Research On Flow Induced Motion Of Multiple Circular Cylinders With Passive Turbulence Control

The vortices shed from a bluff body is a kind of widespread physical phenomena,which involves many complex flow mechanisms. Vortex shedding results in fluctuatingforces acting on the bluff body, which can cause vibrations when the bluff body iselastically mounted or allowed deformation. This vibration further changes the nature ofthe vortex formation. The interaction between the fluid and structure is called FlowInduced Motion (FIM). FIM is typically treated as destructive phenomenon because ofthe fatigue damage may be potentially introduced. With entirely opposite objective ofprevious efforts, which are mainly focused on reducing FIM effects, VIVACE (VortexInduced Vibration for Aquatic Clean Energy) converter is designed to generate powerby utilizing this potentially disastrous phenomenon. With the rapid development ofocean engineering, wind engineering, aerospace, and nuclear engineering, FIM ofmultiple cylinders has attracted a wide spread attention and becomes one of theimportant subjects in the theory and engineering applications of the hydrodynamics. Butthe research on this topic is still in the stage of exploration and needs to be intensivelystudied. Research on FIM of multiple cylinders has great significance in theory andapplication.In this thesis, the FIM of multiple cylinders with Passive Turbulence Control (PTC)are simulated using2-Dimensional Unsteady Reynolds-Averaged Navier-Stokesequations with the Spalart-Allmaras one equation turbulence model. Numerical resultsare compared with experiments conducted in the Marine Renewable Energy Lab(MRELab) of the University of Michigan in the USA. The effects of the parameters ofPTC-cylinders on the vortex shedding, wake vortex patterns, characteristics of bodymotion, hydro-oscillation force, and energy conversion are investigated in the presentstudy. The mechanisms of FIM, such as VIV and galloping, are explored as well. Theenergy conversion efficiency of VIVACE converter is estimated.Firstly, the flow and body kinematics of the transverse motion of a spring-mountedcircular cylinder are investigated. The simulated Reynolds number range for whichexperiments were conducted in the MRELab is30000<Re<130000, which falls in thehigh-lift TrSL3regime, where TrSL indicates Transition in Shear Layer. PTC in theform of selectively distributed surface roughness is used to alter the cylinder FIM. Theamplitude-ratio curve in the simulation results clearly shows three different branches, including the VIV initial and upper branch, and a galloping branch. The numericalbranches are similar to those observed experimentally. The number of the vortices shedfrom the PTC-cylinder in one oscillating cycle increases with flow velocity. Power isharnessed in the entire test ranges of Reynolds number. The optimal energy conversionefficiency of VIVACE is obtained in the starting area of VIV upper branch andgalloping in the simulation results.Secondly, the FIM of two rigid circular cylinders, on end linear-springs, in tandemare numerically studied and verified by experimental data. The key point in thesimulations is the utilization of Topological Mesh Changes combined with General GridInterface. PTC is being used to enhance FIM of cylinders in the VIVACE Converter toincrease its efficiency and power density in harnessing marine hydrokinetic energy. Theamplitude-ratio and frequency results in CFD are in excellent agreement withexperimental data showing the initial and upper branches in VIV, transition from VIV togalloping, and galloping. Vortex structures are studied using high-resolution imagingfrom the CFD results showing typical2S structure in the initial branch and both2P+2Sand2P in the upper branch of VIV. In the galloping branch, amplitudes of3.5diametersare reached before the channel stops are hit.Furthermore, a series of simulations are performed for validating the ability of thecode to predict hydrodynamic loads and response of three spring-mountedPTC-cylinders undergoing flow induced motion using2-D URANS. Validation isperformed by comparing simulation results to the experiments. The results show that thecylinder FIM is enhanced with the application of PTC, the flow over the cylindersurface is altering in a way that generates higher lift which is shown better synchronizedwith the motion. The classical linear viscous damping model used in the simulationsmatches well with the physical damping model because the velocity of oscillations isnot near zero when the PTC-cylinder is in the VIV upper branch or galloping. Forcylinder in FIM, the transition between branches is accompanied by vortex patternchange. In galloping, the driving mechanism is not based on the alternating vortices buton the lift instability caused by negative damping due to the lift force induced by theasymmetry of the geometry of the circular cylinder due to the turbulence stimulation.Finally, the flow induced motions of four cylinders with PTC in tandem are studiedby numerical simulation and experimental analysis. The study results show that thereason for the successful numerical prediction of the experimental results lies in theapplication of the turbulence stimulation in the form of the PTC. The VIV initial branch and upper branches, transition from VIV to galloping, and galloping are observed for allcylinders. The four-cylinder system extracts so much hydrokinetic energy from the fluidflow that a significant drop in free surface level occurs between the first and fourthcylinders. As the velocity increases, so does the response amplitude, making thecylinders reach closer to the free surface and resulting in the strong fluctuations exhibit.For Re=50000, the upstream vortices shedding from the1stcylinder directly and closelyinteract with the downstream cylinders. Especially for the2ndPTC-cylinder, the liftforce is weakened which results in low amplitude and low frequency vibration. Thevariation trends of oscillation frequency for the2/3/4cylinders are similar. The numberof vortex shed from the upstream cylinder is always more than that from thedownstream one, which leads to a more complex vortex pattern for the upstreamcylinder.