Fishes are talented swimmers. Depending on the propulsion mechanisms many fishes can use flapping tails and/or fins to generate thrust, which seems to be connected to the formation of a reverse von Kármán wake. In the present work, the flow past a 2D flapping foil is simulated by solving the incompressible Navier-Stokes equations in the open-source OpenFOAM platform. A systematic study by varying the oscillating frequency, peak-to-peak amplitude and Reynolds number has been performed to analyze the transition of vorticity types in the wake as well as drag-thrust transition. The overset grid method is used herein to allow the pitching foil to move without restrictions. Spatial convergence tests have been carried out with respect to grid resolution and the size of overset mesh domain. Numerical results are compared with available experimental data and discussed. The results show that the adopted methodology can be well applied to simulate large amplitude motions of the flapping foil. The transitions in the types of wake are consistent with the benchmark experimental data, and the drag-thrust transition of the pitching foil does not coincide with von Kármán (vK)-reverse von Kármán (reverse-vK) wake transition and it is highly dependent on the Reynolds number.

The flow around a circular section has many applications in engineering, specifically in the offshore area, such as platforms, risers, pipelines, etc. These structures, which are submerged in water, are susceptible to flow-induced vibrations (FIV) and might oscillate at unwanted large amplitudes due to currents and waves. The Computational Fluid Dynamics (CFD) is a tool that allows the investigation of different parameters related to the flow and/or to the body. A CFD analysis is important in the initial design stage of riser arrangements, being a way to predict the best relative distance, since the flow around the upstream cylinder may strongly influence the downstream cylinder forces and movements responses, characteristic of wake-induced vibrations (WIV). In this context, this work carried out simulations to initiate numerical investigations about the flow around tandem arrangements and then properly identify characteristics of the fluid model that generate the results obtained in experiments. Two similar rigid circular cylinders in a tandem arrangement were analysed, in the stationary case and in the case allowed to move in two degrees of freedom (2DOF) — transverse and in-flow directions, with mass ratio m* = 10 and no structural damping, ζ = 0%. The computations were performed using OpenFOAM, a CFD open source software, in a two-dimensional flow and, numerical uncertainties studies were conducted to well define the results, presenting the value plus its uncertainty. There were variations of the tandem distances (T = 2D, 3D, 4D, 5.5D and 6D), at Reynolds number Re = 100. These two-dimensional flow simulations at low Re are important at the initial stage of a project, in order to filter the best results and save time for more advanced analysis. A single cylinder in a turbulent flow was also analysed, with m* = 2.52, varying reduced velocity. Analysis were done comparing forces coefficients and amplitudes with the literature, for both upstream and downstream cylinders. The vorticity was investigated, properly identifying vortex shedding pattern. For stationary cylinders, a critical distance of 4D was found, where the forces coefficients increase. For the 2DOF cylinders in laminar flow, the highest amplitude was 1.04D at V R = 7 for the downstream cylinder. When the cylinders are free to move, the initial tandem distance does not interfere the amplitude results of the upstream cylinder when V R > 2, results also found in experiments at high Re, and, that justify the two-dimensional studies in low Reynolds as being the basis of a project.

Deepwater steel lazy wave risers (SLWR) subject to vessel motion will be exposed to time-varying oscillatory flow, vortices could be generated and the cyclic vortex shedding force causes the structure vibrate, such fluid-structure interaction is called vortex-induced vibrations (VIV). To investigate VIV on a riser with non-linear structures under vessel motion and oscillatory flows, time domain approaches are needed. In this study, a time-domain approach is used to simulate a full-scale SLWR. Two cases with simplified riser top motions are simulated numerically. By using default input parameters to the time domain approach, the key oscillatory flow induced VIV response characteristics such as response frequency, curvature and displacements are examined and discussed. More accurate VIV prediction could be achieved by using realistic hydrodynamic inputs into the time domain model.

