Computation of drag crisis of a circular cylinder using Hybrid RANS-LES and URANS models

Abstract

Numerical simulation of flow past a circular cylinder across the “drag-crisis” region is extremely challenging for turbulence models because the boundary layer undergoes laminar–turbulent transition and variable-locus separation. We investigate the SA-DDES hybrid model along with two variants, namely, SA-kLES and SA-ILES, based on Spalart–Allmaras (SA) model, and include for comparison the SA-BCM transition and the SA-URANS models, for Re ranging from 50,000 to 5 million, using an in-house unstructured grid solver. All hybrid RANS-LES models produced clearly turbulent-like behavior, as evident from the Q-criterion, while the URANS models did not. A decline in the drag coefficient is noticed in all the turbulence models, but not the sharp decrease observed experimentally, with one exception: the SA-BCM transition model, which predicted the drag coefficients much closer to the experiments. The hybrid RANS-LES models outperformed the URANS SA-BCM model only in the fully turbulent trans-critical region and better represent the physics in the wake region for all Reynolds numbers studied. All the hybrid RANS-LES models produced similar results, suggesting comparatively equal performance in predicting separated flows. We believe that the performance of a hybrid model for mid-range Reynolds numbers will be greatly enhanced if the model is equipped to handle the laminar–turbulent transition.