The flow near and downstream of a Gurney flap, of different flap heights and flap porosities, was investigated at Re = 5.4 x 104. For solid Gurney flaps, the downturned flow-induced effective positive camber effects, together with the flow deceleration in the upstream face of the flap, led to an enhanced circulation and lift force. The width and unsteadiness of the wake behind the flapped airfoil were, however, found to increase compared to the unflapped airfoil. The vortex-shedding frequency was also found to decrease with increasing flap height and angle of attack. In contrast to the downturned flow induced by the solid flap, the flap perforation effectively reduced the positive camber effects and also caused an earlier flow separation, especially in the trailing-edge region. The vortex shedding process was disrupted due to the mixing of the perforation generated double or single jet (for sigma = 40 and 23%, respectively) with the reversed flow immediately downstream of the flap. The interaction and roll-up, including the vorticity feeding mechanism, of the two shear layers (separating from the airfoil upper surface and the bottom tip of the flap) was greatly suppressed or nearly eliminated. The reduced upper-surface pressures (due to the reduced camber effects and the disappearance of the vortex shedding) and lower-surface pressures (as a result of the disruption of the cavity existing in the upstream face of the flap) gave rise to a decreased gain in lift compared to the solid flap. The reduction in the drag, however, outweighed the loss in lift and thus led to an improved lift-to-drag ratio in comparison with that of the solid flap. The larger the flap perforation, the smaller the Cl and wake width and Cd but the larger the (L/D)max. Furthermore, in addition to the narrowing of the near-wake width (indicative of a reduced drag) with increasing flap perforation, the fluctuating intensity and the extent of the unsteady wake were also significantly reduced with increasing amount of flap perforation.
PIV investigation of flowfield behind perforated Gurney-type flaps
Experiments in Fluids ; 46 , 6 ; 1005-1019
2009
15 Seiten, 12 Bilder, 1 Tabelle, 17 Quellen
Article (Journal)
English
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