UC Davis

Dynamic stall is a prevalent phenomenon affecting the aerodynamic performance of a wide array of fluid-dynamic machinery, including maneuvering aircraft, fighter jets, helicopters, and wind turbines. The dynamic stall process induces sudden and undesirable variations in aerodynamic forces and pitching moments, critically affecting the controllability and structural integrity. The characterization of dynamic stall is still an arduous and complicated challenge.

This research investigates the effect of airfoil design parameters, effect of airfoil trailing-edge morphing, and the effect of particle induced surface roughness on dynamic stall through the use of high-fidelity computational fluid dynamics (CFD). OVERFLOW Reynolds-averaged Navier-Stokes (RANS) CFD and delayed detached eddy simulations (DDES) CFD are used to simulate the pitching and ramp-up motion of the airfoil undergoing dynamic stall. An airfoil morphing method investigation involving six parametric airfoil design parameters, including the camber, thickness, thickness crest position, leading-edge radius, trailing-edge camber, and boat-tail angle, is undertaken. The CFD numerical simulations are validated against experimental data for analyzing the aerodynamic loading, which includes the coefficient of lift, drag, and pitching moment. Then, the peaks of these coefficients are compared to identify the trends. Morris' sensitivity analysis is also used to analyze the effects of the different design parameters and further rank and assess their relative influences on the dynamic stall characteristics. Overall, it is found that the thickness crest position has a sizable influence on all the dynamic stall characteristics. Additionally, it is found that the design parameter that modifies the trailing-edge (boat-tail angle) is able to to markedly diminish the pitching moment, while concurrently sustaining a comparable lift coefficient. The complex features of the dynamic stall development stages and flow physics are further analyzed. The study reveals that morphed trailing-edge airfoil with the largest positive selected boat-tail angle design parameter causes the laminar separation bubble (LSB) to burst at an earlier stage, resulting in the dynamic stall vortex (DSV) occurring at an earlier stage. Mainly, the results indicate that morphed trailing-edge airfoils exhibit stronger secondary shear layer separation in the middle and aft airfoil sections at higher angles of attack (AoAs), compared to the baseline airfoil. Finally, the effects of particle impact induced surface roughness are examined. Lawrence Livermore National Laboratory's (LLNL) code called ParticleTSim is used to obtain the particle strike maps to record the range of particle strikes during the dynamic stall motion. A comparison study between different roughness height values and impacted surface areas is performed. The results illustrate that, as the surface roughness height and impacted surface roughness area increase, the dynamic stall progression stages develop much quicker, the stall behavior occurs at an earlier AoA, and the accumulation of kinetic energy above the airfoil diminishes. This research dissertation ventures into the comprehension and mitigation methods for the adverse ramifications of dynamic stall.