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LOCAL FORCING OF LAMINAR SEPARATION BUBBLES

E. Kaiser
Department Fluides, Thermique, Combustion, CEAT Institut PPRIME, CNRS-Universite de Poitiers-ENSMA, UPR 3346 43 rue de I'Aerodrome, F-86036 Poitiers CEDEX, France

A. Spohn
Department Fluides, Thermique, Combustion, CEAT Institut PPRIME, CNRS-Universite de Poitiers-ENSMA, UPR 3346 43 rue de I'Aerodrome, F-86036 Poitiers CEDEX, France

Laurent Cordier
Department Fluides, Thermique, Combustion, CEAT Institut PPRIME, CNRS-Universite de Poitiers-ENSMA, UPR 3346 43 rue de I'Aerodrome, F-86036 Poitiers CEDEX, France

Bernd R. Noack
Berlin Institute of Technology MB1 Strasse des 17. Juni 135, D-10623 Berlin, Germany; Departement Fluides, Thermique, Combustion Institut PPRIME, CNRS UPR 3346 CEAT, 43 rue de I'Aerodrome, F-86036 Poitiers, FRANCE

Résumé

Local forcing of laminar separation bubbles has been studied inside a low-speed water tunnel. Flow separation is induced by a canonical pressure distribution along a smooth ramp. Flow instabilities are excited by oscillating a thin horizontal wire which is placed inside the boundary layer upstream of separation. The diameter of the wire measures only 1/100 of the boundary layer thickness and its Reynolds number remains O(1). The spatial and temporal evolution of the separated shear layer is analysed with highresolution visualisations using the electrolytic precipitation technique. Time-resolved flow visualisations obtained with the hydrogen bubble technique and PIV measurements are used to analyse the reattachment zone of the laminar separation bubble.
Using all these techniques the laminar separation bubbles are found to be highly sensitive to local forcing upstream of flow separations. Periodic oscillations of the wire in the wall-normal direction produce significant changes in the spatial and temporal evolution of the separation bubble. In the upstream part of the separation bubble the beginning growth of the most amplified mode can be precipitated close to the separation line. At the same time the bubble length can be reduced up to 50% by periodic forcing with the natural shedding frequency. All these results confirm previous numerical studies of Rist & Augustin (2006) which predict the possibility to control laminar separation bubbles by local forcing upstream of flow separation. In the near future, we shall apply this highly efficient and flexible technique to perform closed-loop control of laminar separation bubbles in combination with optical sensors.