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Vortex-induced oscillations offer a potential means to harness hydrokinetic energy even at low current speeds. In this study, we consider a novel converter where a cylinder undergoes angular oscillations with respect to a pivot point, in contrast to most previous configurations, where the cylinder undergoes flow-induced oscillations transversely to the incident free stream. We formulate a theoretical model to deal with the coupling of the hydrodynamics and the structural dynamics, and we numerically solve the resulting nonlinear equation of cylinder motion in order to assess the performance of the energy converter. The hydrodynamical model utilizes a novel approach where the fluid forces acting on the oscillating cylinder are split into components acting along and normal to the instantaneous relative velocity between the moving cylinder and the free stream. Contour plots illustrate the effects of the main design parameters (in dimensionless form) on the angular response of the cylinder and the energy efficiency of the converter. Peak efficiencies of approximately 20% can be attained by optimal selection of the main design parameters. Guidelines on the sizing of actual converters are discussed.

A self-powered active support system, which produces a control force using the energy regenerated by a transversely mounted linear electromagnetic (EM) damper, is applied for two-dimensional (2D) (streamwise/cross-flow) vortex-induced vibration (VIV) suppression of an oscillating circular cylinder in low Reynolds number flow (R=90). In addition, a passive energy generating system is devised that achieves a proper trade-off between VIV suppression and energy harvesting actions with the EM damper acting in either the regeneration or the dissipation mode. A multifield cosimulation scheme, which links an active proportional-integral-derivative (PID) controller along with the switch-based energy harvesting circuit of the EM damper (implemented in MATLAB/Simulink) to the plant model (constructed via a user-defined function in a commercial finite volume solver), is designed and implemented. Numerical simulations compare the effects of the passive and active EM-generating systems on 2D VIV suppression of the elastic cylinder as well as on the time evolution of cylinder force coefficients. They demonstrate that the adopted self-powered (tuned) active energy harvesting system, which does not require an external power supply and makes intelligent use of the stored electrical energy for producing the required transverse control force, efficiently balances the regenerated and consumed energies by proper switching between three distinct (regeneration, drive, and brake) modes of EM device operation. It can particularly provide improved cylinder VIV suppression with lower power consumption via enhanced collaborative action between different modes of the EM device. Moreover, the selected system is also found to be effective for other Reynolds numbers in the lock-in region. [ABSTRACT FROM AUTHOR]

The flow-induced oscillation (FIO) of bluff bodies is commonly encountered in the fluid structure interaction (FSI) problems. In this study, we use an unstructured moving grid strategy and simulate the FIO of two rigid plates, which are elastically hinged at the two ends of a fixed flat plate in a cross-flow. We use a hybrid finite-element-volume (FEV) method in an arbitrary Lagrangian–Eulerian (ALE) framework to study FIO of the two hinged plates. The current simulations are carried out for wide ranges of flow Reynolds number (50–175), spring stiffness coefficient, and the two hinged plates' moment of inertia magnitudes. The influences of these parameters are investigated on the magnitudes of maximum deflection angle, the amplitude of oscillation, the total lift and drag coefficients, and so on. The study is also carried out in the transition period to describe the in-phase and out-of-phase angular oscillations occurring for the two elastically hinged plates with respect to each other. After the transition period, the two hinged plates eventually arrive to a similar periodic oscillation; however, with some phase lags. We find that the achieved phase lag is equal to the phase lag between the two pairs of flow vortices, which are alternatively shed into the flow from the upper and lower hinged plates. Similar to past FIO problems, the current model also exhibits two important lock-in and phase-switch FSI phenomena; however, in angular directions. There is a phase jump of approximately 170° between the aerodynamic lift coefficient and angular oscillations of hinged plates, which nearly occurs in the middle of lock-in region. Indeed, our literature review shows that this is the first time to report the phase-switch phenomenon in angular oscillations of three-element bluff bodies in a FSI problem. [ABSTRACT FROM AUTHOR]

