An investigation of the dynamic response of a laminar separation bubble on a NACA 0015 wing forced with a periodically unsteady freestream

The aerodynamic performance of conventional wing sections at transitional Reynolds numbers has been shown to be dominated by the formation of laminar separation bubbles (LSBs) on both the suction and pressure sides of the wing. More specifically at low angles of attack LSBs typically form near the wing trailing edge; increasing the effective wing thickness and altering the effective camber, which can cause significant deviations in the lift slope classical theory. The size and position of the LSBs is strongly Reynolds number dependent, which makes them highly susceptible to unsteady variations in the freestream flow. To better understand this dynamical coupling a finite span NACA 0015 wing section at a fixed angle of attack of α = 2° is exposed to an unsteady freestream, driven in a periodic fashion in the range of 75,000 < Re < 200,000 for reduced frequencies from 0.04 < k < 0.12. The LSB dynamics are experimentally captured through the synchronized measurement of time-resolved surface pressure and planar particle image velocimetry measurements. The periodic roll-up and shedding spanwise oriented coherent vortical structures within the LSB are observed whose convective spacing dynamically varies with the unsteady freestream conditions.