Discrete Structures in the Radial Flow Over a Rotor Blade in Dynamic Stall

Abstract

This paper confirms the finding of discrete quasi-periodic streamwise vortical structures in the radial flow behind the separation line over a rotor blade undergoing dynamic stall in forward flight. It follows our finding of such structures over a blade undergoing transient stall induced by an inflow obstructer in a hover facility. Co-rotating structures suggest that the free shear layer formed at stall rolls up into discrete cells. The spanwise spacing of structures is approximately the height of the separated flow region. The presence of these structures helps explain the nature of the radial flow variation from root to tip in the separated flow region. This, as far as we know, is the first capture and measurement of the quasi-periodic radial velocity field on a rotating blade, and in particular of the profile of the strong radial velocity above a rotating blade in stall. Nomenclature ABS = Advancing blade side RBS = Retreating blade side P IV = Particle Image Velocimetry P LSI = Pulsed Laser Sheet Imaging u, v, w = Velocity in the x, y, z directions x, y, z = Cartesian coordinate system in the stream, normal and span directions U = Velocity Y AG = Ytrrium Alumnium Garnet crystal for a solid state laser µ = Advance ratio. The ratio of freestream speed to blade tip speed.

Experimental Investigation of Dynamic Stall Performance for the EDI-M109 and EDI-M112 Airfoils

“…17) suggest that the dynamic stall results are similar except near the blade tip, even though 3D stall (Ref. 18) exhibits the development of far more complex structures than two-dimensional (2D) stall (Ref. 19).…”

Effect of 3D stall-cells on the pressure distribution of a laminar NACA64-418 wing

“…When extended to airfoils and wings in dynamic regimes, Carta (1975) measured an additional non-uniform load distribution determined by these cellular flow structures, actively contributing to the presence of stall-delay. The presence of these stall-cell structures has been investigated as well in helicopter applications (Ottavio et al 2008). In these high aspect-ratio rotating-wings, the radial flow is the driver of the instability which brings to the development of cellular structures preventing the centrifugal increase of the radial-velocity distribution up to the tip.…”

Advance ratio effects on the flow structure and unsteadiness of the dynamic-stall vortex of a rotating blade in steady forward flight

“…The pitch angle of the blade as a function of azimuth was manually set to follow the function θ(ψ) = θ o + θ c sinψ, where θ o is the collective pitch angle and θ c is the cyclic pitch angle. DiOttavio et al 21 please refer to DiOttavio et al 21 for more details on the kinematics of the experimental setup. Before and after each experimental run (total number of experimental runs = 102), the pitch angle was measured using a digital protractor at intervals of ψ = 30 • to verify the prescribed pitch angle variation with azimuth.…”