| The Gurney flap can provide an increase in lift with a small drag penalty up to a flap height of approximately 3% of the chord. For flap height larger than 3% chord (herein referred to as Gurney-type flap), there is a significant increase in drag, causing a deteriorating lift-to-drag ratio. In this work, perforation was introduced in large Gurney-type flaps in an attempt to improve their aerodynamic performance. A study of different flap heights and perforation porosities were performed by using force balance, surface pressure and hot-wire measurements in conjunction with particle image velocimetry (PIV). Results show that flap porosity reduced lift in comparison to the solid flap due to the decrease in positive camber effects and decompression of the cavity flow up stream of the flap. The corresponding reduction in drag, however, outweighed the loss in lift and rendered an improved lift-to-drag ratio. Detailed PIV flow field behind the perforated flap revealed that the existence of perforation-generated jets are responsible for the observed differences in aerodynamic performance. If strong enough, perforation- generated jets could eliminate the vortex shedding process behind the flap. Furthermore, the near wake was found to be disrupted and narrowed, drastically suppressing fluctuation intensity. |
