Computational Fluid Dynamics Study of a Flexible Flapping Hydrofoil Propulsor

A CFD study to understand the hydrodynamics and fluid flow around a chordwise flexible hydrofoil with combined sway and yaw motion which imitates the caudal fin flapping in thunniforms, is presented. The dependency of motion parameters of the flexible flapping hydrofoil to its propulsive performance is studied by carrying out the analyses over a Strouhal number range of 0.1 to 0.4 in steps of 0.025 at three maximum angle of attacks viz. 10°,15°,20°. Qualitative observations of the wake field and trailing jet is presented using velocity magnitude contours and vorticity contours. The analyses carried out at 40,000 Reynolds number and sway amplitude of 0.75 chordlength, revealed that the average thrust coefficient increases with increase in Strouhal number and maximum angle of attacks. The highest efficiency is achieved when the maximum angle of attack is 15° and Strouhal number is 0.225.

On the Effects of Turbulence Modeling on the Fluid-Structure Interaction of a Rigid Cylinder

The cylinder flow is a canonical problem for Computational Fluid Dynamics (CFD), as it can display several of the most relevant issues for a wide class of flows, such as boundary layer separation, vortex shedding, flow instabilities, laminar-turbulent transition and others. Several applications also display these features justifying the amount of energy invested in studying this problem in a wide range of Reynolds numbers. The Unsteady Reynolds Averaged Navier Stokes (URANS) equations combined with simplifying assumptions for turbulence have been shown inappropriate for the captive cylinder flow in an important range of Reynolds numbers. For that reason, recent improvements in turbulence modeling has been one of the most important lines of research within that issue, aiming at better prediction of flow and loads, mainly targeting the three-dimensional effects and laminar-turbulent transition, which are so important for blunt bodies. In contrast, a much smaller amount of work is observed concerning the investigation of turbulent effects when the cylinder moves with driven or free motions. Evidently, larger understanding of the contribution of turbulence in those situations can lead to more precise mathematical and numerical modeling of the flow around a moving cylinder. In this paper, we present CFD calculations in a range of moderate Reynolds numbers with different turbulence models and considering a cylinder in captive condition, in driven and in free motions. The results corroborate an intuitive notion that the inertial effects indeed play very important role in determining loads and motions. The flow also seems to adapt to the motions in such a way that vortices are more correlated and less influenced by turbulence effects. Due to good comparison of the numerical and experimental results for the moving-cylinder cases, it is observed that the choice of turbulence model for driven and free motions calculations is markedly less decisive than for the captive cylinder case.

Numerical Modelling of Breaking Wave Interaction With a Group of Four Vertical Slender Cylinders in Square Arrangements

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters and an accurate numerical modelling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source (Computational Fluid Dynamics) CFD model REEF3D is used to simulate the breaking wave interaction with the multiple cylinders. The numerical model is based on the incompressible Reynolds Averaged Navier-Stokes (RANS) equations, the level set method for the free surface and the k–ω model for turbulence. The model uses a 5th-order conservative finite difference WENO scheme for the convective discretization and a 3rd-order Runge-Kutta scheme for time discretization. The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is seen between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. The breaking wave forces on each cylinder in the group are computed for the two cases and the results are compared with the breaking wave force on a single isolated cylinder. Further, the study investigates the water surface elevations and the free surface flow features around the cylinders. In general, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. The results of the present study show that the interference effects from the neighbouring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.

Passing Ship Effects in Non-Uniform Water Depth

Waves generated by passing ships have potential adverse impacts on the environment (beach erosion, ecological disturbance, structures damage) and other waterway users (navigations, moored ships) in the coastal and sheltered areas. But issues related to waves of ships were addressed rarely in China until now. Accurate prediction of wash waves is the first step to control the washes from passing ships and it’s significant to reduce the effects of washes. A coupled method is used in this paper to simulate the washes and its effects caused by the passing ship. A potential flow theory method is adopted as the stationary wave generation model; a non-hydrostatic wave flow model is used as the wave propagation model; a time domain method is chosen as the model for simulating the forces and moments of mooring ship. The waves calculated by a potential flow theory method in the near field are used as the input for the non-hydrostatic wave-flow model to obtain the far field waves. A time-domain representation of the wave-cut at the location of the passing vessel is transformed to the frequency-domain and is used as the input for the diffraction computations. Parts of the calculated results are validated experimentally, satisfactory agreement is demonstrated.

Numerical and Experimental Investigations of Ship Maneuvers in Waves

Assuring a ship’s maneuverability under diverse conditions is a fundamental requirement for safe and economic ship operations. Considering the introduction of the Energy Efficiency Design Index (EEDI) for new ships and the related decreasing installed power on ships, the necessity arose to more accurately predict the maneuverability of ships in severe seas, strong winds, and confined waters. To address these issues, extensive experimental and numerical investigations were performed within the European funded Project SHOPERA. Here, second order forces and moments for a containership and a tanker were measured in model tests and computed by solving the Reynolds-Averaged Navier-Stokes (RANS) equations. Generally, these measured and computed second order loads (drift forces and yaw moments, added resistance) compared favorably. Furthermore, the effects of waves on zig-zag and turning circle maneuvers were investigated.

