Separated flow in a three-dimensional diffuser (“Stanford Diffuser”)
The flow in a three-dimensional asymmetric diffuser (see schematic in Figure 1) has been studied in the experiments performed at Stanford University (Cherry et al, 2006, 2007). It is a model for investigation of flow separation problems commonly encountered in many technical devices, in particular, in gas turbine engines, where the annular diffuser past the air compressor operates very near separation for some part of the engine's operating envelope. Another area where corner separation is important is near wing-body or pylon-nacelle junctions, where separation from the corner can seriously affect airplane drag. Therefore, the simplified 3D diffuser configuration can be considered as a representative stepping stone to industrially relevant flows.
Figure 1: Schematic of flow in 3D asymmetric diffuser, reproduced from Cherry’s et al. presentation;
Figure 2: Secondary flow of second kind on square duct.
The main shortcoming of currently used RANS Eddy-Viscosity turbulence models is their inability to predict the secondary flow due to anisotropic normal stresses (Prandtl’s secondary flow of second kind – see Figure 2). This secondary motion generates vortices in square ducts which drive momentum into the corner. It is assumed that the increase in near wall momentum in the corner allows the flow to overcome stronger pressure gradients than without such secondary features. Since secondary flows of this kind cannot be accounted for in linear eddy-viscosity models (LEVM), it is anticipated that properly calibrated Reynolds-Stress Models (RSM) or Explicit Algebraic RSM (EARSM) will perform consistently better than LEVMs.
The main objective of the discussed experimental study was to perform detailed volumetric measurements of the flow field in a truly 3-D diffuser with simple and well-specified boundary conditions, thus providing a good test for objective validation of numerical predictions. The experimental setup was designed to provide a challenging test case for numerical models: it involves a well-defined 3D recirculation region, and a considerable amount of data was collected at realistic Reynolds numbers. In addition, two diffuser geometries were considered and the effect of a small change in the expansion ratio was used to evaluate the ability of the numerical methods to predict trends and sensitivity to the geometry. In the experiments, both diffuser flows exhibited three-dimensional boundary layer separation but the size and shape of the separation bubble exhibited a high degree of geometric sensitivity dependent on the dimensions of the diffuser.
- Cherry, E. M., Iaccarino, G., Elkins C. J., and Eaton, J. K. (2006): "Separated flow in a three-dimensional diffuser: preliminary validation", Center for Turbulence Research, Stanford University, Annual Research Brief 2006, pp. 31-40. This reference is available online at http://www.stanford.edu/group/ctr/ResBriefs/ARB06.html
- Cherry, E.M., Elkins, C.J. and Eaton, J.K. (2007): "Geometric Sensitivity of 3-D Separated Flows", Proc. of 5th International Symposium on Turbulence and Shear Flow Phenomena - TSFP5, Munich, August 27-29.