Etude des écoulements de jets coaxiaux et leur simulation par LES/RANS.

Abstract

The work presents a numerical study of a compressible and turbulent coaxial jet,using a Large Eddy Simulation type approach. This study focuses on the implementation of a high-performance and modern numerical method to meet the requirements in terms of results accuracy and calculation cost. The code was parallelized using the Message Passing Interface (MPI) library. Inviscid fluxes are evaluated using a new linearization solver for the equations characteristic of approximate Riemann problem and the solution has been advanced over time using an explicit two-step McCormack method. The purpose of this study is to understand the initial stage of the transition process from laminar to turbulent flow in coaxial jets and study the mixing process in a compressible coaxial jet, whose primary jet is hot and whose secondary jet is unheated, through a scalar passive injected into the jet, where two cases were considered. Case 1 is a scalar passive injected into central jet and Case 2 is a scalar passive injected into the secondary jet. Particular attention is paid to the role of coherent vortices flow in the mixing process. Therefore, it has been determined that the first step in the transition process from laminar to turbulent flow in coaxial jets is initiated by the development of Kelvin Helmholtz (primary vortices) in sheer layers, while three-dimensionalization turbulence is initiated in the second step with the emergence of secondary vortices. The transverse evolution of Reynolds tensors showed that the axial Reynolds tensor contribution is associated with primary instability while the transversal Reynolds tensor contribution is associated with secondary instability. Flow self-similarity state is obtained in fully developed turbulence region. Axi-symmetric mode predominates on each shear layer in the initial flow region. Mixing process was studied using the spatial and temporal resolution of the Navier-stockes equations and the equation transport of the mixture fraction. The mean and instantaneous mixture fraction were traced and analysed. Then, using the probability density function, mixture quality was studied. Results of this study provided useful information. In particular, active mixing regions were associated with high concentrations of Root Mean Square (RMS) values. Coherent vortices play a very important role in the mixing process, and the efficiency of the mixture is maintained when the passive scalar is injected into the central jet. All results obtained are in agreement with similar experimental results of simulation jets in this study.