Flow in a 3D diffuser

Confined flows

Underlying Flow Regime 4-16

Abstract

The incompressible flow developing fully in a three-dimensional duct and then expanding into a diffuser, whose upper wall and one side wall are appropriately deflected, has been investigated experimentally (Cherry et al., 2008, 2009) and computationally by means of DNS (Direct Numerical Simulation; Ohlsson et al., 2009, 2010), LES (Large-Eddy Simulation) as well as by different hybrid LES/RANS and RANS (Reynolds-Averaged Navier-Stokes) models. The results of the computational studies were analysed along with the experimental reference database in the framework of two workshops on “Refined Turbulence Modelling” (Steiner et al., 2009 and Jakirlić et al., 2010b) organized by the ERCOFTAC Special Interest Group on Turbulence Modelling (SIG15). The flow was also a test case in the European ATAAC project. Two three-dimensional diffuser configurations differing in terms of the values of the expansion angles — the upper-wall expansion angle is reduced from 11.3° (diffuser 1) to 9° (diffuser 2); the side-wall expansion angle is increased from 2.56° (diffuser 1) to 4° (diffuser 2) — were considered. These slight modifications in the diffuser geometry led to substantial changes in the flow structure with respect to the onset, location, shape and size of the three-dimensional separation pattern associated with the corner separation and corner reattachment, Fig. 1. The inflow in both considered cases is characterized by a Reynolds number Re_h=10000, based on the inlet duct height.

The primary objective of the present contribution is twofold:

to provide further insight into the physics of the separation of the three-dimensional boundary layer generated at the intersection of two deflected walls. This separation is the consequence of an adverse pressure gradient evoked by the duct expansion under different conditions and
a comparative assessment of different modelling approaches in terms of their capability to accurately capture the size and shape of the three-dimensional flow separation pattern and associated mean flow and turbulence features.

Figure 1: Instantaneous velocity field in both diffuser configurations, diffuser 1 (upper) and diffuser 2 (lower), obtained by a zonal hybrid LES/RANS model, illustrating different flow separation patterns (from Jakirlić et al., 2010a). Whereas the separation zone spreads over the upper wall in the diffuser 1, it occupies the deflected side wall in the diffuser 2