Experimental study of vortex shedding and subharmonic lock-on for a rotationally oscillating Aat plate

This work investigates the subharmonic lock-on in 1/2 and 1/3 harmonic band s of vortex shedding induced by rotational oscillations of a flat plate nor mal to a uniform stream in its neutral position. The lock-on boundary, surf ace pressure and vortical structure of the near wake are also described. In addition, the onset of lock-on, which depends on the combination of the fr equency and amplitude of the forcing, is examined by applying spectral anal ysis of the wake velocity. The forcing frequency is set at a fixed value, i n the range 15-120% of the natural shedding frequency of the stationary nor mal flat plate. Meanwhile, the angular forcing amplitude is varied to deter mine the lock-on regime. During the subharmonic lock-on, more than one vort ex is shed in an oscillation cycle of the plate; each individual vortex inf luences the base pressure distribution as well. As the lock-on of 1/3 harmo nic band is approached at the lower frequency limit, the mean base pressure appears to be uniform along the plate with a level exceeding the unforced plate. For the lock-on of 1/2 harmonic band, the base pressure level reache s a value significantly higher than the unforced plate, i.e. appreciable re duction in drag. Flow visualization suggests that these differences are lar gely owing to the formation of shed vortex. (C) 2000 Elsevier Science Ltd. All rights reserved.