Vortex-induced vibrations (VIV) of two elastically mounted circular cylinders with different diameters in tandem arrangement are investigated in a two-dimensional (2D) numerical simulation. The fluid domain is simulated by solving 2D Reynold-Averaged Navier-Stokes equations. Meanwhile, the VIV response of the structures is obtained by solving the motion equation using the 4th Runge-Kutta method. The parameters of the cylinders are designed according to an experimental study of flexible risers. Simulation of an elastically mounted single cylinder is firstly carried out and compared with published experimental results to verify the method utilized in the paper. The results of single cylinder show the response frequencies of the bluff cylinders and the flexible cylinder are comparable. In the simulation of the tandem cylinders, a “frequency capture” phenomenon that the oscillation frequencies of downstream cylinder are locked on to that of the up-stream one although they are with different diameters is observed. It also occurs in the experimental study of flexible cylinders. The mechanism behind is analyzed in the paper.

The Flow-Induced Motions (FIM) is an essential topic on multi-column platforms due to the effect on the mooring line fatigue life. Vortex-Induced Motions (VIM) or galloping behavior can be observed for an array of four columns with square sections. The presence of pontoons showed to be important for changing the flow around the array and promoting different amplitude behaviors of the motions in the transverse direction mainly. This article aims to understand the effect of the presence of four pontoons on the FIM of a semi-submersible platform (SS) with four square section columns. Model tests of a floating system supported elastically utilizing four springs were performed in a towing tank. Five different pontoon ratios were tested, namely P/L = 0, 0.25, 0.50, 0.75, and 1.00; where P is the pontoon height (the dimension in the vertical direction), and L is the length of the square column face. The draft condition was kept constant as H/L = 1.5; where H is the draft of the platform. The spacing ratio of the columns was S/L = 4; where S is the distance between column centers. Two incidence angles of the current were carried out, namely 0 and 45 degrees. The amplitudes in the transverse direction (direction perpendicular to the incidence current) decreased by increasing the pontoon ratio for 0 and 45-deg incidences. The pontoons positioned aligned to the flow significantly reduced the amplitudes in the transverse direction since the pontoon presence in this position modified the incident wake in the downstream columns. The pontoon presence needs to be well investigated to choose the best condition to avoid raising the FIM or mitigating the FIM.

The present work deals with wave generation in fully nonlinear numerical wave tanks (NWT). As an alternative to modelling a moving (physical) wavemaker, a two-dimensional (2D) potential-flow NWT is coupled with an external spectral wave data (SWD) application programming interface (API). The NWT uses the harmonic polynomial cell (HPC) method to solve the governing Laplace equations for the velocity potential and its time derivative, and has previously been extensively validated and verified for numerous nonlinear wave-propagation problems using traditional wave-generation mechanisms. Periodic waves of different steepness generated with a stream-function theory as reference solution in the SWD API are first considered to investigate the method’s numerical accuracy. Thereafter, with a higher-order spectral method (HOSM) as the SWD API solution, irregular waves with different wave heights and water depths relevant for e.g. aquaculture and offshore structures are simulated. Differences between the HPC and HOSM solutions in and near steep crests are investigated. The study aims to demonstrate a robust method to generate and propagate general wave fields for further studies of nonlinear waves and wave-body interaction in both two and three dimensions.

This paper presents a numerical study of the flow normal to a triangular plate. A total of four plates with the same frontal area A r and different curved edges are used. The curvature of edges is determined by the compression ratio k ( k = 0.3, 0.4, 0.5, 0.8; the large value of k corresponds to the large curvature of the edges). A disk of χ = 50 ( χ is the diameter-thickness aspect ratio) is used as the reference disk. The Reynolds number Re based on the characteristic length is up to 250. Four states are observed and denoted as: (I) steady and geometric symmetry state (SG); (II) steady and reflectional symmetry state (SR); (III) reflectional symmetry breaking with periodic flow (RSB); (IV) chaotic state (CS). The critical Reynolds numbers at the first two stages ( Re c 1 , Re c 2 ) decrease with the increasing k , indicating that flow of the plates with a larger curvature is more unstable. Therefore, we believe that the flow around a triangular plate is more stable than that around a circular disk.

