Response of the Separated Flow over an Airfoil to Actuator Bursts

TL;DR

This study experimentally investigates how single and multiple actuator bursts influence separated flow over an airfoil, revealing vortex dynamics, pressure changes, and lift variations through advanced flow analysis techniques.

Contribution

It provides new insights into the flow response to burst actuation, including vortex behavior and the effects of burst interactions on lift control.

Findings

Single bursts cause vortex roll-up and lift reversal within 2.0t+

Multiple bursts interact nonlinearly, affecting separation bubble dynamics

Linear interactions contribute to high-frequency lift variations

Abstract

The separated flow response to single and multiple burst mode actuation over a 2-D airfoil at $12^o$ angle of attack was studied experimentally. For the single-burst actuation case, surface pressure signals were correlated with the flowfield observations of the roll-up and convection of a large-scale vortex structure that follows the actuator burst input. A spatially localized region of high pressure occurs below and slightly upstream of a "kink" that forms in the shear layer, which is responsible for the lift reversal that occurs within $2.0t^+$ after the burst signal was triggered. Proper orthogonal decomposition of the single-burst flow field shows that the time-varying coefficients of the first two modes correlate with the negative of the lift coefficient and pitching moment coefficient. The dynamic mode decomposition (DMD) of the single-burst flow field data identified the modes related to the kinetic energy growth of the disturbance. The mode with the largest growth rate had a Strouhal number close to that associated with the separation bubble dynamics. However, when multiple bursts are used to control the separation, the interactions between the bursts were observed which depend on the time intervals between the bursts. The convolution integral and DMD were performed on the multi-burst flow field datasets. The results indicate that the nonlinear burst-burst interaction only affects the reverse flow strength within the separation bubble, which is related to the main trend of the lift. On the other hand, the linear burst-burst interaction contributes to the high-frequency lift variation associated with the bursts.