9. Investigation of Transient Regime in Laminar Separation Bubble Formation

The transient flow regime related with the formation of laminar separation bubbles (LSB) is examined by time-resolved measurements of the velocity field. The investigated scenario aims at studying the flow in multiple stage turbines, where the wake of airfoils in previous compression stages induces periodic variation of turbulence level in the subsequent airfoils. This can lead to periodic removal and formation of LSB. This process is simulated here by exciting controlled disturbances with a vibrating ribbon. Experiments are carried out in a laminar water channel at PUC-Rio. Longitudinal PIV measurements are performed on a flat plate subjected to an adverse pressure gradient. The pressure gradient is set by false walls with adjustable geometry. Suction is applied on the false wall, in order to avoid the boundary layer separation at this surface. Bubble topology and disturbance growth along the streamwise direction are measured during the transient of bubble formation. Results show spatial amplification over a narrow frequency bandwidth. Dominant non-dimensional frequencies (St = fδ*_s/U) at late stages of bubble formation are in close agreement with those reported in literature. During the transient the intensity of reverse flow is significantly changed, pointing out for possible changes on the stability mechanisms involved on the bubble reattachment.