Fluid-structure interaction modeling tools based on computational fluid dynamics (CFD) produce interesting results that can be used in the design of submerged structures. However, the computational cost of simulations associated with the design of submerged offshore structures is high. There are no high-performance platforms devoted to the analysis and optimization of these structures using CFD techniques. In this context, this work aims to present a computational tool dedicated to the construction of Kriging surrogate models in order to represent the time domain force responses of submerged risers. The force responses obtained from high-cost computational simulations are used as outputs for training and validated the surrogate models. In this case, different excitations are applied in the riser aiming at evaluating the representativeness of the obtained Kriging surrogate model. A similar investigation is performed by changing the number of samples and the total time used for training purposes. The present methodology can be used to perform the dynamic analysis in different submerged structures with a low computational cost. Instead of solving the motion equation associated with the fluid-structure system, a Kriging surrogate model is used. A significant reduction in computational time is expected, which allows the realization of different analyses and optimization procedures in a fast and efficient manner for the design of this type of structure.

In this work, the roll damping behavior of the offshore heavy lift DP3 installation vessel Orion from the DEME group is studied. Boundary element codes using potential flow theory require a roll damping coefficient to account for viscous effects. In this work, the roll damping coefficient is calculated using the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. The two-phase Navier-Stokes fluid solver is coupled with a motion solver using a partitioned fluid-structure interaction algorithm. The roll damping is assessed by the Harmonic Excited Roll Motion (HERM) technique. An oscillating external moment is applied on the hull and the roll motion is tracked. Various amplitudes and frequencies of the external moment and different forward speeds, are numerically simulated. These high-fidelity full-scale simulations result in better estimations of roll damping coefficients for various conditions in order to enhance the accuracy of efficient boundary element codes for wave-current-structure interactions simulations.

In this paper, we apply a CFD computer code to study the hydrodynamic behavior of a stand-alone cylinder and a dual coaxial-cylinder system (DCCS) via free-decay motion tests. The geometric proportions of a stand-alone cylinder and the inner and outer cylinder of the DCCS are chosen to be the same as those in [1] and [2], respectively, as ocean wave-energy converter (WEC) devices. Overset mesh based on the commercial code ‘Star-CCM+ 11’ is used to simulate the free-decay motion of the two systems. Five parameters chosen for the CFD implementation are: turbulence model, initial displacement, time step, number of prism layers and mesh size. Results obtained from using different values of these parameters are compared so as to confirm the validity of choices made. The hydrodynamic performance of the stand-alone cylinder and outer cylinder in the DCCS are compared with the experimental results to assess and validate the CFD models. In addition, the heave hydrodynamic coefficients, namely, the added mass and total damping, and ‘resonance’ frequency of the stand-alone cylinder and those of the inner cylinder of the DCCS, with the outer cylinder being fixed, are obtained by using the CFD procedure. The hydrodynamic coefficients of another stand-alone cylinder with the same dimensions as the inner cylinder of the dual-coaxial cylinder are also obtained by simulations. The vorticity-contour plots for the stand-alone cylinder and the outer cylinder in the DCCS in free-decay motion are presented and analyzed. Finally, the results of the three cases are compared to examine the effect of the outer cylinder on the heave hydrodynamic coefficients of the inner cylinder.

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.

In this study, four approaches are investigated to predict the motions and structural loads on a containership in waves. The Flockstra (1974) containership model is used as the benchmark for this study as extensive experimental data is available to compare to the predictions. The hydrodynamic loads and motions are predicted using strip theory, a zero speed Green’s functions panel method with forward speed correction, a fully unsteady 3D panel method and unsteady RANSE simulations for limited cases. Simulations are performed at Fn = 0.245 in head, stern quartering, and bow quartering seas for wave length to ship length ration λ/L of 0.35–1.40. The accuracy of each method, relative to experimental results, in predicting the amplitudes of heave, pitch, and roll are investigated. Vertical and horizontal bending moments, shear forces, and the torsional moment on the hull at midships and 0.25LBP forward and aft of midships are also calculated and compared with the measured values. Through comparison with experimental data, the relative uncertainty of all four methodologies in predicting both motions and structural loads are assessed and discussed. Overall, all linearized potential flow methods show a large discrepancy with the experimental loads, motivating the need for further studies on non-linear effects for this particular ship type. This paper has been prepared in the framework of the ISSC-ITTC special joint committee on uncertainty quantification in wave load estimation.