Vortex-Induced Motion of a Square Cylinder at Moderate Reynolds Numbers

Bluff body structures exposed to ocean current can undergo vortex-induced motion (VIM) for certain geometric and physical conditions. Recently, the study of VIM has been gaining attention for many engineering applications, in particular offshore structures such as buoys, FPSOs, semi-submersibles, Spars and TLPs. The present work is a part of a systematic effort to investigate the VIM response of multi-columns floating platform. In real sea condition, floating platforms are in high Reynolds numbers region and flow patterns around structures are turbulent in nature. For the purpose of investigating and simulating accurately the nonlinear dynamic processes of vortex shedding, transport and wake interactions with the bluff body, the fundamental study of VIM around a square column at moderate Reynolds numbers (1500 ≤ Re ≤ 14000) is firstly investigated. In the present work, the transient flow pattern around a free vibrating square cylinder at moderate Reynolds numbers is numerically investigated by an open source CFD toolbox, OpenFOAM. Good consistency and agreement are found between the present numerical findings and that of experiments. The cylinder, with a blockage area of 4.2%, is mounted on an elastic support for free vibration in the transverse direction. Hybrid RANS-LES turbulence models are considered here for accurate prediction of massively separated turbulent wake flow while maintaining the reasonable computational cost. Three hybrid turbulence models, the DDES (Delayed Detached Eddy Simulation, the k-ω SST-DES (Detached Eddy Simulation), and the k–ω SST-SAS (Scale Adaptive Simulation), are studied and their results are compared with the reported experimental measurements. It is shown that the result of simulation with the k–ω SST-SAS model is closer to the reported literature than the other two and therefore, subsequently adopted for all the simulations of our study in this paper. The scaling effect of cylinder length in the spanwise direction is also studied with the objective to reduce the computational cost. From the comparison with the recent experimental measurements, the discrepancy between the present simulations of reducing cylinder length and the experiment increases only when Re ≥ 4000. This might stem from the increase in wavelength of some vortex shedding modes and turbulence intensity variation in the spanwise direction near the cylinder as Re ≥ 4000. The detailed flow patterns, 3D vortex structures (based on Q-criterion) and vortex-shedding modes are presented in this work as well.

Experimental Study About the Influence of the Free End Effects on Vortex-Induced Vibration of Floating Cylinder With Low Aspect of Ratio

Experiments regarding free-end effects on vortex-induced vibration (VIV) of floating circular cylinders with low aspect ratio were carried out in a towing tank. Four cylinders with low aspect of ratio, L/D = 2 (Length / Diameter) were tested with different free end corner shape types, namely by the relation between chamfer rounding radius (r) divided by the radius of cylinder (R) (r/R = 0.0, 0.25, 0.5 and 1.0). For the initial case, r/R = 0.0 represents flat tip and r/R = 1.0 the hemispherical tip. The aims were to understand the effect of different free-end types on VIV behavior of cylinders. The floating circular cylinders, i.e. unit mass ratio m* = 1(structural mass/displaced fluid mass) were elastically supported by a set of linear springs to provide low structural damping on the system and allow six degrees of freedom. The range of Reynolds number covered 3,000 ≤ Re ≤ 20,000. To conclude, cylinder with r/R = 0.25, shows lower amplitudes in transverse direction. The same occurs for the cylinder r/R = 0, but for amplitudes of vibration in in-line direction. Behaviors of the vibration frequencies in in-line and transverse direction don’t have significantly differences. Regarding force coefficient, flat tip cylinder (r/R = 0) presents higher values compared to the others however, for the lift coefficient, results converge in similar values for the same velocities that were observed higher transverse amplitudes. The visualization experiments show an expressive reduction of the recirculation bubble for r/R = 0.5 model compared with the flat tip, can therefore justify the lower values for this model obtained in draft amplitudes and drag coefficient compared with the flat tip model.

Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part II — Effects of Rounded Edges

Experiments regarding flow-induced vibration on floating rounded squared section cylinders with low aspect ratio were carried out in an ocean basin equipped with a rotating-arm apparatus. Floating squared section cylinders with rounded edges and aspect ratios of L/D = 2.0 were elastically supported by a set of linear springs in order to provide low structural damping to the system. Two different incidence angles were tested, namely 0 and 45 degrees. The Reynolds numbers covered the range from 2,000 to 30,000. The aim was to understand the flow-induced vibrations around single columns, gathering information for further understanding the causes for the Vortex-Induced Motions in semi-submersible and TLP platforms. Experiments on circular and squared sections cylinders (without rounded edges) were also carried out to compare the results with the rounded square section cylinders (with rounded edges). The amplitude results for in-line, transverse and yaw amplitude for 0-degree models showed to be higher for squared section cylinders compared to those for the rounded square section cylinders. No significant difference between the 45-degree models was observed. The results of ratio between frequency of motion in the transverse direction and natural frequency in still water confirmed the vortex-induced vibration behavior for the squared and rounded square section cylinders for 45-degree incidence; and also the galloping characteristics for 0-degree incidence cases. The rounded effect on the square section cylinders showed to be important only for reduced velocity larger than 8, which is probably related to the position of the separation point that changes around the rounded edge, behavior that did not occurr for the squared edge that fixed the separation point for any reduced velocity.

Multi-Scale Numerical Analysis of the Field Efficiency of an Ocean Plastic Cleanup Array

The impact of plastic pollution on marine ecosystems and global economy has been drawing public concern since the end of the 20th century. To mitigate this issue, The Ocean Cleanup (TOC) Foundation is developing technologies to extract, prevent, and intercept plastic debris from coastal and oceanic environments. The core technology being optimized is the use of floating booms placed perpendicular to the main ocean plastic flow so it can concentrate plastic debris to a point where it can be extracted, shipped and processed in a cost-effective manner. In order to optimize the system’s field efficiency (i.e. mass of ocean plastic captured per length of floating boom), a multi-scale approach has been elaborated, where temporal and spatial scales span over several orders of magnitude. Here we introduce this general multi-scale method alongside its assumptions and multi-scale models. We then describe two application examples, the first corresponding to our coastal pilot in the Japanese island of Tsushima and the second related to our main cleanup target area: the so-called Great Pacific Garbage Patch, situated between Hawaii and the US west coast.