we studied the sludge coming from ve treatment plants in northern Algeria. We determined parameters that account for the agronomic quality of sludges and total content of Ag, Cd, Co, Cr, Cu, Ni, Pb, Ti and Zn. We evaluated the bioavailability of theses metals by determining their speciation by sequential extraction, each metal being distributed among ve fractions: easily exchangeable, acido-soluble, bound to carbonates and partly to Fe-sulphides, bound to Fe-and Mn-oxides, bound to organic matter or sulphides, contained in poorly soluble minerals. We found that all the analysed sludges had satisfactory properties from an agronomic quality point of view. High total Ni content indicated that three sludges were not spreadable under French or Chinese regulations. Metal speciation, however, showed that Ni was contained in very poorly bioavailable fractions, and therefore presented a low risk in the soils concerned. In contrast, the total Cu was below the regulatory limit values, but contained in very bioavailable fractions with a risk of toxic effects by accumulation over less than 10 years. These results showed that regulations must take into account the bioavailability with regard to the characteristics of the soils on which sludge will be spread. Metal speciation in the sludge also made it possible to identify the zone of the sewerage network on which the sources of contamination must be sought and gave indications on the nature of these sources.

Introduction

Increasing urbanization, industrialization, and population pressures have resulted in growing challenges in wastewater management on a global scale [START_REF] Allaoui | Good practices for regulating wastewater treatment: legislation, policies and standards[END_REF]. The disposal of residual sewage sludge issued from wastewater treatment plants (WWTP) in dedicated land lls or, by incineration, are options adopted in many parts of the world, but both strategies are expensive and can lead to environmental problems [START_REF] Walter | Heavy metal speciation and phytotoxic effects of three representative sewage sludges for agricultural uses[END_REF][START_REF] Amir | Sequential extraction of heavy metals during composting of sewage sludge[END_REF]. Spreading dewatered sewage sludge (DSS) on agricultural land is another disposal option used in several countries because of its potential for improving soil physical properties and for containing signi cant nutrients for plant growth (Hernàndez et al. 1991;[START_REF] Martìnez | Biowaste effects on soil and native plants in a semiarid ecosystem[END_REF][START_REF] Singh | Potential bene ts and risks of land application of sewage sludge[END_REF]. This practice aims to provide a sustainable solution while allowing a reduction in the use of fertilizers, but provided that two important constraints are met: the conservation of the agronomic properties of the soil [START_REF] Tarchouna | Effects of long-term irrigation with treated wastewater. Part I: Evolution of soil physico-chemical properties[END_REF][START_REF] Cherfouh | Long-term, low technicality sewage sludge amendment and irrigation with treated wastewater under Mediterranean climate: impact on agronomical soil quality[END_REF]) and the absence of accumulation of contaminants such as heavy metals [START_REF] Alloway | The behavior of heavy metals in sewage sludge amended soils[END_REF][START_REF] Mcbride | Toxic metal accumulation from agricultural use of sludge: are USEPA regulations protective?[END_REF] or organics [START_REF] Lamastra | Sewage sludge for sustainable agriculture: contaminants' contents and potential use as fertilizer[END_REF][START_REF] Alvarenga | Sewage sludge, compost and other representative organic wastes as agricultural soil amendments: Bene ts versus limiting factors[END_REF].

When DSS is used as organic amendments for agricultural purposes, its large amount of organic matter can indeed improve the biological activity and the physical properties of the soil [START_REF] Clapp | Sewage sludge organic matter and soil properties[END_REF][START_REF] Cherfouh | Long-term, low technicality sewage sludge amendment and irrigation with treated wastewater under Mediterranean climate: impact on agronomical soil quality[END_REF], but the presence of heavy metals is probably the main critical impediment to overcome (Mo et al. 2000). Thus, it was noted that applying sewage sludge to soil might provide potentially toxic metals under labile forms [START_REF] Mcbride | Toxic metal accumulation from agricultural use of sludge: are USEPA regulations protective?[END_REF]. Several studies proved that urban sewage sludge contains signi cant concentrations of heavy metals because municipal sewage treatment plants are likely to receive industrial and other wastewater. Heavy metals in sludge constitute a relevant constraint limiting their valorization on agricultural soils, in particular, because of the high contents and their bioavailability to the soil-plant system.

Several studies deal with the content of heavy metals and their potential mobility in DSS. There are, however, surprisingly few studies which deal, within a given economic and climatic zone, with the variability within DSS with regard to these parameters, despite the fact that this essential information has to be taken into account in public policies that guide the use of sludges.

In Algeria for example, the quantity of sewage sludge produced annually by around 200 WWTP [START_REF] Bouchaala | Ressources hydriques: traitement et réutilisation des eaux usées en Algérie[END_REF]) is not known with certainty but certainly higher than 200 000 tons of which only 25% are spread on agricultural lands. As the DSS have been shown to have agricultural value (McConnel et al. 1993), it would be economically pro table to increase this percentage. The use of DSS in excess, however, can have adverse effects such as water and soil toxicity and reduced yields [START_REF] Chouial | Performance and limits of use of sewage sludge for seedlings production: case of maritime pine (Pinus pinaster Ait.)[END_REF]. It is therefore necessary to assess the toxic potential of DSS and its variability between treatment plants.

The total heavy metal contents of a DSS are easy to obtain by chemical analysis, usually after sample acid digestion and subsequent Inductively Coupled Plasma measurement. They are useful to assess a potential contamination of a DSS and are usually the only parameters considered by public regulation.

They are, however, insu cient to deliver indication about the bioavailability of the metals, which depends on their chemical species [START_REF] Ma | Chemical fractionation of cadmium, copper, nickel, and zinc in contaminated soils[END_REF][START_REF] Su | Chemical speciation and phytoavailability of Zn, Cu, Ni and Cd in soil amended with y ash-stabilized sewage sludge[END_REF][START_REF] Amir | Sequential extraction of heavy metals during composting of sewage sludge[END_REF][START_REF] Walter | Heavy metal speciation and phytotoxic effects of three representative sewage sludges for agricultural uses[END_REF]).

These can be determined by sequential extraction, a functional technique which can be applied to soil, sludge or sediment samples for determining the distribution of metals in various fractions [START_REF] Tessier | Sequential extractions procedure for the speciation of particulate trace metals[END_REF][START_REF] Lemière | Guide sur le comportement des polluants dans les sols et les nappes[END_REF], some of which correspond to potentially bioavailable metals (Fluentes et al.

