3D Structure of a Rotor Wake at Mid-span and Tip Regions

Abstract

*† ‡ § This paper presents the results of Stereoscopic PIV measurements of a rotor near wake at the tip and mid-span regions. The rotor blade row operates behind a row of Inlet Guide Vanes (IGVs). The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and IGV and a fluid that has the same optical index of refraction as the blades. The data is acquired at a specific rotor phase and at mid-span and tip regions on very closely spaced 10 radial planes as well as on additional planes that are distributed at larger intervals from mid-span to the tip. In the tip region, the tip vortices dominate the flow field, and their interactions with the rotor blades and wakes generate a complex three-dimensional flow. The rotor wake is confined between two tip vortices; one that rolls up from the same blade, and the other that rolls up from the previous blade in the same rotor row. The velocity field induced by these vortices and the low axial momentum in the entrained casing boundary layer, create highly turbulent regions with low axial, high lateral and high radially inward momentum on the pressure side of rotor blade. The effects of the tip vortices are diminished below 75% span, where the dominant flow features are the rotor wakes and the upstream IGV wakes. Within the rotor wakes, the radial velocity is more than an order of magnitude lower than the axial and the lateral velocity components. Within the chopped IGV wake segments, the radial velocity is slightly elevated, but it is still 5 times and 3 times lower than the axial and lateral velocity components, respectively.