Post-stall flow control with upstream flags

Abstract

The post-stall flow control using compliant flags of varying thickness and length, placed upstream a NACA0012 airfoil, was shown to be possible at an airfoil chord Reynolds number of 100,000. The flag wakes produced substantial increase in the stall angle and the maximum lift coefficient of the airfoil placed at optimal cross-stream locations from the wake centerline. Oscillating flags could generate periodic wakes with better spanwise coherence than the stationary bluff body. This resulted in the excitation, formation and shedding of the leading-edge vortices periodically, providing mean lift enhancement. There is an optimal range of the flag mass ratio for which the flag frequency coincides with the natural frequency of the vortex shedding instability or its subharmonic of the baseline airfoil wake. The flag dimensionless frequency is a function of the mass ratio only, which can be predicted by a reduced order model in the limit of very large mass ratio and by using the modified free-streamline theory for the separated flow. There is also an optimal range of the flag dimensionless frequency. Graphical abstract

Post-stall flow control on aerofoils by leading-edge flags

“…2022 a ), flags in a uniform free stream (Zhang et al. 2022 b ) and oscillating aerofoils (Turhan et al. 2022 a ; Turhan, Wang & Gursul 2022 b ).…”

“…In these cases, a phase-averaging method based on the POD analysis can be adapted (Zhang et al. 2022 a , b ). In this method, a smaller PIV measurement domain just downstream of the flag is analysed by the POD method to obtain a periodic reference signal.…”

“…The dashed red line in figure 28 represents a reduced-order model (Zhang et al. 2022 b ) for rigid flags that are placed in the free stream and free to roll around its leading edge. The natural frequency is predicted as The model assumes that the equation of the motion of a rigid flat plate free to roll around its leading edge can be written by considering the moment about the leading edge and equating it to the moment of inertia times the angular acceleration where θ is the angular displacement (see figure 29 a ) and C M is the pitching moment coefficient.…”

“…For the fully separated flow behind the rigid flag in free stream, Zhang et al. (2022 b ) used the free-streamline theory first proposed by Kirchhoff (1869) and Rayleigh (1876). The linearised solution of (3.9) provides the natural frequency of the rigid flag as in (3.8).…”

“…Similar effects are observed when the stationary aerofoils and wings are placed in an unsteady free stream that has time-periodic unsteadiness. Examples include the cases of harmonic variations of free stream (Gursul & Ho 1992; Gursul, Lin & Ho 1994; Choi, Colonius & Williams 2015) and wings in the wakes of bluff bodies or flags (Zhang, Wang & Gursul 2020, 2022 a , b ). In the latter examples, quasi-periodic coherent vortices that exist in the turbulent wakes may promote formation of leading-edge vortices.…”