An adaptive fuzzy sliding mode control (AFSMC) scheme is applied to actively suppress the two-dimensional vortex-induced vibrations (VIV) of an elastically mounted circular cylinder, free to move in in-line and cross-flow directions. Laminar flow regime at Re=90, low non-dimensional mass with equal natural frequencies in both directions, and zero structural damping coefficients, are considered. The natural oscillator frequency is matched with the vortex shedding frequency of a stationary cylinder at Re=100. The strongly coupled unsteady fluid/cylinder interactions are captured by implementing the moving mesh technology through integration of an in-house developed User Define Function (UDF) into the main code of the commercial CFD solver Fluent. The AFSMC approach comprises of a fuzzy system designed to mimic an ideal sliding-mode controller, and a robust controller intended to compensate for the difference between the fuzzy controller and the ideal one. The fuzzy system parameters as well as the uncertainty bound of the robust controller are adaptively tuned online. A collaborative simulation scheme is realized by coupling the control model implemented in Matlab/Simulink to the plant model constructed in Fluent, aiming at determination of the transverse control force required for complete suppression of the cylinder streamwise and cross-flow oscillations. The simulation results demonstrate the high performance and effectiveness of the adopted control algorithm in attenuating the 2D-VIV of the elastic cylinder over a certain flow velocity range. Also, the enhanced transient performance of the AFSM control strategy in comparison with a conventional PID control law is demonstrated. Furthermore, the effect of control action on the time evolution of vortex shedding from the cylinder is discussed. In particular, it is observed that the coalesced vortices in the far wake region of the uncontrolled cylinder, featuring the C(2S)-type vortex shedding characteristic mode, are ultimately forced to switch to the classical von Kármán vortex street of 2S-type mode, displaying wake vortices of moderately weaker strengths very similar to those of the stationary cylinder. Lastly, robustness of AFSMC is verified against relatively large structural uncertainties as well as with respect to a moderate deviation in the uniform inlet flow velocity. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the effects of controlled damping on the amplitude and frequency response profiles of an elastically mounted cylinder in cross-flow. The dimensionless damping parameter, , which is closely related to the traditional “mass-damping” parameter, , was varied over a wide range of values through the use of a variable magnetic eddy current damping system. For low damping and sufficiently high Reynolds number we observe the previously described large-amplitude, three-branch (initial, upper, lower) response profile, and for high damping or low Reynolds number we observe the small-amplitude, two-branch (initial and lower) response profile. However we find that, because of the influence of Reynolds number, the traditional labels of “high mass-damping” and “low mass-damping” are incomplete with regard to predicting a large or small-amplitude response profile. In our experiments, as damping is systematically increased, we observe a transition between these two profiles characterized by a gradual “erosion” and eventual disappearance of the large-amplitude section (upper branch) and the scaling down of the lower branch region. We find that jumps from the upper to the initial branch originate on the 2S/2P boundary in the Williamson–Roshko plane. Another new finding is a hysteresis between the lower branch and the desynchronized region, which only appears at low Reynolds numbers. We also explore changes in the frequency response profile, which are connected with the changes in the amplitude profile, for our upper branch cases. We observe that analogous to the three amplitude branches, there are three distinct branches for the frequency response. [Copyright &y& Elsevier]

Abstract: The response of a freely oscillating circular cylinder (“free vibration”) in cross-flow has been studied experimentally using controlled magnetic eddy current to provide variable damping. In general, the nondimensional response amplitude, A*, and dominant frequency, ω*, depend on the Reynolds number, Re, and the nondimensional mass, m*, damping, b*, and elasticity, k*, of the system. The main objective of this study is to characterize the maximum amplitude that is achieved for a given system as cross-flow velocity is varied. We find that this maximum amplitude, , occurs within a small range of values of . For values of Reynolds number in the range 525

Vortex-induced oscillations offer a potential means to harness hydrokinetic energy even at low current speeds. In this study, we consider a novel converter where a cylinder undergoes angular oscillations with respect to a pivot point, in contrast to most previous configurations, where the cylinder undergoes flow-induced oscillations transversely to the incident free stream. We formulate a theoretical model to deal with the coupling of the hydrodynamics and the structural dynamics, and we numerically solve the resulting nonlinear equation of cylinder motion in order to assess the performance of the energy converter. The hydrodynamical model utilizes a novel approach where the fluid forces acting on the oscillating cylinder are split into components acting along and normal to the instantaneous relative velocity between the moving cylinder and the free stream. Contour plots illustrate the effects of the main design parameters (in dimensionless form) on the angular response of the cylinder and the energy efficiency of the converter. Peak efficiencies of approximately 20% can be attained by optimal selection of the main design parameters. Guidelines on the sizing of actual converters are discussed. [ABSTRACT FROM AUTHOR]

Flow-induced oscillations of three identical circular cylinders, arranged in tandem configuration, are studied via two-dimensional finite element computations. Cylinders are mounted on elastic supports in the streamwise and transverse directions. Each downstream cylinder is placed at a distance of 5 diameters (5 D) from its immediate upstream neighbor. Nondimensional mass ( m ∗) of 2, and zero damping (ξ) are considered. Simulations are performed with the objective of understanding dynamic response of cylinders, as they are subject to wake interference, at constant Reynolds number, R e = 150 , and for varying reduced velocity (U ∗) in the range of U ∗ = 2 − 14. Similar to the behavior of an isolated cylinder at low R e , oscillation response of the upstream body exhibits initial excitation and lower lock-in regimes. However, for both the downstream cylinders, depending on their amplitude and frequency responses, the synchronization regime of U ∗ can be categorized into three distinct regions: initial excitation, upper and lower lock-in regions. First downstream cylinder undergoes gradual transition between initial excitation and upper region, whereas the second downstream body exhibits this transition in two phases, which are separated by a periodic oscillation response regime. Upper lock-in region is characterized by periodic oscillations, while amplitude is modulated in the lower region. Only 2S shedding mode is observed for all the U ∗ considered. [ABSTRACT FROM AUTHOR]