Experimental Study of the Structure of a Rotor Wake in a Complex Turbomachinery Flow

Abstract

Unobstructed PIV measurements within complex turbomachinery flow fields are performed in an optical-refractive-index-matched facility consisting of a 2-stage axial turbomachine. Two different test setups are utilized to demonstrate wake-wake, wake-blade interactions and the associated flow non-uniformities and turbulence. The flow consists of a lattice of interacting wake segments, which are being chopped by the rotor and stator blades. The wake fragments become discontinuous due to the velocity differences across the rotor blades. Striking flow phenomena that occur as a result of this non-uniform flow field, such as turbulent “hot spots” and kinking of the rotor wake are presented at high magnification and samples that are large enough to obtain converged statistics. In this paper we focus on the flow field and turbulence within the rotor wake. One thousand instantaneous realizations at the same phase are used for determining the phase averaged flow and turbulence statistics including Reynolds stresses, turbulence spectra, production, dissipation, and mean strains. Three methodologies are adopted to investigate the details of the rotor wake structure: 1. Local maximization of 2-D shear strain and Reynolds shear stress; 2. 1-D energy spectral analysis; and 3. Subgrid-Scale (SGS) energy budget. Alignment of the local coordinates with direction that maximizes the local shear strain shows that except for the hot spot regions the rotor wake consists of two parallel layers exposed to planar shear strain. The normal strains in this system are significantly lower, indicating that out-of-plane normal straining is much weaker than the in-plane shear (except near the hot spot and close to the trailing edge of the rotor). Significant differences exist in several regions between the orientation of a coordinate system that maximizes the shear strain, and the system that maximizes the Reynolds shear stress, particularly around the hot spot, near the trailing edge, and within the stator wake segments on both sides of the rotor wake. 1-D spectral analysis reveals that the turbulence near the trailing edge is anisotropic and highly dissipative. The dissipation decreases and turbulence becomes more isotropic further away from the trailing edge, but becomes anisotropic again near the hot spot. Spatial filtering of data and measurement of the resulting SGS stresses enable us to examine and compare energy fluxes from the mean flow to the resolved and subgrid scales, as well as from the resolved to the subgrid scales. Due to the limitation in resolution, the present filter scale is 50% of the integral scale (∼wake width). Consequently, the energy flux from the mean flow to the subgrid scales is much higher than flux from the resolved turbulence to the subgrid scales. The production term, representing the energy flux from the mean flow to the resolved scales is typically higher than the flux from the resolved to the subgrid scales. Thus, build-up of large-scale energy occurs in substantial part of the near wake. The dissipation rates estimated from the spectra are everywhere (including the hot spot) of the same order as the overall SGS dissipation rate.