PIV measurements of flow structure and turbulence in a rotor wake within a multi-stage turbomachine

Complex multi-stage turbomachines cause optical obstructions to PIV measurements. This problem is overcome by constructing a refractive-index-matched facility, composed of acrylic blade rows and a sodium iodide water solution as the working fluid. Flows generated by two blade geometries are investigated. Results obtained using the first geometry provide an overview of the flow complexity in a multi-stage turbomachine. For example, as an upstream wake is chopped by a rotor blade, the segments are advected at different velocity on the suction and pressure sides of the blade. The resulting phase shift persists downstream of the blade and shears the rotor wake, causing striking flow phenomena such as wake kinking and generation of turbulent "hot spots". Using the second geometry, where a rotor is operating behind a row of inlet guide vanes, the flow non-uniformities and turbulence of rotor near wake are studied in detail. Planar transformation of local coordinates that maximizes the local mean shear strain shows that except for the hot spot region the rotor wake consists of two parallel layers experiencing planar shear strain. One-dimensional spectral analysis and production-dissipation budget reveal that the turbulence near the trailing edge is anisotropic and highly dissipative. The turbulence becomes more isotropic with decreasing dissipation away from the trailing edge until reaching the hot spot region, where it becomes anisotropic again.; The rotor near-wake data are also used for examining elements of the ensemble averaged and spatially filtered kinetic energy. These two averaging processes decompose the kinetic energy into four parts. Their evolution equations contain five energy flux terms among them. The results elucidate the fundamental difference between the filtered Reynolds production and the ensemble averaged subgrid scale (SGS) dissipation rates. They also highlight the key role played by the subgrid part of the mean-flow kinetic energy in highly non-uniform flow regions. A new method for determining the scaling parameters of rotor wake is introduced. The kinetic energy parts of mean flow scale with the modified velocity defect squared, whereas the parts of turbulence scale with other parameters with slower-decay rates.