Cross-flow Motion Research Articles

Flow-induced vibrations of a twistable two-tandem cylinder system across varied gap ratios

This paper presents a simulation study on the flow-induced vibrations (FIVs) of a two-tandem cylinder system with three degrees of freedom, encompassing in-flow, cross-flow, and torsional motions at a Reynolds number of 150. We specifically focus on investigating the impact of gap ratios (G*) between the cylinders, considering values of 0.5, 2, and 3. The ratio of torsional natural frequency to translational natural frequency was maintained at 12.5, while the mass ratio between the cylinder system and the fluid was set to 2. Our findings reveal a notable sensitivity of the FIV responses to the gap ratio between the cylinders. It was observed that the cylinder system's vibrations can undergo locking phenomena at both the translational and torsional natural frequencies, with maximal cross-flow amplitudes occurring within the torsional locked region. Particularly, at G* = 0.5, the maximal cross-flow amplitude reached approximately 21.4 times that of the cylinder system without torsional vibration, posing potential risks of fatigue failure in offshore equipment. Conversely, at G* = 2, an unusual ultra-low frequency was detected in the in-flow vibration. This phenomenon significantly magnified the FIV of the cylinder system, despite the absence of frequency locking. Furthermore, at G* = 3, we observed an expanded range of vibrations with ultra-low frequency compared to G* = 2. Our analysis identified the differential development speed of the shear layers around the upstream and downstream cylinders as a key factor contributing to varying vortex shedding frequencies. This discrepancy led to periodic changes in the vortex shedding mode of the cylinder system over time, thereby introducing low-frequency components into the flow-induced vibration. These insights deepen our understanding of the complex dynamics governing the FIV of tandem cylinder systems, which holds implications for the design and maintenance of offshore structures.

The dynamic response of floating offshore wind turbine platform in wave–current condition

In this paper, the fluid–structure interaction of floating offshore wind turbine (FOWT) platforms under complex ocean conditions is investigated using OpenFOAM and in-house developed models. Two types of FOWT platform, i.e., a semi-submersible platform and a barge platform, are studied for their dynamic responses to either wave or current. The results reveal that a semi-submersible platform exhibits larger cross-flow motion and lock-in phenomenon, while a barge platform experiences smaller motion with no significant lock-in within the velocity range examined. The combined wave–current conditions are further studied for the semi-submersible platform, with different angles between wave and current, the current speeds, and wave parameters. Unlike other investigations focusing on colinear wave–current interaction, in which the waves usually mitigate vortex-induced motion (VIM); here, we find that waves might lead to an enhanced VIM with a large angle between current and wave. The evaluation on the interaction effect factor shows that the largest wave height in the lock-in region does not lead to the most dangerous scenario, herein, the largest platform motion. Instead, a smaller wave height with a large wave period can induce even larger motion.

Experimental and numerical study of the hydrodynamic features of a cylindrical FPSO considering current-induced motion

Hydrodynamic features of a cylindrical FPSO in the wave, current and coupled wave and current load conditions were experimentally and numerically studied in high sea states. In the wave condition, a large low frequency IL (inline, along the environmental load's direction) motion is observed which is mainly dominated by the 2nd order difference frequency wave loads but no CF (crossflow, perpendicular to the environmental load's direction) motion, and the heave motion is excited by the 1st order wave force. In the current condition, obvious motions are observed in the IL and CF directions, and the dominant frequency of the inline motion is about twice of that of the crossflow motion. Vortex-induced forces are supposed as the main reason. The motion amplitudes in the coupled load condition are larger than those in pure wave condition in 6 DOFs, especially for an obvious CF motion, and the mean offset is about the sum of those in the wave and current conditions. Numerical simulation is then conduced with the identified damping coefficients (from model test) and considering the current-induced motion. The wave motions can be correctly revealed in the numerical simulation by a modified panel model (including an additional damping). For the coupled-load condition, by a simplified time-domain VIM prediction method, the IL and CF motions can be revealed but the amplitude is a little overestimated. The experimental findings and numerical results reveal hydrodynamic features of a cylindrical FPSO, which also provide a reference for the design of it.

Asymmetric vortex pair induces secondary traveling wave vibration of a flexible cylinder from still water to incoming flow

As one of the fundamental problems in fluid mechanics, the flow-induced vibration of a flexible cylinder helps shed light on various complex fluid–structure interaction phenomena, such as the coupling effect of the cross-flow and in-line motions in the relationship between external fluid forces and vortical wake patterns. This paper devised a non-uniform in-flow condition (partially uniform flow and partially still water) for flexible cylinders in experimental and numerical conditions. Consistently, a new phenomenon is observed in different scale experiments and simulations where secondary traveling wave vibration of the flexible cylinder is excited from the still water part to the uniform inflow part due to the positive external fluid energy input in the still water. Furthermore, the detailed flow visualization on the vortical wake patterns around the vibrating flexible cylinders reveals that the external fluid force sources in the still water are due to the existence of the attached vortex pair with an uneven strength, which has been observed before in the still rigid cylinder in the oscillatory flow or the rigid cylinder oscillating in the still water at Keulegan–Carpenter number from 4 to 7.

