Tip-Vortex Dynamics of a Pitching Rotor Blade-Tip Model

Abstract

The tip vortex of a finite rotor blade tip model undergoing pitch oscillations is investigated by means of a wind tunnel study. The motion and flow parameters as well as the model geometry were adapted to rotorcraft applications, including test cases with fully attached flow, light dynamic stall, and deep dynamic stall. High–speed Particle Image Velocimetry was used to study the spatio–temporal behavior of the tip wake at different streamwise positions. Combined to surface pressure measurements, the data establishes a connection between the sectional lift of the model and the vortex parameters (for example, swirl velocity and circulation), including dynamic phenomena like hysteresis and vortex breakdown during stalled flow conditions. For pitch oscillations below static stall limit, only small hysteresis effects were observed, and the vortex structure is similar to static reference measurements. Beyond static stall, the tip vortex retains its general structure in a time–averaged or phase–averaged frame, but the instantaneous flow fields reveal an increasing influence of turbulence. Dynamic stall cases exhibit a circulation overshoot beyond static levels during the upstroke and a disorganized and less compact vortex structure during the downstroke.