A Novel Experiment to Understand the Dynamic Stall Phenomenon in Rotor Axial Flight

Abstract

A rotor test facility was developed to investigate dynamic stall under optimized boundary conditions compared to conventional hover chambers. lt features a defined axial inflow, reduction of ground effect and recirculation of the rotor wake and good optical access to apply non-intrusive measuring techniques. The rotor consists of two blades with an aspect ratio of 6.8 and a tip radius of 0.65m. lt was operated at a Mach number of 0.21 and a chord based Reynolds number of 350000, both at 75% radius. The flow and blade deformation were analyzed by means of unsteady blade pressure transducers, particle image velocimetry and tip deflection measurements covering the whole azimuth and different radii. Three measurement approaches to detect flow separation: tufts, differential infrared thermography and surface pressure analysis showed comparable results and flow separation over one half of the cycle. A radiusdependent separation point in time and an altering of the dynamic stall behavior near the tip were observed. The recirculation area remained close to the blade's surface with radial jet flow increasing towards the tip. A maximum increase of the effective angle of attack of 1.4° was observed due to blade deformation. For a fully attached flow case the laminar-turbulent boundary layer transition differed along the radius and moved over 70% of the blade's chord.