Separation Control in a Plane Asymmetric Diffuser by means of Streamwise Vortices - Experiment, Modelling and Simulation

Authors: Törnblom, O., Herbst, A. H., Johansson, A.V.J.
Document Type: Conference
Pubstate: Published
Journal: The 5th Symposium on Smart Control of Turbulence
Year: 2004


This investigation is a continuation of our previous work on the plane asymmetric diffuser (see e.g. Gullman-Strand et al. (2004) and Törnblom et al. (2003)) which aimed at studying the uncontrolled flow in the plane asymmetric diffuser, in order to produce a reliable experimental database and test turbulence models. The previous work, on the uncontrolled case, showed that a fluctuating separated region was formed on the inclined wall with a mean separation point at x/H = 9 and a mean reattachment point at x/H = 31. The aim now is to investigate ways to reduce the size and/or the motion of the separated region. It is well known that vortex generators are efficient when it comes to delay separation and preliminary tests of vortex generators (VGs) in our experiment have been very successful. By placing a spanwise row of VGs slightly above (at x/H = 7) the mean separation point we managed to increase the efficiency (as compared to an ideal, frictionless case) from 0.76 to 0.84. Vortex generators introduce streamwise vortices which bring fluid with a high level of streamwise momentum down close to the wall. When the streamwise vortices are introduced the spanwise coherence of the flow is broken which is a key element in our control strategy. The DNS results presented in the present paper (see also Herbst & Henningson (2003)) show that control with a spanwise periodic input is more effective in the control of flow separation than a spanwise homogeneous control input. In the DNS the separation is induced on a flat plate by a pressure distribution. In the present paper we describe the first steps of the experiments with vortex generators and a first attempt to compute the controlled flow by means of a simplified modelling approach. We also illustrate the control strategy by means of some new DNS results of control of a separation bubble on a flat plate.