Direct numerical simulation of a jet controlled by fluid injection

Abstract

This study is concerned with direct numerical simulations of jet control carried out by means of secondary jets. The use of realistic inflow conditions enables us to examine the influence of the side jets on the vortical structure dynamics in the main jet. Four control configurations are presented for several positions and flow rates of the control jets. It can be seen that the jet expansion is strongly modified by fluid injection as previously observed experimentally and numerically, with a significant reduction in the potential core length. Secondary jets induce strong anisotropy on the mean flow, whereas the global effect on turbulent fluctuations is more limited. Contrary to an a priori point of view, structural modifications of the main flow due to a fluid injection do not lead to an important increase in the enstrophy production. The strong distortion of the main flow seems to be related to the appearance of large-scale vortices. These structures, organized in two counter-rotating vortex pairs, are created by main vorticity tilting into streamwise vorticity. The effects of the control on the mixing properties of the main flow are also considered by solving a passive scalar equation. Mixing efficiency is seen to be increased considerably by the control action. The use of pulsating secondary jets appears to be the most favourable condition for reducing the fluid fraction of high scalar values, and, more generally, to increase the injected fluid penetration in the core of the main jet.