ANISOTROPIC EDDY VISCOSITY IN THE SECONDARY FLOW OF A LOW-SPEED LINEAR TURBINE CASCADE

The final losses within a turbulent flow are realized when eddies completely dissipate to internal energy through viscous interactions. The accurate prediction of the turbulence dissipation, and therefore the losses, requires turbulence models which represent, as accurately as possible, the true flow physics. Eddy viscosity turbulence models, commonly used for design level computations, are based on the Boussinesq approximation and inherently assume the eddy viscosity field is isotropic. The current paper compares the computational predictions of the flow downstream of a low-speed linear turbine cascade to the experimentally measured results. Steady-state computational simulations were performed using ANSYS CFX v12.0 and employed the shear stress transport (SST) turbulence model with the gamma-Re-theta transition model. The experimental data includes measurements of the mean and turbulent flow quantities. Steady pressure measurements were collected using a seven-hole pressure probe and the turbulent flow quantities were measured using a rotatable x-type hotwire probe. Data is presented for two axial locations: 120% and 140% of the axial chord (C-x) downstream of the leading edge. The computed loss distribution and total bladerow losses are compared to the experimental measurements. Differences are noted and a discussion of the flow structures provides insights into the origin of the differences. Contours of the shear eddy viscosity are presented for each axial plane. The secondary flow appears highly anisotropic, demonstrating a fundamental difference between the computed and measured results. This raises questions as to the validity of using two-equation turbulence models, which are based on the Boussinesq approximation, for secondary flow predictions.