Flow-induced vibrations of a hinged cavity at the rear of a blunt-based body subject to laminar flow

Abstract

We perform numerical simulations to characterize the flow-induced vibrations (FIV) of a rear cavity with elastically hinged rigid plates, placed as a passive device at the base of a blunt body that is subject to a laminar flow of Reynolds number Re = 400 . The dynamic response and forcing of plates, wake features and force coefficients are investigated for the range of reduced velocity U^∗= [0 , 30] . Three different regimes of the rotational oscillations are identified. An initial branch of low oscillation amplitude is defined for U^∗< 2 . 5 , where the plates oscillate in counter-phase (varicose mode) with a frequency f_p that corresponds to the harmonic of the wake vortex shedding frequency f_p ≃ 2 f_w , and is similar to the natural frequency of the plates, f_p ≃ f_n . For intermediate values of U^∗, the plates oscillate in phase (sinuous mode) at their natural frequency, with respect to a closer averaged location of plates. Such synchronization regime amplifies the vibration magnitude and defines the upper branch in the amplitude response curve, whose maximum is attained at U^∗= 4 . 7. Due to such enhanced vibration, the vortex shedding frequency is now locked-in at the natural frequency of plates, so that f_p = f_n = f_w . Finally, for larger values of U^∗, a lower branch of moderate amplitude response is defined, which is characterized by the in-phase oscillation of plates, with respect to an more open average position, governed again by the shedding frequency, f_p = f_w > f_n . Additionally, a multibody model has been developed to retrieve, from the plates rotational motion, the resultant forces and moments that produce the plates vibration. Such inverse dynamics model is formulated to allow its generalization for configurations of higher dynamical order, and validated against the results obtained from the numerical simulations. The analysis shows that the synchronization regime is mainly promoted by a reduced fluid damping and a forcing moment that acts in phase with the plates motion. The switch in such phase from 0º to 180º occurs after the lock-in, what attenuates the plates response at large U^∗. In general, the FIV of plates alters the vortex shedding and near wake pressure, especially during the synchronization regime, inducing an overall increase of the global force coefficients with respect to the static cavity.