The research of parametric rolling has encountered many challenging problems due to its nonlinearity. In this paper, an in-house CFD code HUST-Ship based on fully structured overset grids is used for the investigation of KCS parametric rolling, which can overcome the shortcomings of potential flow method. Modeling errors and numerical errors are presented to improve the credibility of CFD simulation. The grid and time step are studied for the quantification of numerical errors. The phenomenon of parametric rolling is successfully captured in the paper and motion responses of KCS have a good agreement with experimental data. The characteristics of motions, forces and moments under parametric rolling are investigated by using fast Fourier transform. The relationship between wave characteristics and motion responses of ship is revealed in this paper. Following, a simulation model with three DOFs’ model (heave-roll-pitch model) is used to investigate the influence of surge motion on parametric rolling. Result shows that the excluding of surge motion has no qualitative influence on the prediction of parametric rolling. In general, the simulation limited to heave-roll-pitch coupling is adequate for its capture.

CFD uncertainty analysis is a process to quantify the accuracy of numerical simulation results, and it is also a research hotspot in the past decades. ITTC(2017) requires uncertainty analysis of ship CFD simulation results, that is, verification and validation. In this paper, with reference to the recommended procedures by ITTC, the uncertainty of the CFD numerical simulation results of ship model resistance was analyzed. Based on the SST k-ω turbulence model, the Y+ values near the wall were set to 60,120,240, respectively. And for each and Y+ value, three different sets of grid densities were set respectively, and the uncertainty was analyzed. The results show that: 1) the results of Y+ at 60 and 120 were not validated, and the results at 240 was validated, 2) the selection of Y+ value has a significant effect on the numerical results, 3) increasing the mesh density can make the result converge, but it is not sure to get the result with the least error. Through the uncertainty analysis of CFD results, it is helpful to find a method to improve the accuracy of the numerical results.

Subsea liquid-gas flows conveyed through a flexible riser or pipeline may develop into various flow patterns including slug flow. In this study, the slug flow-induced vibration (SIV) of an inclined sagged riser conveying upward air-water flows is experimentally investigated. A small-scale experiment is carried out in an air-water test loop with a section of a free-hanging catenary tube made of silica gel. Attention is placed on the effect of superficial gas and liquid velocities on SIV responses. Both pipe motions and flow patterns are recorded using non-intrusive high-speed cameras. Pressure variations are also measured at the pipe inlet and outlet by two pressure transducers. The SIV system is tested by employing different ratios of the superficial gas-liquid velocities. Occurrence of unstable slug flows is captured at the relatively high gas-to-liquid velocity ratios, leading to a large-amplitude SIV. Experimental results of the space-time varying riser responses and oscillation frequencies are reported together with the associated slug flow features. Depending on the gas-liquid superficial velocities, slug flow characteristics are observed to vary significantly. These entail an intermittent SIV with modulated amplitudes and frequencies along riser span, signalling a potential dynamic stress and fatigue-related concern. In all experimental cases, the riser responses are found to be multi-modal and dominated by the fundamental planar mode whereas an out-of-plane vibration is negligible. Experimental observations suggest the key interrelationships of the two-fluid flow conditions, the slug characteristics and the pipe dynamics. This finding is meaningful for a practical design of riser transporting internal multiphase flows.

In this paper, an end-to-end nonlinear model reduction methodology is presented based on the convolutional recurrent autoencoder networks. The methodology is developed in the context of overall data-driven reduced order model framework proposed in the paper. The basic idea behind the methodology is to obtain the low dimensional representations via convolutional neural networks and evolve these low dimensional features via recurrent neural networks in time domain. The high dimensional representations are constructed from the evolved low dimensional features via transpose convolutional neural networks. With an unsupervised training strategy, the model serves as an end to end tool which can evolve the flow state of the nonlinear dynamical system. The convolutional recurrent autoencoder network model is applied on the problem of flow past bluff bodies for the first time. To demonstrate the effectiveness of the methodology, two canonical problems namely the flow past plain cylinder and the flow past side-by-side cylinders are explored in this paper. Pressure and velocity fields of the unsteady flow are predicted in future via the convolutional recurrent autoencoder model. The performance of the model is satisfactory for both the problems. Specifically, the multiscale nature and the gap flow dynamics of the side-by-side cylinders are captured by the proposed data-driven model reduction methodology. The error metrics, the normalized squared error and the normalized reconstruction error are considered for the assessment of the data-driven framework.

