Boundary Layer Transition on Airfoils and Wings in Unsteady Low-Reynolds Number Flows

Abstract

Laminar separation bubbles (LSBs) often form on wings and turbine blades that operate at aerodynamically low chord Reynolds numbers. Changes in the oncoming velocity magnitude, direction, and disturbance amplitudes may cause bursting of the LSB and a severe reduction in wing or blade performance. The objective of this thesis is to understand the transient dynamics of LSBs in unsteady conditions that are relevant to practical applications of low chord Reynolds number wings and turbine blades. To this end, a series of wind tunnel experiments were performed on a set of NACA 0018 airfoil and wing models at chord Reynolds numbers ranging from 40000 to 100000. The influence of isolated ramp changes in Reynolds number, isolated ramp changes in angle of attack, and large-scale high-intensity free-stream turbulence on the LSB development and aerodynamic loading were investigated. Three-dimensional end effects on LSB development, which are present for all real wings and turbine blades, were also studied in unsteady operating conditions. Two-component particle image velocimetry measurements were performed to investigate the suction surface boundary layer development on the wing and airfoil models. Direct force measurements were employed to quantify the effects of unsteadiness on the lift coefficient.

Transient changes in free-stream velocity and angle of attack leading to LSB formation or bursting were found to produce lift coefficient hysteresis. Stronger hysteresis effects were observed for more rapid changes in operating conditions, and for two-dimensional airfoils relative to finite wings. Although the dynamics of boundary layer transition affect the conditions at which LSB bursting or formation initiates, the temporal evolution of the aerodynamic forces after the onset of LSB bursting or formation were similar across widely varying flow conditions. When undergoing an increase in angle of attack that leads to bursting, the LSB initially contracted. In contrast, the LSB continuously expanded prior to bursting during a decrease in Reynolds number. However, LSB formation was followed by a contraction of the LSB during both decreases in angle of attack and increases in Reynolds number. The frequencies of unstable disturbances that lead to transition in the separated shear layer varied continuously during the LSB formation and bursting transients. The duration of the LSB bursting and formation transients was on the order of 10 global convective timescales. On a finite span wing, end effects caused spanwise expansion of the separated flow region during bursting. Conversely, the separated flow region contracted in the spanwise direction during LSB formation on a finite wing. The gradual spanwise progression of bursting on a finite wing in unsteady conditions caused changes in aerodynamic loads that were more gradual than those experienced by a two-dimensional airfoil. In large-scale, high-intensity free-stream turbulence, fluctuations in the instantaneous effective oncoming velocity direction may lead to large variations in transition location and intermittent boundary layer separation. These findings elucidate the physical mechanisms responsible for the performance characteristics of wings and turbine blades in unsteady low chord Reynolds number flows.