The Effect of Gurney Flap Height on Vortex Shedding Modes Behind Symmetric Airfoils

A NACA 0015 airfoil with and without Gurney flaps (a small tab 1% to 4% of the airfoil chord that protrudes 90° to the chord at the trailing edge) was studied in a wind tunnel with Rec = 2.0 ◊ 10 5 in order to examine the evolving flow structure of the wake through time-resolved PIV and hot-film anemometry frequency measurements. Multiple vortex shedding modes were observed and related to the flap height and angle of attack. Previous studies have shown the Gurney flap to increase the lift coefficient through an increase in circulation around the airfoil. This study examines the vortex shedding interactions that make this possible, and also shows that the ratio between the two dominant shedding modes is approximately 0.8 for both the 2% and 4% flap cases. NACA 4412 airfoil that showed a significant increase in the lift coefficient, shifting the lift curve up by 0.3 for a Gurney flap of 1.25% of the chord length, and providing a greater maximum lift. There was no appreciable increase in drag until the Gurney flap was extended beyond about 2% of the airfoil chord length, at which point the flap extended beyond the boundary layer thickness. Jeffrey et al. (2000) studied the flap through surface pressure, LDA measurements, and flow visualization. The time-averaged velocity fields revealed a pair of counter-rotating vortices downstream of the flap, and spectra from the LDA measurements and smoke visualizations documented the presence of a Karman vortex street. Two possible causes were postulated for the increase in lift caused by the flap: periodic vortex shedding downstream of the flap served to increase the trailing-edge suction of the airfoil, and the deceleration of the flow directly upstream of the flap contributed to a pressure difference acting across the trailing-edge. Time- and phase- averaged PIV analysis by Solovitz and Eaton (2004a and b) provided additional data on the flow pattern around static and dynamically-actuated Gurney flaps. Previous studies (Troolin et al., 2006) conducted at Re = 2.0 ◊ 10 5 identified two dominant shedding modes in the region downstream of the Gurney flap. The primary mode resembles the common Karman vortex street located downstream of bluff bodies. This shedding mode has a strong periodicity and contributes the strongest peak to the frequency spectra measured downstream of the airfoil. The secondary shedding mode results from fluid in the cavity upstream of the flap that is intermittently expunged into the downstream wake. The secondary shedding occurs less frequently and less periodically than the primary shedding; however a peak in the frequency spectra can be seen that corresponds to this shedding mode. This study examines the effects of Gurney flap height on the vortex shedding and overall wake