2004

). Sequential extraction consists of the sequential use of speci c reagents under different pH conditions [START_REF] Gleyzes | Fractionation studies of trace elements in contaminated soils and sediments: a review of sequential extraction procedures[END_REF]. One of the most widely used protocols [START_REF] Tessier | Sequential extractions procedure for the speciation of particulate trace metals[END_REF]) de nes ve geochemical fractions: easily exchangeable, bound to carbonates (acid-soluble), bound to Fe and Mn oxides (reducible), bound to organic matter or sulphides (oxidizable) and contained in primary and secondary minerals (residual). Depending on the physicochemical conditions of the soil, the metals found in the rst four fractions can be mobilized and made bioavailable.

In this context, the objective of this work was to evaluate the characteristics and variability of ve urban residual sludges coming from urban WWTP located in the same Mediterranean area through the determination of the relevant agronomic parameters, the concentration and the speciation of heavy metals (Ag, Cd, Co, Cr, Cu, Pb, Ti, Zn) by sequential extraction. The indirect question was raised whether the origin of potential contaminations in the wastewater network can be determined.

Material And Methods

The DSS samples studied were taken from drying beds from ve activated sludge of WWTP whose capacity ranges from 5,000 to 120,000 inhabitant equivalent, located in Algeria (Fig. 1). The technical characteristics of the WWTP are given in Table 1. All the sludges were of urban origin and resulted from an activated sludge process. For each WWTP, 5 samples taken on the same day were mixed to form an average sample. Samples were air-dried, then dried in oven at 55°C, grinded in an agate mortar, sieved through a 160 µm mesh sieve and stored in polyethylene bottles at 4 ° C. For each sample, an aliquot was weighed for determining water content after drying at 105°C. For each analysis, three analysis replicates for each sample were performed in order to meet statistical acceptance requirements. All extraction solutions were prepared using ultrapure water (conductivity = 18 µS.cm - 1 ) and trace metal grade chemical reagents (Merck).

Electrical conductivity and pH were determined using a 1/10 [w:w] sludge-to-water ratio. Cation exchange capacity (CEC) was determined using the cobalthexamine method [START_REF] Aran | A spectrophotometric measurement of soil cation exchange capacity based on cobaltihexamine chloride absorbance[END_REF][START_REF] Tarchouna | Effects of long-term irrigation with treated wastewater. Part I: Evolution of soil physico-chemical properties[END_REF]). Total organic carbon (TOC) and total N (tN) were measured on solid samples using a CHNS (Shimadzu, Flash2000). The organic matter content was calculated using the formula MO (%) = CO (%) x 1.72, based on the assumption that average soil organic matter is composed by 58% of carbon. (Pribyl, 2010). Aqueous extracts were realized using a 1:10 [w:w] sludge-to-water ratio, agitation for 16h and 0.45 µm ltration. On aqueous extracts, the water-extractible organic carbon (WEOC) was measured using a TOC-meter (TOC-V CSH, Shimadzu) by difference between total carbon and inorganic carbon. Ions concentrations in solution (nutrients and other major species) were measured by ionic chromatography (Dionex-DX-120).

Sequential extraction was performed using a ve steps procedure modi ed from [START_REF] Tessier | Sequential extractions procedure for the speciation of particulate trace metals[END_REF]. Centrifugation was performed after each step at 4500 rpm for 15 min, the supernatant was ltered at 0.45µm on pre-washed nitrocellulose membrane before to be placed in polypropylene bottles where pH was adjusted at pH 2 with concentrated HNO 3 (72%) in order to prevent hydrolysis of metals before analysis. The centrifugation pellet was recovered, washed by stirring 2h at room temperature with 40ml of ultrapure water then centrifuged for 15 mn. The obtained pellet was then submitted to the next step. All bottles and asks were previously cleaned with HNO 3 10% and rinsed.

Step 1 -Fraction F-ex. 2g of soil or sludge were shaken in 20mL of KNO 3 1M for 16h at RT. The supernatant obtained after centrifugation corresponds to an easily exchangeable fraction.

Step 2 -Fraction F-ac. The pellet obtained after Step 1 was stirred for 3h at RT with sodium acetate 1M adjusted at pH = 5. The supernatant obtained after centrifugation corresponds to a strongly adsorbed fraction, bound mainly to carbonates or to part of the Fe-sulphides [START_REF] Flyhammar | Use of sequential extraction on anaerobically degraded municipal solid waste[END_REF].

Step 3 -Fraction F-red. The pellet obtained after Step 2 was stirred for 2h at 65°C in 80mL of 0.25 mol.L - 1 hydroxylammonium chloride at pH 1.5. The supernatant obtained after centrifugation corresponds to a fraction bound mainly to Fe and Mn (hydr)oxides.

Step 4 -Fraction F-ox. The pellet obtained after Step 3 was suspended in hydrogen peroxide 30% (30mL, pH = 2), stirred 5h at 85°C and centrifuged. The obtained pellet was stirred for 0.5h in 20mL of 3.2 mol.L - 1 ammonium acetate acidi ed at pH = 2 with HNO 3 20% v/v. The supernatant obtained after centrifugation corresponds to a fraction bound mainly to organic matter or sulphides.

Step 5 -Fraction F-res. The residual fraction was obtained after aqua-regia extraction assisted by micro-wave (120°C, 900W, 1h) of the pellet obtained after Step 4.

The concentration of metals in the extraction solutions of the different fractions was measured using the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. Statistical analysis was conducted using the XLSTAT software.

Results And Discussion

Agronomic quality of sludges

The main agronomic parameters are given in Table 2. All the studied DSS had signi cant OM and tN contents, which offers good prospects for agricultural use. The cation exchange capacity (CEC) was in the range expected from the high OM content. The pH values were standing between slightly acid 6.4 and alkaline 8.1. Such values were not expected to cause problems, particularly with regard to the mobility of trace metals [START_REF] Yong | pH in uence on selectivity and retention of heavy metals in some clay soils[END_REF]. The tN content ranged between 3.0 and 4.4 %, a result similar to that of other studies [START_REF] Stark | Residual nitrogen availability from soils treated with sewage sludge in a eld experiment[END_REF]. The C/N ratio ranged between 5.2 and 7.6. Some authors consider that such low values may promote nitrogen leaching. C/N ratio should ideally be raised to around 20 by adding bulking agents having a high C/N ratio [START_REF] Tisdale | Soil fertility and fertilizers[END_REF]. Others, however, using the same type of sludge, showed that, in Mediterranean areas, biomass production was better improved by using sludge-alone amendment rather than sludge added to bulk agent amendment [START_REF] Bousselhadj | Nitrogen fertilizer value of sewage sludge co-composts[END_REF]).

