Transient dynamics of stall and reattachment at low Reynolds number

Wind tunnel experiments are performed to investigate stall and reattachment transients for an aerofoil and wing model at low chord Reynolds numbers ( $8\times 10^4\leqslant {{Re}}_c\leqslant 1\times 10^5$ ) where a laminar separation bubble (LSB) may form on the suction surface. Direct force measurements and particle image velocimetry (PIV) are employed simultaneously to characterise the transient aerodynamic loading and flow field development. The imposed changes in operating conditions leading to stall and reattachment include changes in angle of attack at multiple pitch rates and changes in Reynolds number. The evolution of the lift coefficient is consistent with dynamic stall at higher Reynolds numbers, with a reduction in time delay between the passing of the static stall condition and the loss of lift for increasing pitch rate. During an increase in angle of attack, the separation bubble moves upstream prior to rapidly bursting, whereas for a decrease of Reynolds number, the LSB undergoes a more gradual monotonic increase in length prior to bursting. In contrast to notable differences in the aerodynamic loading and flow field development for different types of transients leading to LSB bursting, the process of LSB formation is less sensitive to the type of imposed change in operating conditions. Spanwise PIV measurements on the aerofoil and wing models indicate that the spanwise flow development is also insensitive to the type of imposed transient during LSB bursting and formation.