The flow-induced vibration (FIV) of a cylinder cluster is often observed in subsea pipelines and marine risers. The motion of the cylinder clusters was often modeled as the equation of motion of a particle, which has ignored the torsion freedom. Hence, the answers to the following questions are still unavailable—what are the impacts of torsion on the FIV of a cylinder cluster? Does torsional freedom facilitate the development of new features in FIV? In the present work, the FIV of a two-tandem cylinder system was numerically studied under the consideration of torsional freedom. It was found that six well-defined regimes were formed in the FIV of the cylinder system with the increase in the reduced velocity. The formation of different regimes mainly occurred by the accouplement of two frequencies—the vortex shedding frequency and the natural frequency of the cylinder system. A quirky low frequency dominated the cross-flow vibration at some reduced velocities, significantly magnifying the cross-flow vibration and the torsional vibration. The two cylinders acted as two feet treading water alternatively, enabling the cylinder system to move further on one side at such low frequencies. The characteristics of the amplitudes and frequency responses vs Ur* were qualitatively similar when the stiffness ratio k* was in the range of 10 ≤ k* ≤ 50, where Ur* was the torsional reduced velocity which was defined by the uniform velocity U divided torsional natural frequency frn and diameter of the cylinder D. 

Study on the flow-induced vibration of a two-tandem cylinder system in rigid body motion

In this study, the effect of asymmetric wake flow regime of two side-by-side cylindrical bluff bodies on power output is experimentally examined by using a piezoelectric flag. Different synchronization modes of the flag with wake flow are observed. It is demonstrated that the streamwise gap between the flag and cylinders (Gx), and the center-to-center gap between cylinders have a significant impact on the flag's dynamical behavior that results in a fluctuation in the power output of the piezoelectric flag. The levels of output power are analyzed by varying the Gx and the cross-stream or lateral gap (N/d) between the two cylinders. N/d values from 1.0 to 2.0 for different values of Gx (2.0 ≤ Gx ≤ 4.0) are experimentally tested. The comparison of the flapping response at each point is made to ascertain the impact of the harvester's dynamic behavior on the output energy. The power generated at each point is recorded for all cases and a comparative analysis is made to find the optimal configuration. Limited research is conducted in the past to enhance the energy output by using the bluff body with the improved wake dynamics. Hence, two cylinders are employed in a uniform flow and crosswise gap between cylinders is varied to change the characteristics of the wake region. The cylinder arrangement with N/d = 1.0, shows continuous oscillations and higher output power persisting for 2.0 ≤ Gx ≤ 4.0. The monotonic rise in power output is observed till Gx = 4.0. The stated configurations with N/d = 1.0 gives a significant advantage over a single-cylinder-based energy harvester as a kinetic source of fluid energy harvester from the flowing fluid. The output power became almost doubled with an increase of 95% approximately using side-by-side arrangement. 

Synergistic analysis of wake effect of two cylinders on energy harvesting characteristics of piezoelectric flag

Abstract The flow around a cylinder with slits has been investigated numerically. The main purpose of this study is to show the effect of the slits on the vortex detachment behind a cylinder smooth, and stationary using the finite element approach to solve the fluid governing equations system at Re = 200. The variation of the spacing ratio (D/S) of the slit is strongly influenced both the various physical parameters and the vortex detachment behind a cylinder. We found that the drag coefficient varies between 1.43 and 1.24 in the case of the horizontal slit and between 1.45 and 1.39 in the case of the vertical slit. In the case of the double horizontal-vertical slit, the values vary between 1.47 and 1.38. We also found that the values of the Strouhal number vary between 0.201 and 0.268 according to the spacing ratio (D/S) of the slit. A comparison was made with experimental works for the Strouhal number. The results are very important, and the error found is less than 7%. 

Numerical simulation of the flow around circular cylinder with slits

In this work, nonlinear radiative flow over a vertical cylinder moving with a nonlinear velocity is presented. The mathematical model of the problem is formulated to include many parameters such as the thermal radiation parameter, the nonlinearity parameter, the Prandtl number, etc. Mathematica program is built to numerically solve the system of ordinary differential equations together with boundary conditions that is derived from the partial differential equations governing the fluid motion through using suitable similarity transformations. Numerical results obtained for the case of linear cylinder velocity are compared with analytical results obtained for the same case to validate the numerical method used in this work. Profiles of the fluid velocity and fluid temperatures are introduced and discussed to explore different parameters effects.

Nonlinear Radiative Flow over a Vertical Cylinder Moving with Nonlinear Velocity

Received: 3 April 2019 Accepted: 26 December 2019 The fluid flow around a cylinder is one of the classic issues in fluid mechanics because of its various applications. Cylindrical structures, both single and in the group patterns, are present in the design of cooling systems of nuclear power plants, hydro-structures, heat exchangers, chimneys, high buildings, power lines, cables and networks in air and water. In many engineering applications, Karman's vortex shedding produces flow-induced vibrations. To comprehend the fluid structures surrounding the cylinders, it is vital to understand some fundamental issues such as boundary layers, flow separation, free shear layer, sequence and dynamics of vortices. According to the limited studies conducted for the vertical arrangement of two cylinders in a flow with the high Reynolds number, there is no evidence of oblique flow or bi-stable pattern except in the case where cylinders are close to each other. In this paper, the impact of the vertical arrangement of two cylinders on flow with high Reynolds number has been investigated via numerical modelling. The results indicated that the flow pattern is irregular and unstable for denser arrangements while the propagation of vortices does not have any tendency in different spacing.

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Yaser Esmaeeli

Papers

Numerical Simulation of Flow Around Two Side-by-Side Circular Cylinders at High Reynolds Number