This paper addresses the performance evaluation of an empirical time domain Vortex Induced Vibrations (VIV) model which has been developed for several years at NTNU. Unlike the frequency domain which is the existing VIV analysis method, the time domain model introduces new vortex shedding force terms to the well known Morison equation. The extra load terms are based on the relative velocity, a synchronization model and additional empirical coefficients that describe the hydrodynamic forces due to cross-flow (CF) and In-line (IL) vortex shedding. These hydrodynamic coefficients have been tuned to fit experimental data and by considering the results from the one of existing frequency domain VIV programs, VIVANA, which is widely used for industrial design. The feature of the time domain model is that it enables to include the structural non-linearity, such as variable tension, and time-varying flow. The robustness of the new model’s features has been validated by comparing the test results in previous researches. However, the riser used in experiments has a relatively small length/diameter (L/D) ratio. It implies that there is a need for more validation to make it applicable to real riser design. In this study, the time domain VIV model is applied to perform correlation studies against the Hanøytangen experiment data for the case of linear sheared current at a large L/D ratio. The main comparison has been made with respect to the maximum fatigue damage and dominating frequency for each test condition. The results show the time domain model showed reasonable accuracy with respect to the experimental and VIVANA. The discrepancy with regard to experiment results needs to be further studied with a non-linear structural model.

Recent market trends in the cruise industry aim to provide land-based attractions on mega-cruise liners deck amusements. The structures used are mega-lightweight structures that comprise of slender beams to reduce added weight. They may be subject to dynamic wind-induced resonance as a result of fluctuating forces due to vortex shedding occurring close to the natural frequency of the structure. This paper investigates the structural responses due to wind-induced vibrations in the context of a ship deck outfitting by coupling CFD and FEA. The simulations are performed using one- and two-way coupling FSI to gain insights into the differences between the predicted responses. A deck amusement structure is idealized as an aluminum portal frame subjected to a constant head wind. Transient one- and two-way coupled simulations, based on Reynolds-Averaged Naiver-Stokes (RANS) and linear elastic 3D FEA are conducted. The predictions are compared against quasi-dynamic beam element idealizations.

Flow-structure interactions of submerged or floating bodies can lead to undesired behavior in many marine and offshore engineering applications. In this paper, we consider a complex nonlinear dynamical system of unsteady wake flow interacting with a freely moving tugboat in open water. To meet the operational demands of compact, agile, and high power ship-handling, new hull forms of tugboats are designed with low length-to-beam ratios and rounded sterns. While the hydrodynamic design with low length-to-beam ratios provides improved directional controllability, it can be challenging for tractor tugboats due to the massively separated wake flow with the vortex shedding. These wake vortices can cause large fluctuating yaw moments and possibly a strong fluid-structure coupling via synchronization or lock-in. Of particular interest, the proposed study will focus on the physical mechanism and control of flow-induced oscillations with free-surface effects via our in-house fully-coupled three-dimensional fluid-structure-free-surface interaction solver. Stabilized finite element based methods will be employed to discretize the partial differential equations that arise from the mathematical modeling of the physical phenomena considered. We will begin with a fundamental understanding of coupled dynamics of a canonical geometry of a freely vibrating sphere at free-surface. The physical insight gained will then be applied to a realistic tugboat configuration. We aim to understand the fundamentals of vortex-shedding modes and the coupled dynamics pertaining to the flow-induced vibration (FIV) response of a freely vibrating sphere (a prototypical problem for a rounded tugboat) in all three spatial directions. To predict and analyze the vortex synchronization regimes and the wake patterns, the FIV response of the sphere at a low mass ratio is investigated over a broad range of reduced velocities and Reynolds numbers. We find that the sphere begins to move along a linear trajectory with hairpin vortex-shedding mode, eventually transforming into a circular trajectory with spiral mode in its stationary state for Re ∈ [2000–6000]. We systematically examine these mode transitions and the motion trajectories in the three degrees-of-freedom for higher Reynolds number up to 15,000 which has not been studied in detail in the literature. Finally, we will look into the effect of free surface on the FIV response of the sphere piercing the free surface and will link our fundamental results with a realistic configuration of tugboat undergoing vortex-induced oscillation with free surface effects.

