Modeling capabilities of unsteady RANS for the simulation of turbulent swirling flow in an annular bluff-body combustor geometry

Abstract

In this paper, we investigate the capabilities of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations for capturing the helical structures found in a three-dimensional, incompressible and isothermal annular swirling jet undergoing vortex breakdown. The flow topology is representative of a bluff-body combustor operating at a Reynolds number Re = 8500 and swirl number S = 0.39. As a turbulence model, the SSG Reynolds stress model (RSM) was chosen. The numerical simulation is validated by means of tomographic particle image velocimetry measurements of the same flow configuration. To detect the coherent structures in the flow field, the recently introduced spectral proper orthogonal decomposition technique is adopted, which gives both temporal and spatial information on the large scale coherent structures. The spectral analysis of the sampled velocity signals identified a precessing vortex core with a frequency of 24.3 Hz. In particular, two different large-scale helical flow structures are identified: a single and a double helix. Both of them are wound in the counter-swirl direction and wrapped around the central breakdown bubble. Those findings are quite similar to recent experimental tomographic particle image velocimetry results (Vanierschot et al., Phys. Rev. Fluids (2018) & J. Fluid Mech. (2020)). This study demonstrates that the unsteady RANS approach with RSM is able to predict the coherent structures found in an annular swirling jet undergoing vortex breakdown both temporally and spatially with reasonable accuracy and this approach can hence be used in the design of bluff-body combustors, where a multi-dimensional parameter space may require many simulations to find the optimal design.