Experimental-numerical investigation of mixing enhancement in under-expanded jet

Experimental and numerical means were used to investigate the mixing enhancement of an under-expanded supersonic jet subjected to radial secondary injections. Measurements allowed to confirm the efficiency of such a device. Several configurations were tested to assess the influence of the nozzle pressure ratio, the number of active injectors and their flow rate. A further analysis was conducted with two numerical methods. First, RANS computations permitted to understand the modification of the inside supersonic waves and to prove that they do not contribute to the mixing enhancement. The mixing process is only actuated by longitudinal vortices. Then, several mixing criteria were proposed and a parametric study was carried out. It gave some indications for designing an efficient mixing device. Finally, Zonal Detached Eddy Simulations proved to correctly capture both the compressibility and the turbulent processes. Contrary to RANS computations, they provided a fair quantitative reproduction of the jet farfield, and highlighted the existence of an unsteady instability mechanism which may be responsible for the observed fast vortex decay.