A Statistical Approach to the Identification of Vortical Structures During Dynamic Stall with Flow Control

Abstract

Understanding dynamic stall is of critical importance to controlling unsteady loading during the operation of rotary systems. While in the phase averaged sense, a picture can be created of how flow control affects the recirculation region on an airfoil undergoing dynamic pitching, any given instantaneous flow field captured with Particle Image Velocimetry exhibits two issues. The first is that instantaneous flow fields vary significantly from the phase averaged flow fields. The second is that vortex identification methods fail in adequately describing the vortical structures present in the instantaneous field, due to the high shear that exists between the free stream and the recirculation region. Instead, a method whereby the centers of the vortical structures are identified, and then the circulation about these structures is measured and is used to present how the evolution of dynamic stall occurs in a statistical sense. In this way, it can be shown how the dynamic stall process is altered when flow control is applied vis a vis the change in the extent of vortical structures shed during dynamic stall, with the eventual goal of replacing a full 3-D reconstruction of flow fields under dynamic conditions with a simpler, sparser data set.