Regarding available P and K values, signi cant variability was observed between DSS samples (Table 2).

Total content of heavy metals

The total content of the studied heavy metals did not vary signi cantly between DSSs ( Cr and Ni compounds may come from metal surface treatment or damascening industries. Ni compounds may also come from ceramic manufacturing industries and Cr compounds from textiles and tanning industries [START_REF] Islam | Heavy metals in the industrial sludge and their ecological risk: A case study for a developing country[END_REF]). In our study, the high Ni and Cr content in DSSs must be related to the fact that in the studied area there is no strict quality control of e uents from small industries which discharge directly in the sewer network. [START_REF] Braun | Titanium dioxide -a review[END_REF]. Poorly controlled house painting activity may be the cause of the high concentrations observed here.

Considering the total metal content, sludge from BKH and EST sewage treatment plants can be recommended as a fertilizer or as an organic soil amendment. For Ni, AZF sludge exceeds the Chinese standard, TDM and BDM sludges exceed both French and Chinese standards and therefore cannot be applied without costly prior treatment [START_REF] Veeken | Removal of heavy metals from sewage sludge by extraction with organic acids[END_REF][START_REF] Xiang | Removal of heavy metals from anaerobically digested sewage sludge by isolated indigenous iron oxidizing bacteria[END_REF]. It would be, however, more relevant to trace the origin of nickel in the corresponding localities to avoid its transfer to the sewer network. Anyway, these data underline the importance of quality control of sludges with regard to heavy metals.

Statistical analysis (PCA, principal component analysis) (Fig. 2) was able to identify two groups of metals with differentiated behaviours. Thus, PCA shows good correlations between metals in the (Cu, Ni, Cr, Cd, Ti) group, as well as in the (Pb, Zn) group, these two groups not being correlated with each other.

Co was anticorrelated with Pb and Zn. The two rst components explained 83,6% of the variance. The position of the BKH and TDM points is explained by lower and higher values, respectively, for metals from the (Cu, Ni, Cr, Cd, Ti) group. The position of the EST point is explained by higher values for Pb and Zn and low value for Co; the opposite being observed for the AZF point.

Speciation of heavy metals

In Mediterranean soils where the sludges are spread, the fractions most likely to release bioavailable metals are the F-ex, F-ac fractions, as well as the F-oxi fraction. The latter corresponds to metals bound to sulphides, which are rapidly oxidized in a dry environment and, in the case of organic matter, of pH higher than 6.5 [START_REF] Cherfouh | Long-term, low technicality sewage sludge amendment and irrigation with treated wastewater under Mediterranean climate: impact on agronomical soil quality[END_REF]).

The studied metals showed differentiated behaviours with, except for Ag, a great consistency between the WWTPs (Fig. 3, numerical values are given as Supplementary Material). For all metals, the exchangeable fraction (F-ex) was lower than 3.5 %.

Ti, Cr and Ni were found mostly in the residual fraction (F-res), i.e. contained in minerals (more than 90% for Ti and Cr and more than 80% for Ni), therefore poorly bioavailable [START_REF] Sims | Chemical fractionation and plant uptake of heavy metals in soils amended with co-composted sewage sludge[END_REF]. Small amounts of these metals (<10%) were in the oxidable fraction (F-oxi), i.e. mainly bound to organic matter or sulphides and, for Ni, 8-12% were in the reducible fraction (F-red), i. Cu was mainly distributed between F-red, F-oxi and F-res fractions, with a F-ac fraction ranging from 1.7 to 7.3%. Compared to other metals, Cu is well known to be mainly associated with the oxidizable fraction, which is con rmed by our results with best a nity of this fraction comparatively to Cr, Zn, Ni, Pb and Co. This behaviour is similar to that observed by other authors (Chen et Cd was the only metal having a low or under detection limit residual fraction (F-res). Most Cd was distributed between the F-ac and the F-red fractions, with a signi cant part (10.3 to 15.8%) in the Foxi fraction. Similar distribution has already been described by in sludge from in middle-south region of China [START_REF] Chen | Total concentrations and speciation of heavy metals in municipal sludge from Changsha, Zhuzhou and Xiangtan in middlesouth region of China[END_REF]. The low Cd contents in the fractions were low enough to rule out a longterm risk.

Ag was the only metal whose distribution was quite different from one sludge to another.

The speci c behavior of each metal with respect to each fraction results in similarities between stations in the distribution of metals, as shown in gure 4. There were, however, signi cant differences between WWTPs, which are best highlighted by statistical analysis (Fig. 5). The BKH and EST sludges were characterized by lower values for most of the metal fractions F-red, F-oxi and F-res, and proportionally higher values for the exchangeable fractions F-ex and F-ac. In contrast, the AZF, BDM and TDM sludges were characterized by higher values for F-res, F-oxi and F-red fractions of metals. AZF and sludges were separated from the TDM sludge mainly due to a higher Co values in the F-oxi fraction.

The potential risk highlighted above due to high Ni and, to a lesser extent, Cr, can be put into perspective by the speciation data. More than 80% of Ni and Cr were found in the F-res fraction, the least likely to be made bioavailable by a change in acidity or redox potential in the soil. The pH usually greater than 6 of the soils of the region where these sludges can be spread is not favourable to the solubilization of Ni-or Cr-bearing metallic particles or minerals [START_REF] Smith | Effect of soil pH on availability to crops of metals in sewage sludge-treated soils. I. Nickel, copper and zinc uptake and toxicity to ryegrass[END_REF]. Here the sum of the F-wat, F-ac and F-oxi Ni fractions, all capable of rapid release of bioavailable Ni, ranged from 6 (BDM) to 13 (TDM) mg.kg -1 .

