Active control of wakes and vortex-induced vibration of a circular cylinder at Re = 100
Date of Issue2015
School of Mechanical and Aerospace Engineering
For years appropriate control methods have been actively sought to mitigate vortex-induced vibration (VIV) through either altering the shedding frequency or completely eliminating the vortex shedding. Among these methods, synthetic jets (SJs), as a promising active-flow-control means, are a good candidate for the control. As zero net-mass-flux jets, SJs have been widely applied in flow separation control and drag reduction for bluff bodies. However, their implementation for VIV control is rare and still remains not fully understood. This research aims to explore the control of asymmetric vortex shedding and the resulting VIV of bluff bodies using SJs and one of their variants, with the focus being placed on the change of wake dynamics, aerodynamic forces, and motions of a circular cylinder due to the control at a specific low Reynolds number of 100. To facilitate this study, a lattice Boltzmann method (LBM) based framework is established for fluid-structure interaction (FSI) simulations. In this framework, the multi-block scheme and an improved overlap-mesh approach are adopted to balance the computational accuracy and efficiency, the interpolated half-way bounce-back scheme is incorporated to deal with the curved body surfaces, and the corrected momentum exchange method is employed for accurate evaluation of aerodynamic forces experienced by bodies. In addition, the fly-by-wire vortex tracking method and dynamic mode decomposition (DMD) method are integrated in this framework. Three fundamental problems are investigated. First, the capability of a pair of in-phase SJs in suppressing the shedding of asymmetric vortices is studied systematically at different momentum coefficients, frequencies, and actuating locations, where the SJ excitation frequency of interest is roughly above five times the natural vortex shedding frequency and the lock-on phenomenon does not occur. The wakes, lift and one-dimensional VIV of the cylinder are examined in detail. It is confirmed that the in-phase SJ pair are able to completely suppress the asymmetric wake and the resulting VIV of the cylinder at certain SJ parameter combinations. Meanwhile it also reveals that the asymmetric wake suppression is usually accompanied by an increase of drag. To address this issue, the second study is targeted at suppressing two-dimensional VIV of the cylinder. A variant of the SJ concept, i.e. the windward suction combined with leeward blowing (WSLB), is implemented. The WSLB actuator consists of four continuous jets, two in suction at the windward of the cylinder and two in blowing at the leeward. The control effect of this new concept, with both open-loop and closed-loop schemes, is investigated in detail. Third, the effects of phase difference and excitation frequency of a pair of SJs on the wake and resulting aerodynamic forces are studied, where the SJ excitation frequency of interest is roughly below five times the natural vortex shedding frequency. Four different lock-on regimes are identified, i.e., the primary, secondary, tertiary and subharmonic lock-on. The corresponding wake structures and force characteristics are discussed. In addition, the effective SJ arrangement for minimizing the standard deviation of lift and drag oscillation is obtained. Through this research, a better understanding on the use of SJs for the control of asymmetric vortex shedding and VIV of bluff bodies is achieved. Although this research only focuses at a relatively low Reynolds number, it provides useful information for future extension to the flows of higher Reynolds numbers.