Decay of laminar separation bubbles under decelerating inflow conditions*

The formation of a laminar separation bubble on the suction side of a lifting surface is common when airfoils are operated at low Re. The investigation of unsteady inflow conditions, however, is relatively rare, although these conditions typically occur in practical applications. The work presented here explores the effects of deceleration on laminar separation bubbles. Experiments were conducted in a water towing tank using an SD7003 airfoil model decelerated from a constant chord Reynolds number to rest. Quantitative flow field measurements were performed using time-resolved two-component Particle Image Velocimetry over a range of decelerations. The aim is to gain insight into the time-dependent topology of laminar separation bubbles. The results show that deceleration of the model leads to a gradual shift of the separation point and vortex roll-up towards the leading edge, which is in contrast to the trends expected for a quasi-steady decrease in Reynolds number. At lower deceleration values in the range considered here, the vortex shedding frequency decreases. The effect of deceleration on the pressure gradient indicates that the deviation of the results from quasi-steady trends in the cases studied is due to significant inertial effects.