Considering that sludge application is around 20 t.ha -1 , neglecting leaching and plant uptake, and using 1.5 kg.dm 3 as soil bulk density, it would take 230 (TDM) to 500 (BDM) years to accumulate in the topsoil 10 cm the bioavailable Ni content above which toxic effects can be observed [START_REF] Kumari | Toxicity of cadmium and nickel in soil and vegetables[END_REF]. The speciation of Ni can be very different in other places. In a Slovenian sludge studied by [START_REF] Scancar | Total metal concentrations and partitioning of Cd, Cr, Cu, Fe, Ni and Zn in sewage sludge[END_REF], more than 81% of the high Ni content (621 mg.kg -1 ) were contained in potentially labile fractions. In contrast to Ni, while the total Cu content was below the authorized limits, the sum of the F-wat, F-ac and F-oxi fractions, all capable of rapid release of bioavailable Cu, ranged from 26 (BKH) to 102 (TDM) mg.kg -1 . Considering the same hypothesis than for Ni, it would take 3 (TDM) to 11 (BKH) years to accumulate in the topsoil 10 cm the bioavailable Cu content above which inhibition of nitri cation can be observed (3.8 mg.kg -1 soil) [START_REF] Cela | Critical concentrations of copper, nickel, lead, and cadmium in soils based on nitri cation[END_REF]. These considerations underline the insu ciency of the regulations which only consider the total metal contents and not the conditions of their bioavailability.

Whatever the agronomic value of the sludges, the speciation of the metals in sludges makes it possible to identify the sites on which the search for sources of contamination would be necessary. Analysis of the treated wastewater gives information on the contamination at the moment of sampling, while the sludge gives information on the contamination integrated on the entire deposition time. Here the sources must be searched within the sewage network of the AZF, TDM and BDM WWTPs. The analysis showed that the contaminations concerned several metals simultaneously, the higher values of total ). Here we identi ed metal plating industry in the BDM area and metalworking activities in the TDM area. An exhaustive identi cation of the sources is, however, an activity in its own right which must be carried out in collaboration with the competent regulatory services.

Conclusion

The analysis of sludges from various WWTPs made it possible to assess their variability within an area characteristic of a developing Mediterranean country. All the analysed sludges had satisfactory properties from an agronomic quality point of view. Analysis of the total content of heavy metals showed that some sludges were not usable under French or Chinese regulations. The speciation of metals by sequential extraction underlined, however, that these regulations do not take into account the bioavailability of metals in the spreading environment, here Mediterranean soils. For three of the studied sludges, the total Ni content was higher than maximum legal values while Ni was contained at more than 80% in very poorly bioavailable fractions. In contrast, the total Cu content was below the maximum legal values while this metal was contained in fractions capable of rapid release of bioavailable Cu. The risk of plant toxicity in the short and medium term by spreading these sludges was consequently more linked to Cu than to Ni. Considering that sludge application was around 20 t.ha - 1 , without leaching and plant uptake considerations, and using 1.5 kg.dm 3 as soil bulk density, it was possible de calculate the time that would be required to accumulate bioavailable metal in the topsoil before reaching the threshold above which toxic effects appear. It therefore appears necessary to develop standards which take into account their bioavailability in the soils on which they will be spread.

The speciation of the metals in sludges also makes it possible to identify the sewage network area on which the sources of contamination must be sought. The sludge gives information on the contamination integrated on the entire deposition time, and the metal species indicate what type of source should be sought. Note that the same type of approach could be applied to organic contaminants.

Declarations

Author contributions eld sampling, laboratory work, data curation, writing (initial draft); YL, data analysis, writing; AD, doctoral scholarship acquisition; PM, scienti c supervision, conceptualization, methodology, writing (review and editing).

PO 4 3 -

 3 varied from 4.6 to 0.2 and K + from 3.7 to 0.6 g.kg-1 . These values are in the range of total P and K values found in other studies[START_REF] Wen | Potassium availability with application of sewage sludge, and sludge and manure compost in eld experiment[END_REF] Samars et al. 2008;[START_REF] Tavazzi | Occurrence and levels of selected compounds in European Sewage Sludge Samples, Results of a Pan-European Screening Exercise (FATE SEES)[END_REF]), which con rms their useful practical use as fertilizer. The measured electrical conductivity (EC) is only indicative, because there is no standard value for this type of measurement, which, in addition, is quite rare in the literature. It gives an indication of the salinization hazard for soils sensitive to this problem. In our case, the values for a 1:10 water extract varied from 409 to 1844 µS.cm -1 , which would correspond to 762 to 3432 µS.cm -1 for a 1:5 water extract (Perez-Espinosa et al. 2000). These values are lower or in the range of values observed elsewhere (Perez-Espinosa et al. 2000), therefore without speci c salinization hazard. Table 2 Agronomic parameters of the studied urban dewatered sewage sludges depending of the WWTP location (EC: electrical conductivity, tN: total N, CEC: cation exchange capacity), OM: organic matter

  e. mainly bound to Fe or Mn oxides, in agreement with other studies (Ahumada et al. 2004; Islam et al. 2017; Yang et al. 2017).The distribution of Co among the fractions looks like that of Ni, with a greater proportion in the F-oxi and F-red fractions. High values of the latter were observed for samples BDM and AZF. Cumulative content in the F-ex, F-ac, F-oxi fraction (max. 3.3 mg.kg -1 in the AZF sludge) were low enough, however, to rule out long-term risk with regard to plant growth. Zn an Pb were mostly in the F-red fraction, as observed elsewhere[START_REF] Silveira | Heavy metal sequential extraction methodsa modi cation for tropical soils[END_REF]; Chen et al. 2008; Zu aurre et al. 1998), with, for Zn, a non-negligible proportion (4.2 to 15.9%) in the acid-soluble fraction (F-ac).

Figures Figure 1 Geographic localization of treated wastewater plants Figure 2 PCA 3

 123 Figures

Table 1

 1

	Technical characteristics of the wastewater treatment plants (WWTP) (IE: inhabitant
			equivalent)
	WWTP	Date of	Capacity	Discharge	Outlet
		commissioning	(IE)	(m3 day-1)
	Azeffoun (AZF)	07/2006	5 000	/	Mediterranean Sea
	Boukhalfa (BKH)	11/2006	25 000	3 750	Sébaou River
	Tizi-Ouzou (EST)	07/2000	120 000	18 000	Sébaou River
	Tadmaït (TDM)	05/2007	13 000	1 950	Sébaou River
	Boumerdes (BMD)	01/ 2001	75 000	8000	Tatareg River

Table 3

 3 High content of heavy metals in WWTP sludges usually do not originate from domestic e uents, but from industries that discharge e uents in the sewer system or from entry of runoff stormwater into the sewer system[START_REF] Sörme | Sources of heavy metals in urban wastewater in Stockholm[END_REF][START_REF] Singh | Potential bene ts and risks of land application of sewage sludge[END_REF].

