Subgrid-scale modelling in relaminarizing flows

The reversion of a turbulent flow to a laminar or quasi-laminar state may take very different paths, depending on the cause of the relaminarization; modelling these flows is quite difficult. Large-eddy simulations (LES) may be quite effective as the large, momentum-carrying eddies are accurately resolved; the unresolved, subgrid-scale (SGS) stresses, however, may still play an important role. We report here the results of an a priori analysis of the behaviour of the SGS stresses in two relaminarizing flows: a plane channel subjected to spanwise rotation, and a boundary layer subjected to a strong favourable pressure gradient resulting in freestream acceleration. The a priori test results are accompanied by actual LES, which highlight the effect of grid resolution and modelling errors on the dynamics of the relaminarization. Dynamic SGS models predict the overall features of the relaminarization correctly, although some of the details are not captured. At the levels of grid refinement used, however, the SGS contribution to momentum transport is very small, and little difference is observed between the results obtained with the various models.