Adjoint-based passive flow control is applied to damp the instabilities arising in the three-dimensional incompressible flow over a realistic open cavity with geometric ratio 6:2:1 at low Reynolds number. Direct numerical simulation and TriGlobal stability analysis are performed to identify the complex three-dimensional flow structures corresponding to flow instabilities in this flow. The instability onset is caused by the linear growth of a three-dimensional shear layer mode and the structural sensitivity of this leading global mode is identified by means of an adjoint analysis. Based on this information, we perform a small base flow modification inside the structural sensitivity region by collocating a small rod at the interface of the cavity. Results from this passive control systems demonstrate the substantial reduction on the amplitude of the oscillation and the effective suppression of the three-dimensional vortex oscillation.
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