Effects of grooves on the formation of the LEV of an impulsively-started flat plate

Abstract

This experimental investigation examines the effects of longitudinal and transverse square grooves on the growth of the leading edge vortex produced by a two-dimensional flat plate impulsively started to constant velocity in quiescent water. The experimental conditions include Reynolds numbers based on the chord length and average velocity of 1518, 3036 and 6072 and angles of attack of 90° and 75°. The vortex formation time defined by the travel distance of the plate and chord length is 0.6. The DPIV data are reduced to compare the vorticity fields and circulation of the smooth and grooved plates, and to examine the Reynolds number and angle of attack dependency. The comparison is carried out in context of circulation growth rate and LEV size and magnitude. Moreover, the effect of the secondary vortex of opposite sign on the growth of the LEV is also discussed. The smooth plate results indicate that the circulation growth rate is sensitive to the angle of attack only for the higher Reynolds number cases where the growth rates begin to decrease at t/T > 0.5. The decrease in the growth rate suggests reductions in the size and magnitude of the vortex. The results also show that the LEV separates sooner at the higher angle of attack. Negative vorticity induced by the LEV is found in all cases and is conjectured to be associated with a secondary vortex. This secondary vortex plays a role in modifying the growth of the size and magnitude of the LEV. The longitudinal grooves cause this secondary vortex to increase in size and magnitude, whereas the transverse grooves have opposite effect. However, the transverse grooves have a more pronounced effect on the circulation growth rate.