Ground effect on flow past a wing with a NACA0015 cross-section

Wing in ground (WIG) effects is an important aerodynamics phenomenon as it affects the aerodynamics of aircraft (during take off and landing), marine crafts like wing in ground effect (WIG) crafts and racing cars. In the present investigation, wind tunnel experiment was conducted on a wing with a NACA0015 cross-section, a chord of 0.24 m and an aspect ratio of 1.46. The “ground” was simulated by a vertically mounted 0.6 m (streamwise) × 0.457 m (same height as the wind tunnel test section) perspex plate. The wing was mounted vertically on a load cell, and the load cell on a mechanism that could traverse the wing/load cell set-up in a direction transverse to the free stream, thereby varying the distance between the wing and the ground. Measurements carried out include the pressure distribution on the wing surface at mid-span and forces on the wing, and pressure distribution on the ground. Measurements were carried out at various angles of attack, at a constant wind speed of 12 m/s. The corresponding chord based Reynolds Number (Rec) is approximately 1.872 × 105.Mid-span surface pressure distributions were measured on the wing at both positive and negative angles of attack α, and at different ground clearance h/c. It was observed that for both positive and negative angles of attack, the pressure on the upper surface shows a much stronger angle of attack dependence, and only weak h/c dependence when the angle of attack is in the negative range. On the other hand, the lower surface pressure is influenced by both the angle of attack and h/c. Within a certain range of positive angle of attack and h/c, the “ramming effect” which is associated with a local increase in surface pressure that had been reported in the literature was observed.Streamwise pressure distribution on the ground that corresponds to the wing mid-span shows that when the wing is out of ground effect, the effects of wing angle of attack on the pressure distribution on the ground is not significant. When the ground effect are present (small to moderately small h/c), pressure distribution on the ground especially within the range −0.3 < x/c < 1 is affected. Different pressure versus x/c trends were observed, depending on the magnitude and sign of the wing angle of attack.Both lift coefficients obtained from integrating surface pressure distribution and from direct load cell measurements show that as h/c reduces, the sign of the lift coefficient remains unchanged but its magnitude increases. A temporary reduction in the lift coefficient is observed when h/c reduces from about 0.3 to 0.15 in the α = 0–6° range, for both the pressure distribution integration and load cell measurement cases. This is believed to be the consequence of the “convergent–divergent channel effect” reported in surface pressure distribution. Generally speaking the lift estimated from integrating surface pressure distribution is slightly larger than the one measured directly from the load cell. This is consistent with the slightly more 2-D nature of the flow at the wing mid-span.The lift slope from both the pressure and load cell data show h/c dependence, with the former consistently the larger of the two. Their magnitudes generally lie within the magnitude estimated from the Thin Aerofoil Theory (2-D flow) and Prandtl’s Lifting Line Theory (3-D flow).

► Surface pressure on both sides generally shows h/c and AOA dependence, with some exception.
► The “ramming effect” associated with a local increase in the lower surface pressure was observed.
► Pressure on the ground shows ground effect dependence, especially within the range of −0.3; x/c;1.
► As h/c reduces, the sign of the lift coefficient remains unchanged but its magnitude increases.
► The lift slope was found to show h/c dependence.