Impact of actuation frequency on mean topology and breakdown past a Laminar Separation Bubble

Abstract

Laminar Separation Bubbles (LSBs) typically form in a low Reynolds number boundary layer flow involving a strong pressure gradient. These bubbles adversely impact the aerodynamic performance of a body by increasing the drag, susceptibility to stall, and noise emission. Since a large number of unmanned aerial vehicles and small wind turbine blades operate in a similar Reynolds number range, it is crucial to have an understanding of the bubbles' behavior to improve aerodynamic performance in these operating conditions. The frequency at which this bubble sheds due to amplification of disturbances is termed the natural, harmonic, or fundamental shedding frequency. The way in which the shed vortices break down impacts the behavior of the bubble, and hence it is crucial to study this breakdown of the vortices as well. Though previous works have shown that varying external forcing frequency alters the bubbles' behavior, there is no study investigating the variation of bubble topology and the breakdown of vortices in a forcing frequency regime varying from impulsive to fundamental forcing.

An experimental study is performed using a NACA 0018 airfoil in the Anechoic Tunnel at TU Delft. Under the chosen conditions, a short LSB forms on the suction side and is chosen as the baseline bubble for the current study. Using a spanwise uniform Alternating Current-Dielectric Barrier Discharge (AC-DBD) plasma actuator placed upstream of the separation point, two-dimensional forcing is applied. The actuator geometry and voltage are kept fixed while only the actuation frequency is varied from a low frequency regime, where the bubble has enough time to recover close to its natural state between the actuations, up to the regime where the forcing is close to the fundamental shedding frequency of the bubble. Surface pressure and velocity fields are measured using pressure taps and Particle Image Velocimetry (PIV), respectively. PIV measurements are done in both time averaged and phase averaged manner to gain insight into the vortex dynamics and breakdown characteristics. While the surface pressure taps along the airfoil chord provide time-averaged pressure measurements, PIV data captures the bubble topology in the wall normal plane, and the breakdown characteristics of the actuated wave packet in the wall parallel plane.

The LSB topology quantified in terms of bubble area shows a monotonic decrease in area with an increase in forcing frequency. The decrease in area is steep at lower forcing frequencies and gradual at higher forcing frequencies. The displacement thickness moves closer to the wall, and the maximum shape factor value shifts upstream, indicating increased stability of the free shear layer. Analysis of the breakdown characteristics shows distinct breakdown behavior across the forcing frequency ranges. At a lower forcing frequency, the breakdown shows features of the unforced bubble breakdown. In the subharmonic and fundamental forcing frequency range, the wavelength variation of the convected wave packet shows a distinct U-shape, indicating changed stability of the bubble as compared to lower forcing frequency actuation. The results provide valuable insights into the bubble topology and the breakdown characteristics under the considered forcing frequency range and can be useful in designing an active control system in the future for similar low Reynolds number range.