Flow induced vibration (FIV) from high velocity multiphase flow is a common source of vibration concern in process piping, potentially leading to fatigue failures and hydrocarbon leaks. FIV screening methods tend to be conservative for multiphase flows and are typically only validated for simple single bends at low pressure. FE can predict the response of a system if a sensible forcing function is provided. CFD can be used to predict realistic forcing functions in complex combinations of bends and tees, typically seen in process piping systems. FIV studies were performed on a topside production system operated by Equinor, carrying multiphase flow at high pressure (∼69 bara) conditions, where significant vibration was measured. The study assessed different vibration simulation methodologies, combining FE analysis with forcing functions based on both correlations and CFD simulations. The aim was to gain a better understanding of the accuracy and limitations of calculation methods typically used to assess fatigue. CFD simulations predicted similar force magnitudes but higher frequency forcing at 69 bara compared to equivalent simulations at atmospheric pressure (at the same liquid and gas superficial velocities). The forcing function correlations used do not predict higher frequency forcing at high pressure, which has a significant impact on the predicted vibration. Care is required when undertaking this type of analysis. It is important to have an accurate FE model of the as-built pipework and supports as well as a forcing function which accurately represents the fluid forces on the bends. For the case simulated here the magnitude and peak frequency of the forcing function had a significant influence on the response of the structure. Forcing functions based on correlated data from tests at low pressure should be used carefully for high pressure systems. In addition, the inclusion of phasing of the forces at each bend can influence the structural response, and simulations performed in the frequency domain do not consider this. A combination of CFD and FE modelling offers a potentially powerful tool for assessing and diagnosing multiphase FIV problems in hydrocarbon production piping systems.

While passing through a navigable tunnel, a vessel usually undergoes a steady forward motion at low speed. Due to the limited size of the navigable tunnel, the restricted water conditions of small section-coefficient will have an adverse impact on vessel navigation safety. A bottom suction effect on vessels may occur for the risk of grounding and harming the maneuvering performance. Thus, it is particularly important to reveal the effect of the navigable tunnel scale on the vessel sinkage. In this paper, a numerical model of the representative 1000-ton vessel of the Wujiang channel under construction is established. Numerical simulations of the vessel are conducted based on RANS equations in deep water, both in fixed condition and free condition (i.e. the trim and sinkage are allowed). And the validity of the method is verified by comparing the calculation results with the experimental ones. Subsequently, for diverse water depths, water widths and vessel speeds, the parameters such as the vessel resistance and sinkage are predicted and compared. The calculation results are analyzed to obtain the trend of vessel motion at different tunnel scales. In addition, the effect of the tunnel scale on vessel navigation performance is also investigated.

The purpose of this study is to numerically investigate the bed shear stress and near-bed mixing due to coherent vortex structures in the vicinity of a vertically wall-mounted circular cylinder subject to an imposed finite-depth oscillatory sinusoidal flow. Previous studies reveal that the Keulegan–Carpenter (KC) number influences the formation of lee-side wake vortex structures as well as the horseshoe vortex in front of a cylinder. Therefore, parametric studies in a moderately wide range of KC from 5 to 20 are numerically performed. In the present study, Direct Numerical Simulation (DNS) is conducted using the open-source software, OpenFOAM, that solves the three-dimensional unsteady incompressible Navier-Stokes equations using finite volume method. Nondimensional parameters used in the simulations are carefully chosen to represent the real physics. The numerical solutions are first validated using an analytical solution for the oscillating Stokes flow and the results are then systematically and quantitatively compared with the experimental measurements. The results show that the lee-side wake is significantly influenced by KC, and distinctive types of the lee-side wake are generated and classified based on KC. It is also found that both KC and the ratio of the thickness of the Stokes boundary layer to the water depth are heavily associated with the stability of the lee-side wake. In addition, the simulated size and lifespan of the horseshoe vortex agree well with the experimental data.

