Effect of Gurney flap on the vortex-dominated flow over low-AR wings

A Gurney flap is a simple passive flow control device that, when employed on wings, can significantly augment its lift with minor changes in drag and stall angle. In this work, a Gurney flap of height 0.02c was employed on low-AR flat plate wings (AR ≤ 1.5) of different planforms (rectangular, Zimmerman and inverse Zimmerman). Measurements were carried out at a low Reynolds number (1 × 10⁵) to understand the flap's influence on the flow field over the wings. Time-averaged 2D particle image velocimetry (PIV) measurements on the wings' mid-span plane revealed a leading-edge separation bubble. The early reattachment of the leading edge separated flow is consistent with the results obtained from surface oil flow visualization. Stereoscopic-PIV measurements carried out at different cross-flow planes showed an increase in the strength of wingtip vortices and higher downwash for the Gurney flapped configuration. The collective information from these measurements suggests that the Gurney flap increases the strength of wingtip vortices via enhanced pressure difference between the upper and lower surface of the wing. The strong wing tip vortices promote higher downwash between them that reattaches the separated shear layer leaving a leading-edge separation bubble in between. The increased downwash delays complete flow separation to higher angles of attack. The strong tip vortices retain the lift-generating vortical flow closer to the wing. Hence, the maximum lift coefficient and stall angle were significantly increased.