). For metals for which there is a legal maximum value, all SSs values were below the permitted threshold, except for Ni. TDM and BDM samples had Ni content higher than the French and Chinese legal ceiling value and sample AZF had Ni content higher than Chinese ceiling value. TDM and BDM sample had a Cr content lower, but rather close to the Chinese limit value. High Ni and Cr content in DSSs were frequently observed in other studies

[START_REF] Juarez | Intéres agrícola de lodos de depuradoras de águas residuales[END_REF][START_REF] Zu Aurre | Speciation of metals in sewage sludge for agricultural uses[END_REF][START_REF] Lasheen | Assessment of metals speciation in sewage sludge and stabilized sludge from different wastewater treatment plants, Greater Cairo, Egypt[END_REF]

.

Table 3

 3

	Titanium in sludges was more studied due to the increasing use in recent decades of TiO 2 nanoparticles,
	especially in personal care products, and because it is used as a catalyst for photocatalytic degradation
	of organic pollutant in wastewater (Wielinsky et al. 2021). Data on bulk TiO 2 content in DSSs, however,
	are also scarce. Values ranging from 305 to 1800 mg.kg -1 were measured in DSS issued from European
	WWTPs (Johnson et al. 2011; Pradas del Real et al. 2018; Wielinsky et al. 2021). Lazareva and Keller

Total content of heavy metals (mg.kg -1 of dry matter ± std) in sludge and ceiling values application to the land. Fr: France NF U44-041; US: U.S. EPA CRF 503.13; Ch: China GB4284-84. In red, values higher than one of the legal ceiling value # Unavailable data.

No legal maximum value was found for Ag, Co and Ti. Ag content in the studied DSSs ranged from 1.1 to 7.4 mg.kg -1 . These values can be considered to be low in comparison with the few data available in the literature. A study from USEPA (2009) considered 74 sludge samples representative of U.S. WWTPs and found that Ag content was ranging from 2 to 195 mg.kg -1 (20 on average), 50% of the samples having a value higher than 13. Pradas del Real et al. (

2016

) reported a value equal to 14 mg.kg -1 for a sludge from Switzerland.

Cobalt is of special interest because it can act on biogeochemical cycles, in particular with regard to the availability of nitrogen for plants

[START_REF] Perez-Espinosa | Effect of increased cobalt treatments on sewage sludge amended soil: Nitrogen species in soil and transference to tomato plants[END_REF]

. Here the Co content in sludges ranged from 9.5 to 27 mg.kg -1 , which can be considered as high and even outstanding values compared to the scarce data available in the literature, that range from 1 to 13 mg.kg -1

[START_REF] Grummitt | Transfer of cobalt-60 to plant from soils treated with sewage sludge[END_REF]

; Perez-Espinosa et al. 2002; Pradas del Real et al. 2016; Malinowska and Jankowski, 2020). The higher value observed here (27 mg.kg -1 , AZF sample) is, however, unlikely to have an effect on plant growth (Perez-Espinosa et al.

2002)

. A cumulative effect of DSS application over several years, however, is not to be ruled out, depending on the mobility of Co in the soil. (2014) calculated, considering market studies, that the Ti content in sludges from New-York, London and Shanghai would range from 40 to 208 mg.kg -1 . Here the Ti content in DSS ranges between 946 and 1092 mg.kg -1 , values relatively high with regard to the literature. Titanium, mainly as TiO 2 , is widely used products from paints to personal care products

  al. 2008; Walter et al. 2006, Zu aurre et al. 1998).

  Ni being accompanied by higher values for most F-red, F-oxi and F-res fractions of Cd, Co, Cr, Cu and Ti. Possible sources can be industries or cottage industries related to painting (Lokhande et al. 2011; Tesfalem and Abdrie, 2017), metalworking and metal plating (Quin et al. 2018), tannery, textile dyeing (Imtiazuddin and Mumtaz, 2013; Uma et al. 2016

Funding The doctoral scholarship of RC was provided by the Algerian Ministry of High Education and Scienti c Research (MESRS) and the Mouloud Mammeri University. Financial support was provided by a grant from the Université de Toulon.

Data Availability All data generated or analysed during this study are included in this published article and its supplementary information les.

Ethics approval and consent to participate

Not applicable

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

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Keywords: Network simulator, Traffic simulator, Mobility model, intgration, NS-3, SUMO. I

One of the main challenges in the Vehicular Ad-Hoc Networks (VANETs) research domain is the simulation of vehicular communications using realistic mobility models. Several efforts have been put lately in this purpose; yet, the proposed models are either inappropriate, or carry significant disadvantages. In this paper, we propose a novel approach for realistic simulations in vehicular networks, inspired from the hierarchical video and image compression technique. We developed HINTS (Hybrid Integration of Network and Traffic Simulators), the correspondent platform, engendered from the integration of SUMO, a traffic simulator and NS-3, a network simulator after adding some supplemental modules to both simulators. Its efficiency and performance are demonstrated in a proof-of-concept study, based on a realistic traffic management scenario.

INTRODUCTION

Vehicular Ad hoc Networks (VANETs) are a special kind of Mobile Ad-Hoc Networks (MANETs) adapted to the communications between vehicles. The great potential of this technology has been acknowledged with the establishment of ambitious research programs and standardization activities on vehicular communication systems worldwide, such as ISO TC204, IEEE (802.11p and 1609.x) and SAE DSRC in the USA, ETSI TC ITS and CEN WG278 in Europe and ARIB T-75 in Japan. In addition, significant evolution inside the vehicles themselves is observed since they are expected to be equipped with high efficiency computing systems and communication capabilities. According to ETSI, in 2027 almost 100% of the vehicles will be equipped with On Board Units (OBUs). OBUs are devices that provide communications among neighboring vehicles (Vehicle to Vehicle) or between vehicles and nearby fixed equipments (Vehicle to Infrastructure). Vehicular networks attract both research and industry communities. Recently, they focus their efforts to grow vehicular communication and networking into maturity by moving it from research field into real implementation (Field Operational Test, FOT); aiming to provide not only more safety in the transportation systems but also other high quality-of-service (QoS) based applications. However, such applications need to be analyzed, tested and validated using costly real testbeds (FOT), or simply using lower cost simulations.

