Dynamics of laminar separation bubbles under accelerating inflow conditions

Laminar separation bubbles mainly occur at low Reynolds numbers, where they alter the flow significantly. Investigations performed in the past preferably focused on steady inflow conditions. The work presented here examines the influence of acceleration on the formation of a laminar separation bubble. The experiments were performed in a towing tank with an SD7003 airfoil model accelerated from rest to a constant chord Reynolds number and then decelerated. Quantitative flow field measurements were performed using two-component time-resolved Particle Image Velocimetry over a range of conditions. The aim is to provide insight into the mechanism of LSB formation by a detailed analysis of the spatio-temporal flow development focusing on the LSB formation and dynamics. The associated transient flow development is shown to persist over several convective time scales after steady state free stream velocity is reached, with no significant effect of acceleration on the overall transient duration. However, the acceleration rate has a substantial effect on flow development during the acceleration, with a delayed development of shear layer perturbations observed at lower accelerations.