Research on vortex-induced vibrations (VIV) mainly involves experimental science but building laboratory setups to investigate the flow are expensive and time consuming. Computational fluid dynamics (CFD) methods may offer a faster and a cheaper way to understand this phenomenon depending on the solution approach to the problem. The context of this paper is to present the author's computational approach to solve for vortex-induced vibrations which cover extensive explanations on the mathematical background, the grid structure and the turbulence models implemented. Current computational research on VIV for smooth cylinders is currently restricted to flows that have Reynolds numbers below 10,000. This paper describes the method to approach the problem with URANS and achieves to return satisfactory results for higher Reynolds numbers.The computational approach is first validated with a benchmark experimental study for rather low Reynolds number which falls into TrSL2 flow regime. Then, some numerical results up to = 130,000, which falls into TrSL3 flow regime,are given at the end of the paper to reveal the validity of the approach for even higher Reynolds numbers.
Incompressible flow assumption is an essential step that simplifies numerical simulations for objects inside flowing water. However, incompressibility assumption creates a conflicting situation for sound propagation as the propagation speed is given by 0 = √ ⁄ where denotes the pressure and denotes the density. When = 0 is assumed to relieve simulations, speed of sound theoretically becomes infinite and therefore, induced pressures should be corrected with the acoustic analogy. This approach is called the "hybrid method" that combines hydrodynamic solver with hydroacoustic solver. Hydroacoustic solver adds compressibility effects to the incompressible hydrodynamic solver and uses hydrodynamic pressure to calculate acoustic pressure. Time step size is an important parameter to calculate acoustic pressure field in the fluid domain and an approach to determine the minimum value (for at least capturing the first blade passage frequency) is presented in this study. Another purpose of this paper is to investigate the effect of incompressibility on hydroacoustics and to analyze the necessity of utilizing the famous Ffowcs Williams-Hawkings (FWH) equation for predicting marine propeller noise; both for cavitating and non-cavitating cases. Our results are in line with other researchers; hydrodynamic pressure is sufficient to assess the hydroacoustic performance of marine propellers in the near-field due to having very low acoustic Mach numbers. Near-field results from the hydrodynamic solver are then extrapolated to the far-field by adopting ITTC distance normalization equation. However; this equation, which is actually the inverse distance law, is only valid for point noise sources in stationary flow. It is found out that eliminating FWH equation by coupling the incompressible hydrodynamic solver with ITTC distance normalization equation fails to produce satisfactory results. For cavitating cases, numerical results in this study show that implementation of FWH is required even in the near-field.
In two-dimensional experimental setups, tip-flow cannot be eliminated completely. In one degree-of-freedom Flow Induced Motions (FIM) of circular cylinders placed perpendicular to a uniform flow, three-dimensional effects may become significant. An ideal setup extends the cylinder to the limits of the flow-channel to minimize tip vortices, which reduce the effective length of the cylinder. Depending on how close to two-dimensional the experimental setup is, obtained results may differ. It is difficult to avoid the tip-flow in nature as well. Applications involving Vortex-Induced Vibrations (VIV) have more or less three-dimensional flow characteristics and one of the manifestations of three-dimensionality is the tip-flow. In this paper, the effects of tip-flow on VIV are investigated both experimentally and computationally. It is found that the tip-flow reduces the lift force exerted on the cylinder and narrows down the range of synchronization. Two-dimensional computational simulations become insufficient to grasp the effects of the tip-flow for a cylinder in VIV as the Reynolds number increases. Computational results for vortex-induced vibrations at these relatively high Reynolds numbers (up to 1.2 * 10 5) in the TrSL3 flow regime are not satisfactory when compared with experimental results. To improve the CFD predictions by introducing three-dimensional (3D) flow characteristics in a two-dimensional (2D) computational environment, a parameter called tip-flow correction factor is defined and analyzed. This parameter is introduced to compensate for any deviations from 2D flow approximation that might arise due to the 3D nature of the flow. The tip-flow correction factor is implemented as a multiplier of the force term in the vibration equation to represent the lift-force losses caused by the tip vortex. When compared to the results obtained with straightforward use of the vibration equation, it is found that the tip-flow correction factor improves the agreement between 2D computational results and experimental measurements. This 3 method extends the validity of 2D-URANS simulations at least up to = 1.2 * 10 5 for which experimental results are available in this study.
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