The paper addresses the flow-induced response of a rigid spool/jumper. It mainly focuses on its mechanical response resulting from internal intermittent slug flows but also addresses potential coupling with an external excitation due to vortex shedding. These works provide quantitative experimental data that match quite well with existing empirical correlations in terms of slug flow properties. The repeatability of the measurement system has been experienced and underlines promising capabilities. The tests provide exhaustive complementary data regarding the slug flow properties (e.g. pocket length) which will be reused for numerical modelling purpose. The mechanical response of the spool is exhaustively addressed for different regular slug flows. Some correlations are proposed aiming at describing the mechanical responses. The coupling with an additional external current solicitation and Vortex Induced Vibrations (VIV) is discussed and characterized for some conditions. Finally, a discussion on the current Industry Best Practices is introduced in order to challenge the capability of the proposed approaches to described and recover such complex phenomenon. The comparisons underline the weak agreement between experiments and numerical models, opening discussion on the best way to address this physics and the next developments.

In many Engineering applications, the Reynolds-Averaged Navier-Stokes (RANS) equations are still used to simulate high Reynolds numbers (turbulent) flows around complex geometries. In flows that exhibit significant regions of flow separation leading to vortex shedding, it does not make sense to define the mean flow using time-averaging. Therefore, the use of RANS (even if locally) in statistically unsteady flows requires the application of ensemble-averaging to the flow variables and to the mass and momentum equations, which generates the appearance of the Reynolds stresses. Turbulence models available in the open literature have been developed for the simulation of statistically steady flows (mean flow defined by time-averaging). Nonetheless, the same models are used for the simulation of statistically unsteady flows. Therefore, it is not guaranteed that such models provide sufficient diffusion (damping) to capture only the mean flow. In this paper, we have investigated the modeling and numerical properties of RANS supplemented by the k – ω SST eddy-viscosity model when applied to the classical problem of the flow around captive and moving (imposed motion) cylinders with Reynolds numbers ranging from 10 2 to 10 6 . Two and three-dimensional simulations are performed and numerical (statistical, iterative and discretization) convergence properties are assessed for moving and deforming grids techniques. The quantities of interest are the drag and lift coefficients, for which we determine the frequency content of the time signal to assess if the numerical results correspond (as intended) to the mean flow. Results obtained at a Reynolds number of 10 4 are compared with experimental data available in the open literature.

Oscillation induced by vortex shedding often causes fatigue damage of slender cylindrical structures in many engineering fields, so it is significant to study the suppression of vortex induced oscillation. We focus on the methods of experimental test to study on the suppression effect of vortex induced oscillation of cylinder with single freedom at Reynolds number of 2.4 × 10 4 . We designed a set of device with forward water jet to suppress vortex induced oscillation. We found the most notable observation from a serials of experiments: reduction of displacement of cylinder exhibits two distinct modes separated by a sudden and very pronounced decrease in the extent of cross-flow displacement at a critical value of the geometric and flow variables followed by what appears to be a saturation regime where no further decrease is observed. These results will provide some meaningful and evaluable references for practical application in ocean engineering.