Passive fluid-induced vibration control of viscoelastic cylinder using nonlinear energy sink

This study focuses on the performance of the nonlinear energy sink (NES) in passive controlling the cantilever cylinder vibrations subjected to the external fluid flow. The nonlinear differential equations of motion are obtained by considering the large strain-displacement relation and viscoelastic behavior. Wake oscillation in fluid-structure interaction is modeled based on the Van der Pol wake oscillator model with is the classic acceleration coupling between the cross-flow motion and wake. Based on the Von Karman strain-displacement relation, and Euler-Bernoulli beam theory, the nonlinear vibration equations which are coupled with attached NES motion are obtained using Newton's second law, and discretized by applying the Galerkin method. The fluid flow velocity and nonlinear stiffness, damping, and mass of the NES are studied to determine their effects on the vibration response of the system. The present study comprehensively evaluates the effects of adding a NES on the lock-in phenomenon and maximum oscillating amplitudes of a cantilever cylinder, and guides to determine the best design of NES for significant fluid-induced vibration mitigation.

A fluid–structure interaction simulation on the wake-induced vibration of tandem cylinders with pivoted rotational motion

This paper predicts the wake-induced vibration of tandem cylinders with pivoted rotational or translational motion using the fluid–structure interaction technique. Large eddy simulation using a dynamic Smagorinsky subgrid-scale model is applied for the turbulent flow. Structural computation is applied to obtain the displacement of the downstream cylinder in the cross-flow direction. Conditions such as damping ratio, natural frequency, and mass ratio of pivoted rotational motion are the same as those of translational motion. The computational conditions are Re=3790, 8470 and 15 160, corresponding to U*=5.0, 11.2 and 20.0, respectively. The unsteady characteristics of lift, drag, and cross-flow motion of tandem cylinders are investigated. Distributions of pressure, λ2 criterion, vorticity, and streakline are also investigated. Finally, the energy harvesting capability of the pendulum system is compared with the translation system.

A large-eddy simulation study on the similarity between free vibrations of a flexible cylinder and forced vibrations of a rigid cylinder

The strip theory in hydrodynamics has been widely used for predicting complex vortex induced vibrations (VIV) behind bluff bodies, but the question of how accurate such predictions are has not been addressed adequately before. In order to corroborate the application of strip theory in VIV, we present a comparative study between free mono-frequency vibrations of a long flexible cylinder in both uniform and linearly sheared flow and corresponding forced vibrations of a rigid cylinder with prescribed sinusoidal motions. We employ the entropy–viscosity large-eddy simulation (LES) to resolve the vortical flow and the coupled cylinder response, which we validate by companion experiments of the same configuration. We then extract from LES, at the same Reynolds number, the values of the sectional vibration amplitude, frequency, and phase angle (between inline and crossflow motions), and use them as input parameters for the forced vibration case, for which we perform two-dimensional simulations. We show here by systematic simulation studies that the hydrodynamic coefficients exhibit strong similarities between the two cases, and the forced vibration closely resembles the sectional near wake of the free vibration.

Modelling of coupled cross-flow and in-line vortex-induced vibrations of flexible cylindrical structures. Part I: model description and validation

This paper is first of the two papers dealing with the nonlinear modelling and investigation of coupled cross-flow and in-line vortex-induced vibrations (VIVs) of flexible cylindrical structures. As a continuation of the previous work (Qu and Metrikine in Ocean Eng 196:106732, 2020) where a new single wake oscillator model was proposed and studied for VIVs of rigid cylinders, the present paper focuses on applying it to flexible cylinders. In this paper, the structure is modelled as an extensible Euler–Bernoulli beam and its 3D nonlinear coupling motion is described in the absolute coordinate system. The single van der Pol wake oscillator model with nonlinear coupling to the in-line motion of the structure, in addition to the classic linear cross-flow motion coupling, is uniformly distributed along the structure to model the hydrodynamic force acting on it. The finite element method has been applied to solve the dynamics of the coupled system, and the experiments of the VIV of a top-tensioned straight riser subjected to a step flow have been taken for the validation of the model. The model has been shown to be able to capture most features of VIVs of flexible cylinders, and a good agreement between the simulation results and the experimental measurements has been observed with regard to the amplitude, frequency and excited mode of both cross-flow and in-line vibrations, as well as the mean in-line deflection due to the amplified in-line force. While it is conventionally expected that the VIV of a flexible cylinder subjected to a uniform flow is dominated by a single frequency, a multi-frequency response is observed in the simulation results over the range of flow velocities through which the transition of the dominant mode of vibration occurs.