One simple way of evaluating protocols and applications in VANET is by using communication network simulators. However, this solution appears to be inadequate since existing mobility models, implemented in network simulators, are not realistic. To solve this problem, an approach called offline simulation was introduced; it consists of integrating realistic mobility traces into a network simulator. These traces are extracted from movements of real vehicles that were collected by multiple monitoring systems like induction loops and video surveillance cameras installed in real roads. Another way of extracting these traces is by using a traffic simulator with realistic mobility patterns. The traffic simulator generates the realistic mobility traces prior to the network simulator execution. This approach allows the nodes in the network simulator, to move according to a realistic mobility model that reflects the reality. However, this approach only solves the VANET applications evaluation problem partially. In fact, it is suitable for large scenarios without any unexpected events occurring during simulation time, as it allows the communication between the simulators in one way but not in the opposite way. For example, in a traffic efficiency application which informs the vehicles about the state of the road, a vehicle can decide to change its route to avoid the traffic jam; hence, the need for a bidirectional communication. We can think of a solution where mobility traces are generated after the occurrence of each event, but this will require an enormous amount of time, as the generation of the mobility traces is very slow.

The other major advance in this domain is the online simulation approach. In this case, no static mobility traces are generated by the traffic simulator, but instead, the latter is launched simultaneously with the network simulator. Thus, all the alterations caused by the network simulation can be taken into account in real time. This was made possible with the insertion of an efficient feedback loop which consists in sending the required information (position, speed…) to the network simulator on each simulation step. In the following, we refer to the information sent for each vehicle as a subscription for this vehicle. Although this approach seems to be a good alternative to evaluate and validate Intelligent HINTS: A Novel Approach for Realistic Simulations of Vehicular Communications Sofiane Zemouri, Sara Mehar, and Sidi-Mohammed Senouci DRIVE Labs, University of Burgundy, Nevers, France Nevers, France Contact: sofiane.zemouri@gmail.com, {Sara.Mehar, Sidi-Mohammed.Senouci}@u-bourgogne.fr Transportation Systems (ITS) applications, it shows some major drawbacks. In fact, it is more suitable for small scenarios on account of the large number of messages exchanged between the simulators on each time step and their important lengths; not to mention the high computational cost and the consequent long simulation times.

As emphasized earlier in this section, the offline simulation approach is suitable for large scenarios without any unexpected events occurring during runtime; where the online simulation is more appropriate for smaller scenarios with a large number of events. In this work, we advocate a novel simulation approach and develop a corresponding optimized platform for realistic simulations of vehicular communications. Our hybrid approach has the advantage of using both offline and online simulations so it overcomes the limitations of one using the other and vice versa.

The reminder of this paper is organized as follows: The next Section is a quick review of the most popular related works in the literature. We describe our approach and its architecture in Sections III and IV respectively; evidence of its performance is given in Section V. Finally, Section VI presents our conclusion and future perspectives.

II. RELATED WORK

The most common classification for the different integration approaches, briefly addressed in the previous section and referred to in [START_REF] Hartenstein | VANET : Vehicular Applications and Inter-Networking Technologies[END_REF] for coupling network and traffic simulators, sort the literature solutions into three distinct categories: (i) isolated or offline simulators, (ii) embedded simulators, and (iii) federated or online simulators.

In the first category, the interaction between the network and the traffic simulators is petty or non-existent as the mobility model is bounded to a static mobility trace. MOVE (MObility model generator for VEhicular networks) [START_REF][END_REF], is a dedicated open source tool that generates mobility traces readable in network simulators like NS2 [START_REF]The network simulator ns-2[END_REF] and Qualnet [START_REF]Qualnet[END_REF]. SUMO Player [START_REF]SUMOPlayer[END_REF] and Trace Exporter [START_REF]TraceExporter[END_REF] are also tools designed to convert netstate dump files (XML files containing all the simulation's details) generated from traffic microsimilators like SUMO (Simulation of Urban MObility), into mobility traces interpretable by network simulators like NS2 and VISSIM [START_REF] Schroth | Simulating the traffic effects of vehicle-to-vehicle messaging systems[END_REF].

In the embedded approach, the traffic and the network simulators are natively coupled as they are incorporated to form a unique entity. MoVES [START_REF] Bononi | MoVES: A framework for parallel and distributed simulation of wireless vehicular ad hoc networks[END_REF], NCTUns [START_REF] Wang | NCTUns 5.0: A Network Simulator for IEEE 802.11[END_REF] and VCOM [START_REF] Killat | Enabling efficient and accurate large-scale simulations of VANETs for vehicular traffic management[END_REF] the embedded module in VISSIM fall into this category. However, the major limitation of these platforms is the simplified network or traffic capabilities. In fact, most of these platforms were initially meant either for traffic or network simulations but not both; and were provided by additional modules to support VANET simulations.

To overcome this limitation, the third category was introduced. In this case, a bidirectional communication is established between the simulators. The network simulator takes control of the traffic simulator by sending commands to change the nodes behavior. The latter responds to the former by reporting the positions of the loaded nodes. In the following, this two-ways interaction will be referred to as feedback loop. This approach is considered so far as the most suitable solution in the literature for vehicular communications simulations offering an acceptable level of realism. TraNS [START_REF] Pi ´orkowski | TraNS: Realistic Joint Traffic and Network Simulator for VANETs[END_REF], an opensource simulation environment, was the first implementing a federated approach; as it links the traffic simulator SUMO [START_REF]Simulation of urban mobility[END_REF] and the network simulator NS2 using a specific interface called TraCI [START_REF] Wegener | TraCI: An Interface for Coupling Road Traffic and Network Simulators[END_REF], which we will evoke further in Section 3. iTERIS [START_REF] Gozalvez | iTETRIS: the Framework for Large-Scale Research on the Impact of Cooperative Wireless Vehicular Communications Systems in Traffic Efficiency[END_REF] project, also an online approach implementation, aims at providing a framework to combine traffic mobility and network simulations for large scale testing of traffic management solutions. This framework uses SUMO as a traffic simulator and NS-3 [START_REF]The ns-3 network simulator[END_REF] as a network simulator. Although it was a promising project, iTERIS has not been updated in its official website since 2010.

