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

This work investigates the subharmonic lock-on in 12 and 13 harmonic bands of vortex shedding induced by rotational oscillations of a flat plate normal to a uniform stream in its neutral position. The lock-on boundary, surface 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 frequency and amplitude of the forcing, is examined by applying spectral analysis of the wake velocity. The forcing frequency is set at a fixed value, in the range 15–120% of the natural shedding frequency of the stationary normal flat plate. Meanwhile, the angular forcing amplitude is varied to determine the lock-on regime. During the subharmonic lock-on, more than one vortex is shed in an oscillation cycle of the plate; each individual vortex influences the base pressure distribution as well. As the lock-on of 13 harmonic 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 12 harmonic band, the base pressure level reaches a value significantly higher than the unforced plate, i.e. appreciable reduction in drag. Flow visualization suggests that these differences are largely owing to the formation of shed vortex.