Experimental investigation of laminar separation bubbles on a flat plate

Similar to the wind tunnel experiments by Gaster,³ an invertedly mounted airfoil was used to create laminar separation bubbles on a flat plate. The present experiments were carried out in a water tunnel. Suction was applied on the airfoil to prevent separation from its surface. The bubble dimensions were large enough, and the naturally occurring frequencies low enough, to allow highly resolved temporal and spatial PIV measurements. The influence of the momentum thickness Reynolds number at separation (Reθ_s) was investigated. The natural shedding frequencies and the bubble size varied with Reθ_s. Two-dimensional disturbances were introduced into the upstream boundary layer with a vibrating ribbon. The separation bubble was found to be highly susceptible to two-dimensional forcing. When forced with frequencies close to the natural shedding frequency, the bubble length could be decreased by as much as 50%. Forcing of this frequency apparently exploits the natural instability mechanism of the separated shear layer. This enforces the development of vortical structures ("rollers") which are highly effective in bringing high-momentum fluid into the separated region. This leads to an early reattachment of the flow. Using phase-locked PIV measurements, the formation, downstream movement and breakdown of these unsteady flow structures was investigated. The growth of disturbances, which lead to these structures, was studied. A comparison of the amplification rates and disturbance amplitude profiles with Linear Stability Theory showed good agreement. Long separation bubbles (Gaster³) were observed in certain cases. These bubbles were mainly steady, showing only weak, intermittent shedding activity. The reattachment of long bubbles occurs gradually, stretching over a relatively long streamwise extend. Short bubbles are characterized by a more abrupt reattachment, caused by the formation of vortical structures (rollers) in the reattachment region.