Delicacy Management on Kiloton Dry Wet Spinning Bath Liquid

During the dry-jet wet spinning process of polyacrylonitrile carbon fiber precursor, the fluctuation of the coagulation bath liquid level affects the stability of the nozzle directly. The motion trajectory and the motion intensity in all directions of the fluid during the movement of the fiber in the coagulation bath fluid field were studied. A three-dimensional model of the cross flow and jet collision motion trajectory was established, and the impact of fluids with different strengths on the surface of the coagulation bath was analyzed. Solidification of the liquid surface in the fitting strength of the peak effect of the return wave overflow trough coupled superimposed to determine the coagulation bath surface to eliminate interfering fluctuations affecting factors. Based on the above analysis, a smart device for real-time monitoring of the coagulation bath air layer has been developed by using the damping equipment in the coagulation bath, where the impact of various fluid waves on the liquid surface can be eliminated on fixed point or position. According to the understanding of kiloton dry-jet wet spinning precursor fiber production line, in-depth exploration has been made to control the high dynamic coagulation bath level effectively from the perspective of technology and delicacy management.

A practical technique for hydrodynamic coefficients modification in SHEAR7 for fatigue assessment of riser buoyancy modules under vortex-induced vibration

The hydrodynamic coefficients in semi-empirical tool are optimized for a riser attached with staggered buoyancy modules undergoing Vortex-Induced Vibration (VIV) based on the SHELL Oil Experiment. The existing prediction tools are based on data derived from experiments, number of assumptions from strip theory, energy balance between fluid and structure and use of lift coefficient databases. Current advancements in conducting and calculating VIV response from the experiments shows that these assumptions may be invalid. The major difference arises between experimental observations and theoretical estimations is from lift coefficient databases. The databases extracted under laboratory conditions have limited Reynolds number flow conditions and only cross-flow motions were considered. The extracted hydrodynamic excitation coefficient database is posed to optimization problem, where the main objective is to minimize the prediction error of semi-empirical tools when compared with experimental results. We optimize the existing excitation coefficient database to improve the fatigue damage prediction of riser attached with staggered buoyancy modules. Application of modification factors and optimizing the hydrodynamic lift coefficients to the data from SHELL Exploration and Production Test provides the new optimized excitation coefficient datasets which reduce the error in the predicting the VIV response of the riser with staggered buoyancy modules.

A single van der pol wake oscillator model for coupled cross-flow and in-line vortex-induced vibrations

In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-line vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-line motion is coupled with the wake variable in a nonlinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-line vibrations at high frequencies are also discussed along with a possible remedy.

Mapping the properties of the vortex-induced vibrations of flexible cylinders in uniform oncoming flow

Flexible structures placed within an oncoming flow exhibit far more complex vortex-induced dynamics than flexibly mounted rigid cylinders, because they involve the distributed interaction between the structural and wake dynamics along the entire span. Hence, mapping the well-understood properties of rigid cylinder vibrations to those of strings and beams has been elusive. We show here with a combination of experiments, conducted at Reynolds number, $Re$ from 250 to 2300, and computational fluid dynamics that such a mapping is possible for flexible structures in uniform flow undergoing combined cross-flow and in-line oscillations, but only when additional concepts are introduced to model the extended coupling of the flow and the structure. The in-line response consists of largely standing waves that define cells, each cell spanning the distance between adjacent nodes, over which stable vortical patterns form, whose features (‘2S’ versus ‘P$+$S’) depend strongly on the true reduced velocity, $V_{r}=U/f_{y}d$, where $U$ is the inflow velocity, $f_{y}$ is the cross-flow vibration frequency and $d$ is the cylinder diameter, and the phase angle between in-line and cross-flow response; while the cross-flow response may contain travelling waves, breaking the symmetry of the problem. The axial distribution of the highly variable effective added masses in the cross-flow and in-line directions, and the local phase angle between in-line and cross-flow motion determine the single frequency of cross-flow response, while the in-line response vibrates at twice the cross-flow frequency. The cross-flow and in-line lift coefficients in phase with velocity depend strongly on the true reduced velocity but also on the local phase angle between in-line and cross-flow motions. Modal shapes can be defined for in-line and cross-flow, based on the resemblance of the response to conventional modes, which can be in the ratio of either ‘$2n/n$’ or ‘$(2n-1)/n$’, where $n$ is the order of the cross-flow response mode. We use an underwater optical tracking system to reconstruct the sectional fluid forces in a flexible structure and show that, once the cross-flow and in-line motion features are known, employing strip theory and the hydrodynamic coefficients obtained from forced rigid cylinder experiments allows us to predict the distributed forces accurately.