Vertical pipe configuration happens in subsea well re-entry operations for wellbore drilling, subsea equipment installations, deepwater intake risers, among other scenarios, usually by hanging the marine riser at the top from a platform or ship. The simple operational configuration justifies its wide application; however, there are some drawbacks such as stress concentration, fatigue, among other issues. The purpose of this study is to contribute for the overcoming of those problems by improving the understanding of hanging risers through investigations of a vertical pipe forced to oscillate horizontally at the top. For this purpose, laboratory experiments with small diameter pipe model have been developed with oscillatory motions applied at the top termination through a mechanical device. Displacements over time and along the pipe length were measured by an optical measurement system. In the present study, a numerical scheme has been implemented for simulations to support evaluations of the experimental results. A vertical pipe response depends on several parameters, such as pipe overall length, frequency and amplitude of oscillation itself, among others. Due to existing relative velocity between the oscillating pipe and the water surrounding it, vortex shedding occurs along the pipe length which influences riser motion behaviour. The results from experiment and numerical simulations bring important insights for describing and understanding oscillatory vertical pipe behaviour, in the plane and the out of plane of the forced top oscillation. The obtained results provide a further step toward the modelling of Vortex-Induced Vibration (VIV) in marine risers, aiming to contribute to relevant technological advancements.

OpenFOAM (OF) represents an attractive and widely used open-source environment for simulating complex hydrodynamic scenarios with several implemented numerical methods and wide variety of problems it can be applied to. For commercial and open-source solvers, though, expertise and experience are required to get physical and reliable results. Here, without pretending to be exhaustive, we aim to contribute in highlighting advantages and challenges of some key computational fluid dynamics (CFD)-simulation tools, with focus on the OF platform. We examine the effect of grid type, grid size and time-evolution scheme. Dynamic-mesh techniques and their influence on local and global numerical results are discussed, as well as the use of an overset grid versus a deforming mesh. Lastly, possible error sources in CFD simulations are discussed. These numerical studies are performed investigating two complex hydrodynamic problems: 1. a fully-immersed flapping hydrofoil aimed to generate thrust, 2. a damaged and an intact ship section fixed in beam-sea waves, in forced heave and roll motion in calm water. In the first case, vortex-shedding and wake features are crucial; in the second case, free-surface flow effects play the key role while the importance of vortex-shedding and viscous-flow effects depends on the scenario. The first problem is solved with OF and validated with results from benchmark experiments. The second problem is solved using (A) OF, (B) an in-house CFD solver and (C) a fully-nonlinear potential-flow code. A and B assume laminar-flow conditions and use, respectively, a volume-of-fluid and a level-set technique to handle the free-surface evolution. C is considered to examine importance of nonlinear versus viscous effects for the examined problems. The results are compared against in-house experiments.

During single blade installation in offshore wind farms, relative motion between nacelle and blade root due to wind and wave excitation pose a significant challenge. Wave excitation can be modelled considerably well by employing state-of-the-art simulation tools and can, therefore, be included in installation planning. Other phenomena, such as flow-induced vibrations are hard to capture and hence challenging to account for when defining installation procedures and limitations. Here, we present measurements conducted during the installation of an offshore wind farm consisting of multi-megawatt turbines installed on monopile foundations in the North Sea. A custom-built sensor capturing linear & angular acceleration and GPS-data was deployed atop the nacelle. Both partially and fully assembled turbines displayed complex oscillation orbits, swiftly changing amplitude and direction. Mean nacelle deflection correlated strongly with significant wave height as well as mean wind speed. As wind speed and significant wave height showed a strong correlation as well, it is difficult to discern which load drives the observed relative motions. While wind loads are significantly smaller than wave loads on partially assembled turbines under installation conditions, additional momentum induced by vortex shedding may prove sufficient to cause the observed effects.

The stranded rope is one of the important components of the fishery aquaculture equipment. We investigate the fluid flow through two-dimensional stranded rope by direct simulation of the Navier-Stokes equations. We show that for different kinds of stranded rope structures, there are significant differences in hydrodynamic performance. This paper established a numerical model of unsteady flow past the stranded rope based on the Navier-Stokes equation and Morison formulas to study the hydrodynamic characteristics of three-stranded rope, four-stranded rope, and seven-stranded rope, respectively. The turbulence flow was simulated using Standard k-ε model and Shear-Stress Transport k -ω (SST) model. The flow distribution strongly depends on the Reynolds number, a range of 3,900 and 30,000. With increasing Reynolds number, the alternate eddy formation and shedding were repeated behind the stranded ropes. Such parameters of hydrodynamic characteristics of multiple stranded ropes were calculated as the lift and drag coefficients, and vortex shedding frequencies. The numerical simulation results presented flow performances of different cross sections ( a , b , c , d ) at different Reynolds numbers. However, Reynolds number has no significant impact on the Strouhal number for the same attack angle of the stranded rope.

