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.

Vortex-induced vibrations (VIVs) are highly nonlinear and it is hard to approach the problem analytically or computationally. Experimental investigation is therefore essential to address the problem and reveal some physical aspects of VIV. Although computational fluid dynamics (CFDs) offers powerful methods to generate solutions, it cannot replace experiments as yet. When used as a supplement to experiments, however, CFD can be an invaluable tool to explore some underlying issues associated with such complicated flows that could otherwise be impossible or very expensive to visualize or measure experimentally. In this paper, VIVs and galloping of a cylinder with selectively distributed surface roughness—termed passive turbulence control (PTC)—are investigated experimentally and computationally. The computational approach is first validated with benchmark experiments on smooth cylinders available in the literature. Then, experiments conducted in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan are replicated computationally to visualize the flow and understand the effects of thickness and width of roughness strips placed selectively on the cylinder. The major outcomes of this work are: (a) Thicker PTC initiates earlier galloping but wider PTC does not have a major impact on the response of the cylinder and (b) The amplitude response is restricted in VIV due to the dead fluid zone attached to the cylinder, which is not observed in galloping.

In the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan, Flow Induced Motion (FIM) is studied as a means to convert marine hydrokinetic energy to electricity using the VIVACE energy harvester [1–4]. Turbulence stimulation in the form of sand-strips, referred to as Passive Turbulence Control (PTC), were added to oscillating cylinders in 2008 [5]. PTC enabled VIVACE to harness hydrokinetic energy from currents/tides over the entire range of FIM including VIV and galloping. In 2011, the MRELab produced experimentally the PTC-to-FIM Map defining the induced cylinder motion based on the location of PTC [6]. In 2013, the robustness of the map was tested and dominant zones were identified [7]. Even though the PTC-to-FIM Map has become a powerful tool in inducing specific motions of circular cylinders, several parameters remain unexplored. Experiments, though the ultimate verification tool, are time consuming and hard to provide all needed information. A computational tool that could predict the FIM of a cylinder correctly would be invaluable to study the full parametric design space. A major side-benefit of PTC was the fact that PTC enabled computational fluid dynamic (CFD) simulations to generate results in good agreement with experiments by forcing the location of the separation point [8]. This valuable tool, along with experiments, is used in this paper to investigate PTC design parameters such as width and thickness and their impact on flow features with the intent of maximizing FIM and, thus, hydrokinetic energy conversion.