Momentum Diffusion Near Jet Exit in a Round Jet Controlled by Half Delta-Wing Tabs

Direct numerical simulations for a round jet controlled by half delta-wing tabs are performed to elucidate the transition of turbulent diffusion near the jet exit. The aim also involves clarifying the function of the tabs in round jet control. The jet Reynolds number is set as 10,000. The number of the tab is set to 3 and 6. The results reveal that the installation of the tabs generally decreases the turbulent diffusion coefficient except immediately after the jet exit of \(x/d<1.5\), where x is the streamwise direction and d is the diameter of the jet exit. The magnitude relation of the turbulent diffusion coefficient between the behind-the-tab area and between-the-tabs area changes along the streamwise direction in the upstream region of \(x/d<4\). Also turbulent diffusion does not always follow the conventional gradient-diffusion model there. On the other hand, in the downstream region of \(x/d \ge 4\), the difference in terms of the turbulent diffusion coefficient disappears between the two areas. However, the jet shape is triangular in the case of three tabs due to the remaining streamwise vortices generated by the tabs. This fact leads to the larger mean velocity gradient and momentum transport in the behind-the-tab area than those in the between-the-tabs area. Conversely, in the case with six tabs, the turbulent diffusion coefficients at the two areas become almost identical in the downstream region because the jet shape approaches a round shape.