Validation of a Numerical Model for Predicting Stalled Flows in a Low-Speed Fan—Part I: Modification of Spalart–Allmaras Turbulence Model

Abstract

The original Spalart–Allmaras (SA) model is known to predict premature stall when applied to fan or compressor, which is in line with the observation of other researchers who use the SA model. Therefore, to improve the prediction of the stall boundary, the original SA model was modified by scaling the source term based on the local pressure gradient and the velocity helicity of the flow. Furthermore, a generalized wall function valid for nonzero wall pressure gradient was implemented to improve the accuracy of boundary conditions at the solid wall. This work aims to produce a turbulence model which can be used to model flows near the stall boundary for the transonic fan rotors on relatively coarse grids of around 600k points per passage. Initially, two fan rotors with different design and operating speeds were used to optimize the new parameters in the modified turbulence model. The optimization was based on improving the correlation between measured and numerical radial profiles of the pressure ratio. Thereafter, steady computations were performed for two other fans (by using the same parameters), and the predictions were compared with the experimental data for all the four fan rotors. Numerical results showed a significant improvement over those obtained with the original SA model, when compared against the measured data. Finally, for completeness, it was decided to test the performance of the modified model by comparing the result with measured data for a simple canonical case.