Ovnis [START_REF] Pigné | A Platform for Realistic Online Vehicular Network Management[END_REF] and Veins [START_REF] Sommer | Bidirectionally Coupled Network and Road Traffic Simulation for Improved IVC Analysis[END_REF] are the latest attempts to implement an online simulation platform as it couples SUMO with NS-3, and SUMO with OMNeT++ [START_REF]Omnet++[END_REF] respectively.

The reason behind the miscarriage of the latest category, even though it presents the most realistic results one can get from the literature tools, is the long simulation times and the high computational cost. In fact, this constraint narrows the scope of the correspondent platforms to small scenarios, unless a supercomputer is used to perform the simulation. This will continue to restrain the progress of the ITS field as long as VANET simulations are not trivialized enough.

Our approach, named HINTS, advances all state-of-the-art solutions in terms of performance as it uses resources more efficiently and considerably reduces the computational cost and the simulation time. The combination of the online and the offline approaches brought forth a new approach that uses one full mobility trace and a feedback loop, for an active exchange and a real time interaction between the network and the traffic simulators.

III. HINTS DESCRIPTION

HINTS stands for Hybrid Integration of Network and Traffic Simulators. Our approach combines all the benefits from state of the art solutions. We propose an approach that is as flexible as the online approach, but with less computational cost and shorter simulations times by taking advantage of the offline simulation approach benefits also. As a result, our hybrid approach will be suitable for large scenarios simulations and will adapt with the number of events to offer better simulation times and less computational cost. This approach is inspired from the hierarchical approach for image and video compression, where a first image is stored entirely as a background and the next images are obtained from storing the parts of the image that differ from the original one. We noticed many similarities between our problem and the image/video compression problem as both aim for reducing the computational cost and the size of data transferred in each time step, and reducing the size of data stored respectively. We noticed also that if an event occurs, this will be likely to alter the state of only a few nodes in the network while the bigger percentage of nodes will not be affected by this event. This led us to come up with a novel approach in this domain: the hybrid approach for realistic simulations of vehicular communications. In brief, we use a mobility trace as a background image of the network state, generated beforehand by the traffic simulator; and since the alteration in our case comes from the network simulator, we can determine exactly the nodes for which the behavior has changed. In this case, only the positions of the nodes concerned by an event are sent to the network simulator, and these new values will come to overload the values extracted from the original mobility trace.

IV. PLATEFORM ARCHITECTURE

As pointed out in the first section, the analysis of protocols and applications in VANET require a real time bidirectional interaction between a network and a traffic simulator. HINTS consists of three essential blocks which are the traffic simulator, the network simulator and the ITS application. These three blocks interact with each other in real time via some dedicated communication interfaces. To benefit from the best tools in the literature, we chose the widely spread traffic simulator SUMO that has proved its reliability and performance within the Intelligent Transportation Systems community; and the successor of the most used network simulator: NS-3 also validated in the academia. The TraCI API accessible in SUMO has confirmed our selection for the traffic simulator as it allows running SUMO as a server and controlling it by any third-party application using a TCP socket connection. NS-3 was chosen due to its accurate implementation of physical layers and communications standards, its improved scalability (including parallelization capabilities) and its active development community. The accessibility of both simulators' source code in C++ is a further reason that reinforced our choice for these two simulators.

A. The general hybrid platform architecture

Our coupled platform uses SUMO in one hand to provide the required information about the state of the network (position, speed, lanes…) and supply the network simulator with the necessary feedback. In the other hand, NS-3 is used to perform data dissemination in order to inform the ITS applications embedded on each node about the surrounding environment and the state of the network. It is now up to the ITS applications that takes the role of the behavioral model to decide whether a node should change its behavior or not. This efficient feedback loop is extended with the insertion of a mobility trace that includes the movements of all nodes from the entire simulation duration, and which represents a background image of the network; and thereby, considerably reduces the amount of data transferred between the traffic simulator and the network simulator in each time step since only the positions of nodes concerned by a certain event are sent.

Fig. 1 shows the general overview of the architecture. In this figure, the traffic simulator in the leftmost box manages the nodes' mobility and generates both static traces and dynamic micro-traces. The middle box represents the network simulator whose role is handling the data dissemination via wireless communications, in addition to translating received data about the nodes movements; and that, in order to implement its own realistic mobility model based on the decisions taken by the rightmost box, which represents the user level model. This latter block represents the communication protocol or user VANET application to be analyzed. The communication between these three blocks is provided by the TraCI client/server interface, which allows real time, bidirectional communication between the network and the traffic simulators.

B. Simulation flow overview

As the simulation starts, SUMO generates one complete mobility trace to constitute the base structure of the mobility model. This mobility trace is injected into the network simulator and the two simulators are launched simultaneously with NS-3 as the client application and SUMO as the server application. NS-3 reads from the previously generated mobility trace and sends commands to SUMO via TraCI to execute simulation steps in order to stay synchronized in time. Information about the surrounding environment of a node (speed, route, destination…) is provided by SUMO while the cooperative awareness messages (CAM) [START_REF]Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Part 2: Specification of Cooperative Awareness Basic Service[END_REF] exchanged between these nodes (safety messages, traffic state messages…) are provided by NS-3. All these information are operated in the ITS application in order to decide whether a node should change its behavior (speed, trajectory, route or destination). Once the decision is taken for a node to change its trajectory, the corresponding information will be retrieved on each time step and until the node reaches its destination, and overload the values given by the static mobility trace. Fig. 2 shows the flow of events in HINTS.

One of the benefits of using this platform is that the mobility trace generated beforehand, can be reused multiple times with different scenarios of applications. In fact, analyzing an ITS application consists of defining a simulation scenario, testing it with different parameters to finally compare and evaluate its results. However, this will not be possible if we change the initial nodes mobility scenario each time we change a parameter. Another advantage of using this platform is the reduction of the amount of data sent and received between the two simulators. This will considerably shorten the simulation time and decrease the computational cost allowing simulating even larger scenarios.

C. SUMO and NS-3 implementation details 1) Modifications and additional modules

Note that the nodes in direct interaction with the nodes affected by a certain event are likely to change their trajectory too and the corresponding positions need, therefore, to be reported as well. To select these nodes, we used a simple technique that consists of marking each node interacting directly with an altered behavior node. In other words, all nodes in the same lane close enough from a marked node, and all nodes that want to change lane but are blocked by a marked node, are themselves marked. At the end of each time step, the positions of all marked nodes are reported to the network simulator.

