The Importance of Strain-Dominant Regions in Vortical Flows: Application to Dynamic Stall

The pressure loads induced by vortices on immersed surfaces are central to numerous problems in fluid dynamics. While past studies in unsteady aerodynamics have focused mostly on the role of vortices in force production, we show using a force partitioning method (FPM; Menon & Mittal, JFM 918, R3, 2021) that strain-dominated regions associated with vortices can in fact have a significant effect on aerodynamic loads in some situations. FPM allows us to quantify the loads induced on immersed surfaces by individual vortices as well as their associated regions of strain. By analyzing the forces on a pitching airfoil undergoing dynamic stall, we show that our current understanding of vortex-dominated phenomena could be incomplete without considering the substantial, and sometimes dominant, effect of strain-dominated regions that are associated with vortices.