Scale-Adaptive Simulation of the Unsteady Turbulent Flow in a High-Speed Centrifugal Compressor with a Wedge-Type Vaned Diffuser

The present work investigates the details of unsteady flow behavior in a transonic centrifugal compressor with vaned diffuser. The analysis is based on solving three-dimensional, compressible, unsteady Navier-Stokes equations. The computational model is a high-compression ratio centrifugal compressor (4:1) consisting of an inlet duct, impeller, and diffuser vane. The hybrid scale-adaptive simulation (SAS) turbulent model is used to provide detailed flow information and characterize the transient flow structures within the compressor passages. A numerical sensitivity test is performed to validate the computational results in terms of pressure ratio and compressor efficiency. Instantaneous and mean flow field analyses are presented in the impeller and the vaned diffuser passages. Applying transient simulations, it is shown that the interaction between the pressure waves and the surface pressure of the diffuser blades leads to a pulsating behavior within the diffuser. Moreover, spectral analysis is evaluated to analyze the blade passing frequency (BPF) tonal noise as the main noise source of centrifugal compressors. In addition, the current SAS results are compared with those of the URANS-SST (shear stress transport) approach to show the ability of the SAS approach in the prediction of turbulent structures, where the SAS model leads to a much better resolution of the unsteady fluctuations. This study shows that the current SAS approach is promising in terms of the prediction of transient phenomena like LES, but offers a substantially reduced turn-around time.