Vortex control in large-eddy simulations of compressible round jets

We investigate through large-eddy simulations the effects of different types of upstream forcing in subsonic (Mach 0.7) and supersonic (Mach 1.4) round jets. We have reproduced and tested the different methods of forcing developed in incompressible round jets by Urbin and Métais In Direct and Large-Eddy Simulations II, 1997, P. R. Chollet, J. P. Voke, and L. Kleiser, Kluwer: Dordrecht, pp. 539–542, Danaila and Boersma, Physics of Fluids A, 12, 1255–1257, da Silva and Métais Physics of Fluids, 14, 3798–3819, (see also Lee and Reynolds Bifurcating and blooming jets at high Reynolds number 5th Symposium on Turbulent Shear Flows, New York). Our strategy is to search the optimal excitation that maximizes the jet spreading at Reynolds number Re = 36 000. Four different forcings based on information obtained both instantaneously and statistically. In the subsonic case, and as in the incompressible one, we aimed to favour the flow spreading along one particular plane (bifurcating plane), while maintaining a standard or reduced spreading rate along the bisecting plane, perpendicular to the bifurcating one. The flow response to the excitations is analysed both instantaneously and statistically. In the subsonic case, and as in the incompressible one, the maximum jet spreading is obtained with inlet varicose–flapping perturbations at preferred and first subharmonic frequencies, respectively. The potential core length is reduced by 27% with respect to the natural jet. These results are in good agreement with several laboratory experiments and numerical simulations carried out in incompressible round jets. Indeed, the subsonic jet has a convective Mach number of 0.35, and is weakly affected by compressibility. In the supersonic jet case, on the other hand, the highest spreading rate is found with a flapping excitation at the second subharmonic. The potential core length is now reduced by 28% with respect to the unforced jet.