Turbulence Within a Turbomachine Rotor Wake Subject to Nonuniform Contraction

Abstract

The flow and turbulence in a rotor near wake located within a nonuniform field, generated by a row of inlet guide vanes, are investigated experimentally in a refractive-index-matched facility that provides unobstructed view of the entire flowfield. Stereoscopic particle image velocimetry measurements, performed in closely spaced radial planes, enable measurements of all the components of the phase-averaged strain rate, Reynolds stress, and triple correlation tensors. The rotor wake is bent and compressed as a result of exposure to regions with high axial momentum (jets), which fill the gaps between inlet guide vanes wakes. As the rotor wake propagates away from the blade, this process of bending and compression by the jets leads to formation of distinct wake kinks containing regions of high turbulence (turbulent hot spots). We focus on an early stage of hot-spot formation. On the suction side of the wake, compression by a jet increases turbulence production, causing nonuniform and asymmetric distribution of Reynolds stresses. In a curvilinear coordinate system aligned with the wake centerline, all the Reynolds stresses are higher on the suction side of the wake where the decay rate of turbulence is much slower than that expected in wakes. The production rate of streamwise stress is dominated by interactions of the shear stress with the cross-stream gradients of the phase-averaged streamwise velocity. The latter also interact with the cross-stream stress to cause high production of shear stress, especially on the suction side. The locations of peak streamwise and shear stresses are consistent with those of the corresponding production rates. The similar locations of peak streamwise and cross-stream stresses, together with low production rate of the latter, suggest a significant contribution by intercomponent transfer from the streamwise to the cross-stream stress. The effects of streamwise curvature and rotation on the evolution of turbulent stresses are marginal. The paper also provides distributions of advection by phase-averaged flow and turbulent diffusion components. The diffusion terms oppose the peaks of production rates, but also have high values along the perimeter of the wake. We compare the distribution of diffusion of turbulent kinetic energy to a model based on the gradient-diffusion hypothesis. The model predicts the diffusion peak that opposes the production rate maximum quite well, but underpredicts the diffusion along the perimeter of the wake and overpredicts it near the wake center.