Response of a turbulent separation bubble to zero-net-mass-flux jet perturbations

The response of a turbulent separation bubble (TSB) to zero-net-mass-flux actuation is investigated via direct numerical simulations. Rectangular jets with their long axis oriented in the streamwise direction are used to generate unsteady streamwise vortices that mimic the streamwise elongated Gorder vortices found to be associated with the low-frequency unsteadiness of the TSB [Wu et al., J. Fluid Mech., 883, A45 (2019)]. Three sinusoidal actuation frequencies are investigated, corresponding to the two natural frequencies of the undisturbed separation bubble (f(l) and f(h) with a ratio of f(h)/f(l) = 2.5) and a high frequency at 10 f(l) motivated by a harmonic resolvent analysis. The results are compared to the baseline uncontrolled flow. Very-large scale (VLS), spanwise-rotating vortices are formed at f(l) and f(h), causing a 50% reduction in the mean TSB length. A counter-rotating secondary vortex is induced locally by the VLS vortex in the f(l) case and forms a vortex pair with the VLS vortex as they move downstream together. The interaction of the vortex pair facilitates their decay. The VLS vortex generated by the forcing at f(h) is not strong enough to produce such a secondary vortex. Spectral analysis of the harmonic resolvent operator is used to quantify the receptivity of the flow to actuation at different frequencies. The perturbations that excite the most energetic response in the flow are indeed in the form of streamwise-elongated structures in the separation region at f(l) and f(h). Energetic structures corresponding to the temporal mean obtained from the analysis are found to extend to distances far downstream of the separation bubble confirming the great sensitivity of the entire flow to such forcing.