Flow Structure on a Rotating Wing: Effect of Rossby Number

The flow structure on a rotating wing is determined via stereoscopic particle image velocimetry. Sectional and three-dimensional, volumetric reconstructions define the flow patterns as a function of Rossby number \textit{Ro}. An aspect ratio \textit{AR} $=$ 1 rectangular, flat plate is rotated at a geometric angle of attack $\alpha = $ 45$^{\circ}$. The flow structure is determined at various angles of rotation, in order to characterize both the initial development and the fully evolved state of the flow structure. The Rossby number \textit{Ro} $= r_{g}/C$ is varied via alteration of the radius of gyration $r_{g}$ of the wing, to give values from \textit{Ro} $=$ 1.2 to \textit{Ro} $=$ 5.1. Large changes of the flow structure are represented by images of of spanwise vorticity, Q-criterion; spanwise velocity; and downwash velocity. At the lowest Rossby number \textit{Ro} $=$ 1.2, a vortex is attached to the leading edge of the wing; it is present along most of the span. At higher Rossby numbers \textit{Ro} $=$ 2.1 and \textit{Ro} $=$ 5.1, this leading-edge vortex becomes less organized and deflects away from the surface of the wing. At a Rossby number \textit{Ro} $=$ 5.1 the structure of the flow in the vicinity of the leading edge resembles a separated shear layer. The nature of other elements of the three-dimensional flow, such as the root and tip vortices and the downwash velocity, are closely related to the degree of coherence of the leading-edge vortex.