Experimental investigation of the ground effect on a wing without/with trailing edge flap

Abstract

In this study, the ground effect over a wing without/with a flap at low Reynolds numbers was investigated experimentally. Three-dimensional flow fields over and behind a flapped wing and single (without flap) wing in/out of ground effect at several clearances and Re numbers (0.75 × 105, 1 × 105 and 1.5 × 105) were investigated using three different experimental methods. From flow visualization with smoke technique, a succession of recorded photographs allowed to reveal the interactions of flow and ground. The changes caused by the employment of trailing-edge flap and ground effect were visualized and compared with data obtained from Constant Temperature Anemometer (CTA). According to force measurement results, the flapped wing near the ground conduced to the increase of lift and drag forces for all clearances. From smoke wire results, the separation zone above the flap appears and contributes to a higher turbulence level in the wake region. The leading-edge vortices, tip vortices behind the wing interact with the ground boundary layer and the flow coming out from the section between the trailing edge of the wing and the ground. For this, dominant vortex frequencies were detected using the spectral analysis from velocity measurements at the wake region of the wing. Further, the ground effect enabled the control of the laminar separation bubble (LSB) on the wing at low angles of attack in low Reynolds flows, the bubble gets smaller as clearance increased; however, the ground effect is not effective in controlling the LSB at high angles of attack. As the Reynolds number raised, improvement of ground effect on the wing increased. When all these results were considered, the flap should not be brought too close to the ground to benefit from the ground effect. The flapped configuration with/without ground effect presented the best performance at low angles of attack.