3D Measurements of the Mean Velocity and Turbulence Structure Within the Near Wake of a Rotor Blade

Stereoscopic PIV measurements examine the flow structure and turbulence within a rotor near wake located in a non-uniform field generated by a row of Inlet Guide Vanes (IGVs). The experiments are performed in a refractive index matched facility that provides unobstructed view of the entire flow field. The data are acquired at 10 closely spaced radial planes located near mid-span, which enable measurements of all the components of the mean strain rate and Reynolds stress tensors. Chopping and variations of advection speed of the upstream IGV wakes, as they pass along the rotor blade, create a non-uniform flow that shears the rotor wake. However, the phase averaged flow at mid span remains almost two-dimensional. Due to the overwhelming effects of the non-uniform strain field, the presently observed trends of the Reynolds stresses within the sheared wake differ from those measured in previous studies of curved wakes. The axial velocity fluctuations increase along the suction/outer side of the wake, while the other components decay. On the pressure/inner part of the wake the circumferential velocity fluctuations are higher. The shear stress has a complex distribution, but is also higher on the suction side. To explain these trends the stresses and production rates are examined in coordinate systems aligned with the principal strain directions. As expected, the production is high along the compressive directions and low, even negative, in the extensive directions. Accordingly, the compressed normal stress component increases along the wake, while the extended component decays. The in-plane shear stress component and its associated production remain very high in the principal coordinate system of the strain. Projecting the stresses back to the laboratory coordinate system explains the observed inhomogeneous anisotropic distribution of Reynolds stresses within the kinked wake.