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The current work evaluates the effectiveness of two leading-edge dynamic stall criteria in mild to moderately compressible regimes using numerical simulations. The two criteria under consideration, namely, the maximum magnitudes of the leading edge suction parameter (max(𝐿𝐸 𝑆𝑃)) and boundary enstrophy flux (max(|𝐵𝐸𝐹|)), have previously been found to be effective at signaling dynamic stall in the incompressible regime. Based on unsteady Reynolds-averaged Navier-Stokes simulations at a Reynolds number of 2 × 105 and freestream Mach numbers between 0.1 - 0.5, we observe that these criteria are directly applicable in the mild to moderately compressible regimes, since they are reached shortly after suction collapse at the leading edge and well in advance of dynamic stall vortex formation for all the cases. This is attributed to compressibility effects promoting adverse-pressure-gradient(APG)-induced stall for the flow conditions considered. For the highest Mach number of 0.5, shock wave interactions with the separated shear layer are observed. It is noted that although compressibility leads to separation at a lower APG, the maximum APG scaled by the local flow density remains in the same range for all the cases. 

We evaluate two leading-edge-based dynamic stall-onset criteria (namely, the maximum magnitudes of the leading-edge suction parameter and the boundary enstrophy flux) for mixed and trailing-edge stall. These criteria have been shown to successfully predict the onset of leading-edge stall at Reynolds numbers of O(10^5), where the leading-edge suction drops abruptly. However, for mixed/trailing-edge stall, leading-edge suction tends to persist even when there is significant trailing-edge reversed flow and stall is underway, necessitating further investigation into the effectiveness of these criteria. Using wall-resolved large-eddy simulations and the unsteady Reynolds-averaged Navier–Stokes method, we simulate one leading-edge stall and three mixed/trailing-edge stall cases at Reynolds numbers of 200,000 and 300,000. We contrast the progression of flow features such as trailing-edge separation and vortex formation across different stall types and evaluate the stall-onset criteria relative to critical points in the flow. We find that the criteria nearly coincide with the instance of leading-edge suction collapse and are reached in advance of dynamic stall vortex formation and lift stall for all four cases. We conclude that the two criteria effectively signal dynamic stall onset in cases where the dynamic stall vortex plays a prominent role. 

In this paper, we present an approach to obtain a desired leading-edge vortex (LEV) shedding pattern from unsteady airfoils through the execution of suitable motion kinematics. Previous research revealed that LEV shedding is associated with the leading-edge suction parameter (LESP) exceeding a maximum threshold. A low-order method called LESP-modulated discrete vortex method (LDVM) was also developed to predict the onset and termination of LEV shedding from an airfoil undergoing prescribed motion kinematics. In the current work, we present an inverse-aerodynamic formulation based on the LDVM to generate the appropriate motion kinematics to achieve a prescribed LESP variation, and thus, the desired LEV shedding characteristics from the airfoil. The algorithm identifies the kinematic state of the airfoil required to attain the target LESP value through an iterative procedure performed inside the LDVM simulation at each time step. Several case studies are presented to demonstrate design scenarios such as tailoring the duration and intensity of LEV shedding, inducing LEV shedding from the chosen surface of the airfoil, promoting or suppressing LEV shedding during an unsteady motion on demand, and achieving similar LEV shedding patterns using different maneuvers. The kinematic profiles generated by the low-order formulation are also simulated using a high-fidelity unsteady Reynolds-averaged Navier–Stokes method to confirm the accuracy of the low-order model. 

We evaluate two leading-edge-based dynamic stall onset criteria, namely, the maximum magnitudes of the Leading Edge Suction Parameter and the Boundary Enstrophy Flux, for mixed and trailing-edge stall. These criteria have been shown to successfully predict the onset of leading-edge stall at Reynolds numbers >= O(10^5), where the leading-edge suction drops abruptly. However, for mixed/trailing-edge stall, leading-edge suction tends to persist even when there is significant trailing-edge reversed flow and stall is underway, necessitating further investigation of the effectiveness of these criteria. Using wall-resolved, large-eddy simulations and unsteady Reynolds-Averaged Navier-Stokes method, we simulate one leading-edge stall and three mixed/trailing-edge stall cases at Reynolds numbers 2x10^5 and 3x10^6. We contrast the progression of flow features such as trailing-edge separation and vortex formation across different stall types and evaluate the stall onset criteria relative to critical points in the flow. We find that the criteria nearly coincide with the instance of leading-edge suction collapse and are reached in advance of dynamic stall vortex formation and lift stall for all four cases. We conclude that the two criteria effectively signal dynamic stall onset in cases where the dynamic stall vortex plays a prominent role.