Two-degree-of-freedom flow-induced vibration of a rotating circular cylinder

Flow past three identical circular cylinders is numerically investigated using the immersed boundary method. The cylinders are arranged in an equilateral-triangle configuration with one cylinder placed upstream and the other two side-by-side downstream. The focus is on the effect of the spacing ratio , Reynolds number and three-dimensionality on the flow structures, hydrodynamic forces and Strouhal numbers, where is the cylinder centre-to-centre spacing and is the cylinder diameter. The fluid dynamics involved is highly sensitive to both and , leading to nine distinct flow structures, namely single bluff-body flow, deflected flow, flip-flopping flow, steady symmetric flow, steady asymmetric flow, hybrid flow, anti-phase flow, in-phase flow and fully developed in-phase co-shedding flow. The time-mean drag and lift of each cylinder are more sensitive to than while fluctuating forces are less sensitive to than . The three-dimensionality of the flow affects the development of the wake patterns, changing the ranges of different flow structures. A diagram of flow regimes, together with the contours of hydrodynamic forces, in the space, is given, providing physical insights into the complex interactions of the three cylinders.

Numerical simulations of flow past three circular cylinders in equilateral-triangular arrangements

This study performs a comprehensive parametric study and optimization of a piezoelectric wind energy harvester subjected to the conjunction of vortex-induced vibration (VIV) and galloping. The mathematical model of the harvester undergoing both the VIV and galloping is formulated, which is experimentally validated through the wind tunnel tests. The experimental results uncover that different combinations of the aerodynamic parameters of the model significantly influence the system dynamic behaviors. Changing the bluff body's cross-section shape can alter the aerodynamic parameters. In the experiment, a unique hump phenomenon due to the coupled VIV and galloping is discovered, which significantly improves the voltage output. The influence of each aerodynamic parameter on the system performance is revealed, and the aerodynamic parameter's effect is also mathematically interpreted by the terms in the governing equations. Finally, this study presents an effective structural optimization method based on the genetic algorithm (GA) to seek the optimal structural natural frequency matching up the given aerodynamic parameters. Results show that the piezoelectric wind energy harvester with the optimal designed natural frequency outperforms the harvester with other values, and the performance is also robust against a certain range of the parametric uncertainties.

Piezoelectric wind energy harvesting subjected to the conjunction of vortex-induced vibration and galloping: comprehensive parametric study and optimization

The flow-induced vibration (FIV) of multiple cylinders is a common phenomenon in industry and nature. The FIV and energy harvesting of three circular cylinders in tandem are numerically studied by 2D-URANS simulations in Reynolds number range of 30,000 < Re < 105,000. Simulation results match well with experiments in the tested cases. Four branches of FIV are clearly captured in the amplitude and frequency ratio curves of the three cylinders with roughness, including initial branch of vortex-induced vibration (VIV), VIV upper branch, transition from VIV to galloping, and galloping. It is shown that the vortices from downstream cylinder are strongly disrupted and modified by vortices of upstream cylinder. The third cylinder is almost suppressed in VIV initial branch. The 2P vortex pattern is observed for the first cylinder in the VIV upper branch. For Re = 90,000 in the transition regime, the vortex patterns of the first and second cylinders are 2P + 4S and 2P + 2S, respectively. In the galloping branch, the shear layer motion is in synchronization with the motion of the cylinder, and the maximum amplitude of 2.8D is reached by the first cylinder. The total converted power of the three cylinders increases with U*water both in the simulation and experiment. For the three cylinders, the maximum power reaches up to 85.26 W with the increase of Reynolds number. The energy conversion efficiency is stable and higher than 35% in the starting region of VIV upper branch, and the maximum value of 40.41% is obtained when Re = 40,000.

