Numerical and physical aspects in LES and hybrid LES/RANS of turbulent flow separation in a 3-D diffuser

An incompressible fully-developed duct flow expanding into a diffuser, whose upper wall and one side wall are appropriately deflected (with the expansion angles of 11.3° and 2.56° respectively), and for which reference experimental and DNS databases were provided by Cherry et al. (2008, 2009) and Ohlsson et al. (2009, 2010), was studied computationally by using a zonal hybrid LES/RANS (HLR) method, proposed recently by Kniesner (2008) and Jakirlić et al. (2009). In addition a complementary Large-Eddy Simulation (LES) method has been applied. The flow Reynolds number based on the height of the inlet channel is Reh = 10,000. The primary objective of the present investigation was the comparative assessment of the computational models in this flow configuration characterized by a complex 3-D flow separation being the consequence of an adverse-pressure gradient evoked by the duct expansion. The focus of the investigation was on the capability of different modelling approaches to accurately capture the size and shape of the 3-D flow separation pattern and associated mean flow and turbulence features.