Boundary Layer Measurements on a Rotating Disk

Dynamic stall occurs on helicopter rotor blades, wind turbine blades, and even insect and bird wings. Although most studies on dynamic stall are conducted assuming two-dimensional behavior, it has been shown that this phenomenon is highly three-dimensional. Recent studies of dynamic stall on a rotating blade of helicopter in forward flight and wind turbines in yaw have shown that the nature of the radial flow near the surface has first-order significance in the stalled flow field. Past literature suggests that the boundary layer over a rotating disk has been correlated to that over a swept wing. Drawing a similar parallel it is hypothesized that fundamental insights into dynamic stall may be derived from studying the boundary layer over a rotating blade in dynamic stall is similar to that over a rotating disk. Particle image velocimetry (PIV) is used to investigate the boundary layer development on a rotating disk. The first set of measurements was conducted using PIV with a micro lens attachment to the camera. Next, a microscope was used to conduct μ-PIV measurements. The measured radial velocity profiles show substantially higher radial jet peak velocities than the analytical solution for a mirror-smooth disk. This difference is narrowed down to the effect of surface roughness of the painted disk, representative of reality on rotor blades used in PIV. This results in a much higher effective viscosity in the near-surface layer, contributing to additional radial flow, as seen from the centrifugal pump literature. However, the non dimensional radial velocity profile exhibits the expected self similar behavior at various radial locations.