Characterization and Control of Unsteady Separation on Swept Wings

Large transient excursions in angle of attack promote unsteady separation and dynamic stall over a wing, allowing it to briefly exceed static-stall conditions but also inducing potentially undesirable variations in aerodynamic loading or structural response. This phenomenon is prevalent in a number of engineering applications, including helicopter rotors, maneuvering aircraft, wind turbines, and wing–gust/wing-wake encounters. Thus, its prediction and control are of great interest.  Our group at AFRL has performed a large computational campaign over recent years coving a broad range of flow conditions, planforms, and kinematics to uncover the viscous mechanisms preceding stall onset.  In these studies, we have documented the dominant role of a small-scale laminar separation bubble (LSB) on the initiation of dynamic stall, which guided the development of a novel low amplitude, high-frequency control strategy to exploit the LSB dynamics for transient stall suppression.  Although originally designed for nominally 2D wing sections, the control technique was successfully extended to finite wings because by exploiting the mostly spanwise-uniform LSB prior to stall. Swept wings exhibit a similar LSB stall precursor state, yet their eventual stall behavior (transient tip stall) is drastically different than the dynamic center-wing stall of its straight wing counterpart. Despite this, the targeted, high-frequency control is also remarkably successful at also suppressing tip stall, demonstrating the dominant and somewhat universal nature of the LSB across relevant configurations and its potential for manipulation. The presentation will provide an overview of the swept-wing separation control, while also highlighting a promising passive control strategy through micro-cavity actuation.