Numerical Investigation On Flow-induced Vibrations Of Multiple Circular Cylinders In Tandem Arrangement At A Low Reynolds Number

Sea currents and tides can provide an abundant of clean and renewable energy. Inthe past decades, many researchers studied the optimal designs of sea current and tidalenergy converters. Globally, the flow velocity of sea currents is generally below1.5m/s and conventional tidal energy converters utilizing rotating blades to generateelectricity cannot be started. M.M. Bernitsas and K. Raghavan in the University ofMichigan invented a novel sea current energy harvesting device——Vortex InducedVibration Aquatic Clean Energy，VIVACE. The VIVACE converter is based on theidea of extracting energy from the flow-induced vibrations of a circular cylinder or acircular cylinder array and enhancing their amplitude and velocity to improve theenergy conversion rate. However, due to the insufficient understanding on theflow-induced vibrations of a cylinder array at present, the theoretic system is far fromcomplete and perfect, which determines the development and application of theVIVACE converter. In this paper, taking the VIVACE converter as the researchbackground, the flow-induced vibrations of three circular cylinders in a tandemarrangement was numerically investigated and reasonable and systematicalexplanations on the underlying mechanisms of flow-induced vibrations of circularcylinders were presented.The parallel code adopted in this study is CgLes_IBM which has a second orderaccuracy in both time and space. This code combines the direct numerical simulationof turbulent flow and the iterative immersed boundary method on the purpose ofsolving the fluid-structure interaction problems.The contents of this paper includes is the flow-induced vibrations of threecircular cylinders in a tandem arrangement at a low Reynolds number of Re=100.Three cylinders can only oscillate freely in the cross-flow direction and thenon-dimensional mass of the cylinders ism*=2. The structural damping coefficient isset to zero to obtain the max vibration amplitude. The streamwise spaces between thecylinders are equal and the spacing ratio is L/D=2.0,3.0,4.0and5.0and the rangeof the reduced velocities (U r) is from3to10. The results include the lift force, thedrag force and the amplitude of oscillation in the cross-flow direction. Then, thevariation characteristics of the non-dimensional shedding frequency, lock-in, the dragand lift force coefficients and the amplitude of cross-flow oscillations versus thereduced velocity and the spacing ratio were investigated. The phase between the cross-flow oscillation and the lift force and the one between the cross-flow oscillationof three cylinders were also analyzed. It is found that the phase between lift force andcross-flow oscillation changes from0°to180°rapidly at the reduced velocity ofU r=8~10. Complex flow patterns were observed in the wake of three cylinders andtheir effects on the lift and drag forces and cross-flow responses of the cylinders werestudied.