Vortex shedding and eddy generation can provide a substantial contribution to the hydrodynamic roll damping of vessels. While sharp-edge bilge geometries generally result in greater damping, a more rounded bilge may be beneficial for the other ship motions and mooring design. To this end, the effect of the bilge rounding on the vortex shedding dynamics and the hydrodynamic damping of hulls is investigated numerically. A series of new computations of the response of hulls with differing bilge radii and subject to forced oscillatory motion are reported. The severity of the vortex shedding and the magnitude of the damping is shown to depend strongly on the bilge geometry. Computations using an efficient isolated edge discrete vortex model are also found to be generally in qualitative agreement with the CFD results. The results extend the relatively scarce literature on the effect of bilge rounding and can assist in informing hull design and concept selection.

Direct numerical simulations are performed for the flow past an inclined square plate. The side length-thickness ratio of the plate is selected as 50, representing a reasonably thin plate. Various incidence angles of the plate with respect to the flow are considered from 0° to 75°, where 0° refers to the condition in which the flow is normal to the plate. The Reynolds number (Re) based on the flow velocity and the side length in the streamwise direction of the plate is up to 300. The hydrodynamic characteristics, including the force coefficients, the three-dimensional vortical structures and vortex shedding process, are presented. The effects of incidence angle on the wake transition are also discussed. It is observed that as the incidence angle increases, the flow firstly changes from a chaotic state to a periodic state and finally returns to chaos again at high incidence angles.

The prediction of roll motion of a ship section with bilge keels is particularly difficult because the flow separation and vortex shedding under the hull significantly affect the behavior of roll damping. To predict the roll damping and roll motion directly, the numerical models must simulate the fluid viscosity. Recently, Reynolds-averaged Navier–Stokes (RANS) method and Discrete Vortex Method (DVM) have been applied in this area and show promising results. In this paper, we will use both methods to simulate the free roll-decay motion of a ship section with bilge keels. The numerical predictions of the roll time histories will be compared with experimental measurements. Besides, the numerically-predicted vorticity distributions at different time instants near a bilge keel will be shown and compared. Moreover, the computation times for both numerical methods will also be reported. In this work, we will conduct the comparison for a number of cases that are with different bilge-keel heights and bilge-keel installation angles. Thus, the accuracies and the computational efficiencies will be evaluated comprehensively.

The viscous flow past a prolate spheroid is always complicated and often three-dimensional, thereby fascinating scientists in fluid dynamics and aerodynamics. The objective of this paper is to investigate the asymmetric wake behind a 6:1 prolate spheroid at 45-deg angle of attack by means of Large-Eddy simulations(LESs). The LES was performed at a Reynolds number of 10000, based on the free stream velocity U0 and minor axis diameter D. Results in the paper were compared with the previous LES results for different Re. The simulation revealed that there was a pair of counter-rotating vortex in the near wake field, which developed from a pair of vortex sheet separating from the sides of prolate spheroid. The vortex broke and the coherence of vortex tube lost because of the influence of vortex shedding from the tip of the prolate spheroid and energy dissipation. The asymmetry could be observed evidently from several physical quantities, such as velocity, vorticity, pressure and side force. The results showed an obvious skew of vortex structure, the direction of which was random and could be explained by pitchfork theory. The comparison between near field and far field was present to show the development and features of the flow characteristics. The asymmetric wake should be given more attention as it may be detrimental to underwater vehicle performance and submarine maneuverability.

