TY - JOUR

T1 - Response of a turbulent separation bubble to zero-net-mass-flux jet perturbations

AU - Wu, Wen

AU - Meneveau, Charles

AU - Mittal, Rajat

AU - Padovan, Alberto

AU - Rowley, Clarence W.

AU - Cattafesta, Louis

N1 - Publisher Copyright:
© 2022 American Physical Society.

PY - 2022/7

Y1 - 2022/7

N2 - 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 Görtler vortices found to be associated with the low-frequency unsteadiness of the TSB [Wu et al., J. Fluid Mech., 883, A45 (2019)10.1017/jfm.2019.911]. Three sinusoidal actuation frequencies are investigated, corresponding to the two natural frequencies of the undisturbed separation bubble (fl and fh with a ratio of fh/fl=2.5) and a high frequency at 10fl 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 fl and fh, causing a 50% reduction in the mean TSB length. A counter-rotating secondary vortex is induced locally by the VLS vortex in the fl 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 fh 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 fl and fh. 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.

AB - 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 Görtler vortices found to be associated with the low-frequency unsteadiness of the TSB [Wu et al., J. Fluid Mech., 883, A45 (2019)10.1017/jfm.2019.911]. Three sinusoidal actuation frequencies are investigated, corresponding to the two natural frequencies of the undisturbed separation bubble (fl and fh with a ratio of fh/fl=2.5) and a high frequency at 10fl 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 fl and fh, causing a 50% reduction in the mean TSB length. A counter-rotating secondary vortex is induced locally by the VLS vortex in the fl 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 fh 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 fl and fh. 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.

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U2 - 10.1103/PhysRevFluids.7.084601

DO - 10.1103/PhysRevFluids.7.084601

M3 - Article

AN - SCOPUS:85137265928

SN - 2469-990X

VL - 7

JO - Physical Review Fluids

JF - Physical Review Fluids

IS - 8

M1 - 084601

ER -