Assessment of unsteady RANS in predicting swirl flow instability based on LES and experiments

Swirl flows play an important role in many engineering applications such as modern gas turbines, aero propulsion systems etc. While the enhanced mixing and stabilisation of the flame caused by the swirl are desirable features, such flows often exhibit hydrodynamic instabilities called precessing vortex core. For design purposes it is very important to predict such instabilities. Computational fluid dynamics (CFD) using Reynolds-averaged Navier–Stokes (RANS) type turbulence models are state of the art for the prediction of flow properties in engineering practice. The objective of this paper is therefore to evaluate the performance of the unsteady RANS (U-RANS) method in predicting the precessing vortex core phenomenon. To this end, an unconfined swirling flow with precessing vortex core at swirl number 0.75 and Reynolds number ranging from 10 000 to 42 000, investigated by means of both experiments and large eddy simulation, is utilised. The results show that U-RANS is able to capture the precessing vortex core both qualitatively and in parts quantitatively.