Large-Eddy Simulation of Flow Separation and Control on a Wall-Mounted Hump

Abstract

Active flow control techniques such as synthetic jets have been successful in increasing the performance of naturally separating flows on post-stall airfoils, bluff body shedding, and internal flows such as wide-angle diffusers. However, in order to implement robust control techniques there is a need for accurate computational tools capable of predicting unsteady separation and control at high Reynolds numbers. This thesis developed a compressible large-eddy simulation (LES) and validated it by simulating the turbulent flow over a wall-mounted hump. The flow is characterized by an unsteady, turbulent recirculation region along the trailing edge of the geometry, and is simulated at a Reynolds number of 500,000. Active flow control is applied just before the natural separation point via steady suction and zero-net mass flux oscillatory forcing. The addition of control is shown to be effective in decreasing the size of the separation bubble and pressure drag. LES baseline and controlled results are validated against previously performed experiments by Seifert and Pack and those performed for the NASA Langley Workshop on Turbulent Flow Separation and Control. Three test cases are explored to determine the effect of explicit filtering and the Smagorinsky subgrid scale model on the average flow and turbulent statistics. The flow physics and the control effectiveness are investigated at two Mach numbers, M=0.25 and M=0.6. Compressibility is shown to increase the separation bubble length in the baseline case, but does not significantly change the effectiveness of the control. In terms of decreasing drag on the wall-mounted hump model, steady suction is more effective than oscillatory control, but both control techniques are effective in reducing the separation bubble length. Two-dimensional direct numerical simulations (DNS) of the wall-mounted hump flow are also presented, and the results show different baseline flow features than the 3D LES. However the controlled 2D flow gives an indication of the most receptive actuation frequencies around twice that of the natural shedding frequency. Two regimes of reduced actuation frequency are also explored with the 3D LES. It is found that the low frequency actuation is successful in reducing the separation bubble length, but high frequency actuation produces an average flow comparable to the baseline case, and does not result in drag or separation bubble length reduction.