The non-Newtonian shear-thinning fluid widely exists in the industrial process and the rheology exerts a significant influence on the flow pattern transition and flow-induced vibration (FIV). However, studies on the rheology effect of the liquid phase in the vertical upward two-phase flows are quite lacking due to the complexity of non-Newtonian fluid properties. In the present study, the vertical upward gas/shear-thinning liquid flows experiments are conducted on a rigid acrylic pipe with an internal diameter of 20 mm. Three different Carboxymethyl Cellulose (CMC) solutions are used as the non-Newtonian fluid, aimed at capturing a two-phase flow regime transition including the vertical slug, churn and annular flows. The results indicate that the maximum energy spectral densities of vibration occur at the slug-to-churn flow transition boundary at low liquid velocities and the annular flow region under high liquid velocities, respectively. The effects of the rheology of the shear-thinning fluid in terms of the flow patterns and FIV are also presented and discussed.

The vortex induced vibration of slender cylindrical structures is common in offshore structures and marine applications such as risers, towing cables, etc. The VIV response of such slender elements in steady uniform current has been investigated in the past using numerical and experimental studies. Though few numerical studies exist for varying current (sheared flow), experimental studies are limited. Hence, the experimental studies are an essential part of VIV investigation, especially for sheared flow. The experiments were conducted using a specially fabricated circular steel tank of diameter 2.4 m with a central hinge to rotate the pipe horizontally in a water pool of depth 0.7 m. Shear current is simulated by rotating the pipe about the hinge. A pipe of diameter 25 mm (= D ) and length 1 m (= L ) was fixed at one end of the rotating cable support, and the other end was passed over a pulley inside a rotating cylinder. The rotating cylinder is provided with a pulley at the top to tension the pipe. A shear current with a maximum velocity of 1.3 m/s and a minimum velocity of 0.1 m/s is generated using the set up. The VIV response of the pipe was measured using electrical resistance-type strain gauges pasted along the length. The measured axial strain was used to obtain transverse displacements, which was used to determine the response frequency, amplitudes, and forces. The Strouhal number was calculated. The VIV response and the fluid force coefficients obtained from the experiments were compared with Shear7 results.

In this paper, a new multiphase MPS-GPU method is proposed through the combination of moving particle semi-implicit (MPS) method and Graphics Processing Unit (GPU) acceleration technique. The new method not only inherits the advantage of MPS method in capturing complex interface deformations, but also overcomes the limitations of huge computational cost in three-dimensional MPS simulation. By this method, both the two-layer-liquid and three-layer-liquids sloshing problems are simulated three-dimensionally on the GPU device, in which more than one million of particles are included. In simulations, the sloshing patterns of each liquid layer under different external excitations are accurately captured. From the interface elevations and impacting pressures calculated by present method, it is found that an obvious discrepancy exists between the deformations of free surface and phase interfaces. Then, the results obtained by multiphase MPS-GPU method are compared with experimental data and other numerical results in open literature and a good agreement is achieved, which validates the accuracy and applicability of the present method in three-dimensional simulations of multi-layer-liquid sloshing flows.

This paper examines the hydrodynamic problem of a two-dimensional symmetric and asymmetric wedge water entry through freefall motion. The gravity effect on the flow is considered and because of precise simulation close to the real phenomenon, the oblique slamming is analyzed. The defined problem is numerically studied using SIMPLE and HRIC schemes and by implementing an overset mesh approach. In order to evaluate the accuracy of the numerical model, the present results are compared and validated with previous experimental studies and showed good agreement. The results are presented and compared for each symmetry and asymmetry in different deadrise angles, drop heights and heel angles. Based on a comparison of the measured vertical acceleration of the experimental wedge data, it is determined that the proposed numerical method has relatively good accuracy in predicting the slamming phenomenon and wedge response. The influence of viscous regime on water entry simulations is investigated, in according to results, effect of viscosity is negligible. Results show that the heel angle dramatically affects the wedge dynamics, pile-up evolution, and pressure distribution. These results suggest evidence for a complex interaction between geometric parameters on the water entry of rigid wedges, which could finally develop our understanding of planing vessels operating in real sea conditions.