Computational Fluid Dynamics (CFD) are used to predict the behavior of a massive Gravity-Based Structure (GBS) during open-water towing. Generally, it is poorly understood how the hydrodynamic interaction between the shaft (slender portion of the GBS structure at the waterline), free surface and caisson (lower, storage portion of the GBS) influences vortex shedding and potential VIM response. There is also limited ability to quantify the expected squat response of a structure of the substantial mass and bottom profile of a GBS being towed through areas of decreasing under bottom clearance. In order to properly capture all relevant physical effects, the Detached Eddy Simulations (DES) turbulence model is used. The resulting mesh is somewhat coarser and temporal resolution lower than in a previous study [1] which focused on accurate pitch and roll damping estimates. However, new physical decay test data confirm that the current model reproduces the damping in the relevant amplitude range. The planned towing speed results in reduced velocities for the shaft in the range where Vortex Induced Motions (VIM) typically occur. The numerical set-up is qualified by studying the shaft alone in a uniform current; self-sustained VIM is triggered, with behavior similar to known results. Two towing velocities and loading conditions are studied; a higher speed with a large under bottom clearance and a lower speed where the under bottom clearance is just a few meters. When no VIM is simulated for these conditions, a higher speed corresponding to the VIM range for the shaft is studied. Due to the presence of the caisson and the significant total mass of the GBS, no material VIM occurs. This is explained by a simplified one-degree of freedom model combining the excitation force on the shaft with the damping characteristics of the caisson and total GBS added mass and inertia. The simulations are also used to assess if large pitch or roll motions may be triggered.

The safe and efficient offloading of liquid natural gas (LNG) in a side-by-side configuration has emphasised the need to accurately quantify the hydrodynamic responses of floating bodies when located in very close proximity. A new series of computational results are presented for the forced roll of a hull in the presence of a second body representative of a floating FLNG facility. The vortex shedding phenomenon which provides a significant contribution to the damping of an isolated hull, is demonstrated to be modified by the presence of the second body. The total damping coefficients were found to be significantly reduced by approximately 50 percent for the very small gap widths considered. It is shown that in addition to the modification of the local shedding dynamics, the variation in the pressure field owing to the presence of the second body also contributes significantly to the reduction in the damping. The findings will be of particular interest to the offshore industry for improving and optimising offloading practices.

Deep-water flexible risers conveying hydrocarbon oil and gas flows may be subject to internal dynamic fluctuations associated with the spatial variations of phase densities, velocities and pressure drops. Many studies have focused on single-phase flows in pipes whereas understanding of multiphase flow effects is lacking. This study aims to investigate the planar free-vibration characteristics of a long flexible catenary riser carrying the steady-state, multiphase slug oil-gas flows in order to understand how the inclination-dependent internal slug flows affect riser natural frequencies and modal shapes. The influence of slug characteristics such as phase velocities on the riser vibration is also studied. The catenary riser planar motions are mathematically described by a two-dimensional continuum model capturing coupled horizontal and vertical responses. Based on the selected two-phase flow rates at the wellhead, riser geometric configurations and specified slug unit lengths, a steady-state slug flow model is considered by taking into account several empirical closure correlations and riser mechanical properties, solving for the multiphase flow aspects including pressure, velocities, liquid holdup and gas fraction. By assigning an undamped free-vibration shape of an empty catenary riser as initial displacement conditions, the space-time numerical simulations are performed using a finite difference approach. Comparisons of oscillation frequencies, time histories, phase planes, time-space varying responses and dynamic stresses of catenary risers with and without slug flows are presented, identifying the dynamic modifications arising from the internal slug-induced mass momentum change and pressure loss. To understand the influence of slug flow properties, parametric studies are carried out with different gas velocities. Numerical results highlight the reduced riser tensions, decreased oscillation frequencies, multiple oscillation modes, amplified amplitudes and stresses. These key observations will be useful for the forced vibration analysis of catenary risers subject to combined internal (multiphase) and external (vortex-shedding) flow excitations.