Application of a Coupled Reynolds Stress Model to a Swirl-Driven Diffuser Flow

Abstract

Swirling flow is a dominant feature in a significant number of technical applications. Hydraulic turbines at part-load are strongly affected by the related vortex rope phenomenon. Its dynamic behavior has a negative impact on the operating performance and durability of the machine.CFD can be used to get additional insight in this complex phenomenon but requires a valid simulation model able to capture the relevant flow physics, which is driven by highly anisotropic turbulent structures. The simulation results are therefore strongly affected by the turbulence modeling. A swirl apparatus (AC6-14), for which extensive experimental data is available, is used in this work for the assessment and validation of different turbulence models. The state-of-the-art SST k-ω model, with and without curvature correction, is compared to a coupled full Reynolds stress model. All models are integrated into a pressure-based coupled flow solver.The investigation revealed that both, SST k-ω with curvature correction and the full Reynolds stress model better predict the time-averaged velocity profiles in the diffuser compared to standard SST k-ω. The swirl component is thereby best captured with the Reynolds stress model. All models deliver a reasonable frequency spectrum for the dynamic behavior of the vortex rope. However, flow visualization shows that standard SST k-ω is not capable of predicting the shape and size of the vortex rope accordingly. Both, SST k-ω with curvature correction and the full Reynolds stress model, can be used in the future for more detailed flow investigations, which include also the assessment of flow control measures.