INVESTIGATION OF FLOW SEPARATION IN A 3-DIMENSIONAL DIFFUSER USING A COUPLED EULER AND BOUNDARY-LAYER METHOD

In the small scale model of the ETW (European Transonic Wind Tunnel) diffuser, flow separation has been detected in experiments on the empty tunnel. The causes of the separation are investigated numerically. In the numerical study, the inviscid flow is calculated by a three-dimensional finite volume method for the solution of the Euler equations. The viscous flow on the diffuser wall is evaluated by a boundary-layer method for three-dimensional, compressible turbulent flow. Both methods are used iteratively and the boundary-layer information is transferred as a boundary condition to the inviscid method via the outflow concept. The geometry of the diffuser is such that the cross-sections vary from a rectangular shape in the test section to a circular shape in the diffuser exit plane. It is shown that this perculiarity of the diffuser geometry is responsible for the separation of the flow.