DOI: 10.1615/TSFP8
LOCAL FORCING OF LAMINAR SEPARATION BUBBLES
RESUMO
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.