Research on Flow-Induced Vibration and Energy Harvesting of Three Circular Cylinders with Roughness Strips in Tandem

Effects of flow-induced vibration on forced convection heat transfer from two tandem circular cylinders in laminar flow

Vortex-induced vibrations of three tandem cylinders in laminar cross-flow: Vibration response and galloping mechanism

Vortex-induced vibrations of dual-step cylinders in laminar flows at the Reynolds number of 200 based on a larger diameter are investigated through three-dimensional direct numerical simulations. Four larger-to-smaller diameter ratios (D/d) of 4.0, 2.0, 1.43, and 1.19 are considered for the cylinder with a low mass ratio of 2. Numerical results reveal that D/d significantly influences vibration responses and the vortex shedding process. The case with D/d = 1.43 can effectively reduce vibration amplitudes. The wake vortices are shed in cellular patterns along the cylinder span, with a distinct frequency for each cell. A direct and “X-shaped” connection of wake vortex-shedding modes occurs between different vortex cells, and a loop connection appears between the same cell vortices. A new wake pattern entitled the “out-of-phase vortex shedding” is found downstream of the smaller cylinder at D/d = 2.0. This is manifested by the wake vortices not being shed at a fixed frequency and intensity. The essential reason for this phenomenon lies in a non-lock-in condition between the structural vibration and vortex shedding frequencies of the smaller cylinder. As D/d decreases, a coherence between vortex cells is enhanced, leading to a disappearance of the “out-of-phase vortex shedding.”

Vortex-induced vibrations of dual-step cylinders with different diameter ratios in laminar flows

Flow around a dual step cylinder for D/d = 2 and L/D = 5 at a low Reynolds number of ReD = 200 is numerically investigated utilising the immersed boundary method. The vortex dislocation caused by two steps is investigated. It was found that a pair of streamwise vortex - the centre vortex exists at the downstream of the large cylinder. Meanwhile, due to the barrier effect of the centre vortices, a very weak L-cell exists behind the centre of the large cylinder. In a complete dislocation cycle, three positive and negative half-loop connections appear alternately. This cycle is related not only to the phase realignment of the different vortex cells, but also to the connection realignment. Moreover, the lateral velocity is always dominated by the vortex shedding frequency. While in the streamwise velocity spectra, the vortex shedding frequency gradually dissipates in the downstream, leaving lower frequencies of dislocation.

Vortex dislocation characteristics of a dual-step cylinder in low-Re flow

Flows around a dual step cylinder are numerically investigated at a Reynolds number of 200 for three diameter ratios (D/d) of 2, 1.43 and 1.19. Simulation results reveal three distinctive vortex shedding modes: (i) the N-S mode for D/ d = 2 , (ii) the transition mode for D/ d = 1 . 43 , and (iii) the L-S mode for D/ d = 1 . 19 . The vortex shedding frequency transition from the smaller-diameter cylinder to the larger-diameter one, the dominant shedding frequency downstream of the larger cylinder, and the vortex interactions exhibit significant differences in the three vortex shedding modes. As D/ d = 2 , the direct cross-boundary and half-loop connections take place downstream of the step where vortex dislocations are essentially fixed. When decreasing D/d, two new vortex interactions, the double-half-loop and three-half-loop connections, appear with a larger spanwise vortex dislocation occurrence range. In addition, flows in different D/d cases are governed by a distinctive vortex shedding law. A pair of streamwise vortices occur downstream the centroid of the larger cylinder. This reduces the cylinder step effect on the local vortex shedding, absorbing the shear layer of the spanwise N-cell vortices, and making the hydrodynamic force coefficients smaller. Interestingly, these fluid force coefficients further decrease as D/d increases.

Numerical simulations of flows around a dual step cylinder with different diameter ratios at low Reynolds number

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Papers

Vortex-induced vibrations of three tandem cylinders in laminar cross-flow: Vibration response and galloping mechanism

Vortex-induced vibrations of dual-step cylinders with different diameter ratios in laminar flows

Vortex dislocation characteristics of a dual-step cylinder in low-Re flow

Numerical simulations of flows around a dual step cylinder with different diameter ratios at low Reynolds number