Exploration of High-Frequency Control of Dynamic Stall Using Large-Eddy Simulations

Dynamic stall represents a challenge in a number of engineering applications including rotorcraft, maneuvering aircraft, gust encounters, and wind turbines. Delay of the onset of dynamic stall and mitigation of its undesirable transient loading effects have been sought in the past five decades employing a number of passive and active flow control techniques. Development of improved flow control approaches requires the detailed characterization of the underlying boundary-layer processes preceding the onset of abrupt unsteady separation. Such information is difficult to acquire experimentally, given the spatial and temporal resolution requirements and the difficulties encountered with near-wall measurements over a moving wing. In this regard, modern computational approaches offer a complementary research tool for advancing active flow control. In the present paper, a flow control strategy for the delay of dynamic stall on a pitching airfoil is developed and demonstrated, employing high-fidelity wall-resolved large-eddy simulations and stability analysis. The point of departure is a detailed analysis of the uncontrolled dynamic stall case revealing the critical role played by the laminar separation bubble (LSB). This observation motivated the exploration of very high-frequency [St∼O(102)] actuation targeting the convective instabilities present in the LSB. Provided that the imposed forcing frequency or its harmonics are high enough for amplification within the LSB, very small forcing amplitude is required for control effectiveness. This approach, guided by linear stability theory and detailed flowfield information, is successfully demonstrated for the delay of dynamic stall over a pitching NACA 0012 wing section.