One inconvenient of the offline approach is the time taken to parse the generated mobility trace and convert it to a format understandable by the network simulator. For example, in the case of one simple intersection simulation, the generation of the raw output produced by SUMO for 1h30 scenario with 3400 nodes takes 10 seconds; while the conversion of this raw trace to a tcl format with two of the most efficient tools for this purpose, SUMO Player and Trace Exporter, takes 15 and 18 minutes respectively.

To tackle this shortcoming, we modified SUMO so it produces an optimized generic format of mobility traces (tcl format) directly usable in NS-3. This way, the mobility trace generation is shortened to the traffic microsimulation time, which is relatively short. In addition to that, we have optimized NS-3 so it reads through the produced mobility trace more efficiently by preventing it from parsing the file multiple times, and reading only the desired portions of the trace instead, at each time step, allowing gaining considerable time.

To furnish our platform with an efficient feedback loop (Fig. 1), we have provided NS-3 with a TraCI client that takes control of the SUMO simulation via the TraCI server, and therefore, supplies NS-3 and the ITS application with the needed data about the network and the nodes mobility; with the help of value retrieval methods, state changing methods and control-related methods. The latter category performs SUMO simulation steps, and subscriptions for the concerned nodes (speed and value retrieval on each time step).

2) Specifications about HINTS

We have classified the events that are susceptible to alter the behavior of a node into two essential classes: primary events and secondary events. The first class touches the infrastructure and affects the mobility of nodes in a major way, while the second class only concerns a group of vehicles, and affects the traffic in a more microscopic manner. The reason behind this appellation is due to the nature of the events. In other words, the occurrence of a primary event will lead to the occurrence of multiple secondary events. As an example for a primary event, we can cite a blocked road. This event will most probably lead to traffic congestion and a number of vehicles changing their routes, which in this case, are considered as secondary events. Based on this classification and the VANET application taxonomy proposed by the C2C-CC [START_REF]CAR 2 CAR Communication Consortium[END_REF], we decided to provide the TraCI client with five vehicle state changing mobility primitives: Stop, Change speed, Change lane, Change route and Change target. These five primitives are sufficient to handle mobility alteration in a microscopic level from any scenario, and thereby, increase the efficiency of our feedback loop.

HINTS was designed to reduce the simulation complexity procedure from a developer point of view. Apart from the XML files needed to generate the mobility trace in SUMO, and the ITS application written in C++ and executed in NS-3; no additional files will be required for the simulation execution. Also, most of this simulation process will be automated. From NS-3 side, a simple call for the method "Install ()" which configures the positions of nodes, will trigger the synchronized execution of SUMO time steps. In addition to that, subscription messages are sent to SUMO automatically with each call for a state changing method of the corresponding node. The novelty for the developer is the use of the methods provided by our platform to supply the ITS application with the required data (get speed, get lane, get route…) and influence the mobility model (stop node, set speed, reroute…) as wished.

V. PERFORMANCE EVALUATION In order to prove the efficiency and the performance of the developed platform, we considered testing it on a modified Manhattan grid scenario depicted in Fig. 3 and representing a city of 20000 inhabitants. We used a 2 km² network with 3 types of roads (1 lane, 2 lanes and 3 lanes) to show the importance of each road. The represented city has 4 main entrances. In the following, we will briefly introduce the operating mode of the tested ITS application. The performance of HINTS is justified using the percentage of subscriptions metric while the efficiency is substantiated using a metric called Mean Sojourn Time or the Mean Travel Time metric in our case: the average time vehicles spend on the road before reaching their destinations, calculated for the arrived vehicles at the end of each simulation step.

A. Application's operating mode

In the following, we assume that vehicles are equipped with interfaces for wireless communications, and also with an onboard GPS that keeps track of the current road id. Each vehicle follows its planned route; if any unexpected event occurs (accident, blocked road), the vehicle in front will emit warning messages within a range of 250 meters. This warning message is composed of the current road id and time. Vehicles that receive the warning message can then check if this road is part of the initially planned route with the help of the onboard GPS, and can then change their route if possible. The following algorithm shows the iterative behavior of the application embedded on vehicles:

B. Results

The simulated scenario counts an overall of 7730 vehicles running for 5400 seconds. The simulation starts at time 25000, which corresponds to the morning rush hour (around 7:00 AM). At time 26000 seconds, we block one of the main roads of the city. The road remains blocked for 2000 seconds then it is opened again. In Fig. 4, we show the performance of our platform by illustrating the decreased number of subscribed vehicles (vehicles for whom the speed and positions are retrieved from SUMO on each time step). We can notice that the number of subscriptions doesn't exceed 25% of the loaded vehicles over the event occurrence period; while an online implementation requires the subscriptions of 100% of the loaded vehicles. This allows deceasing the simulations' computational cost letting the simulation run faster. The efficiency of our platform is shown using the Mean Travel Time metric. The results reflecting the fluidity of road traffic obtained after launching the tested application are illustrated in Fig. 4. The minimum interval between two packets sent by a vehicle was set to 5 seconds and the time to live for a packet is 1 second. In the observed period, the number of vehicles and the mean travel times increases starting from around 25600 seconds. Fig. 5 (a) shows the mean travel time of vehicles without the application reaching higher than 4000s in some cases. The enhancement can be observed in Fig. 5 (b), showing the mean travel times with the application launched, as the maximum travel time is slightly higher than 3000s. In other terms, vehicles spend less time on the road and reach their destinations faster compared with the scenario wanting the application.

VI. CONCLUSION

In this paper, we introduced a novel approach for realistic simulations of vehicular communications, and the design of the platform HINTS, the corresponding implementation. It reposes on SUMO for the realistic mobility of nodes, and on NS-3 for the wireless communications. The goal of this new approach is to decrease the simulations' times and their computational costs in order to trivialize VANET application simulations. Future works will focus on improving the open source platform HINTS, and using it for its main purpose, that is, to evaluate VANETs protocols and applications.

Figure 1 .

 1 Figure 1. Platform general architecture

Figure 2 .

 2 Figure 2. Sequence diagram in HINTS

Figure 3

 3 Figure 3. Topology

Figure 4 .

 4 Figure 4. Percentage of the subscribed vehicles

Figure 5 .

 5 Figure 5. Mean travel time (a) without the applicantion, and (b) with the application