Streamline Computation Study on Rotation Aerodynamic Noise Prediction of Cross-flow Fan

The approach of resolving analysis the cross-flow relative motion streamline on the cross-flow straight blade is presented to predict the rotation aerodynamic noise performance of cross-flow fan by solving the dipole source term of Ffowcs Williams and Hawkings equation, while the cross-flow fan is applied in the indoor unit of split-type air-conditioner and operated in the rated condition. The calculating results of the method are respectively drawn by programming in Matlab computational language, and compared with the results of CAA numerical simulation and noise experiment. There are some differences in the distribution condition of aerodynamic force fluctuation amplitude on the blade surface, and sound pressure in the related frequency on the indoor unit casing, or on the sphere far field, between the streamline computation approach and numerical simulation. The orders of magnitudes of these calculating results solved by the analysing streamline method are similar with that attained by the numerical simulation. The resolving analysis approach has the characteristics of decreasing the computing cost and not constructing the acoustics grid model of fan, compared with the numerical simulation. The error between the result of numerical simulation and noise test is larger than that between the result of theoretical calculation and noise test, so the approach could be used for the rotation aerodynamic noise analysis of the cross-flow fan.

Nonplanar multi-modal vibrations of fluid-conveying risers under shear cross flows

Risers/pipes conveying fluid are a typical kind of slender structures commonly used in marine engineering. It is of great academic significance and application value for us to evaluate and understand the vibration characteristics and nonlinear responses of these risers under the combined action of internal and external fluid flows. In this paper, the nonplanar vibrations and multi-modal responses of pinned-pinned risers in shear cross flow are numerically studied. With this objective in mind, the van der Pol wake oscillators are used to simulate the dynamical behavior of the vortex shedding in the wake. Two nonlinear equations of motion of the riser are proposed to govern the lateral responses of the riser structure. The nonplanar nonlinear equations for the riser and wake are then discretized by employing Galerkin's method and solved by using a fourth-order Runge–Kutta integration algorithm. Theoretical results show that the coupled frequencies for cross-flow (CF) and in-line (IL) motions and the corresponding coupled damping ratio could be influenced by the external and/or internal fluid velocities. Based on extensive calculations, the dynamical behavior of the riser with various internal and external flow velocities are presented in the form of bifurcation diagrams, time traces, phase portraits, oscillation trajectories and response spectrum curves. It is shown that some interesting dynamical phenomena, such as ‘lock-in’ state, ‘figure-of-eight’ trajectory and quasi-periodic oscillation, could occur in such a fluid-structure interaction system. Our results also demonstrate that the shear parameter can significantly affect the dynamic responses of the riser. When the shear parameter of the cross flow is large, multi-modal quasi-periodic responses of the riser can be excited, showing some new features undetected in the system of fluid-conveying risers in uniform cross flow.

One- versus two-degree-of-freedom vortex-induced vibrations of a circular cylinder at [formula omitted

The one- versus two-degree-of-freedom vortex-induced vibrations of a circular cylinder are investigated on the basis of direct numerical simulation results. The Reynolds number, based on the oncoming flow velocity and cylinder diameter, is set to 3900. Three cases are examined: the elastically mounted body is free to oscillate either in the direction aligned with the current (in-line direction; IL case), in the direction normal to the current (cross-flow direction; CF case), or in both directions (IL+CF case). In each case, the behavior of the flow–structure system is studied over a range of values of the reduced velocity (inverse of the oscillator natural frequency). The in-line and cross-flow responses observed in the IL+CF case substantially differ from their one-degree-of-freedom counterparts, especially in the intermediate reduced velocity region. In this region, no vibrations develop in the IL case and in-line oscillations only occur if cross-flow motion is allowed. These in-line oscillations are accompanied by a major increase of the cross-flow responses, compared to the CF case. The two-degree-of-freedom vibrations are associated with the emergence of large-amplitude higher harmonics in the fluid force spectra. These aspects and more specifically the impact of the existence of a degree-of-freedom and oscillations in a given direction, on the fluid force and structural response in the perpendicular direction, do not seem to be systematically connected to changes in wake topology. Here, they are discussed in light of the orientation and magnitude of the instantaneous flow velocity seen by the moving body.

The SCR Wave-Rigid Body Rotation Model Maximum Amplitude Analysis Influenced by Z Direction Motion Based on Cable3D

Rigid body rotation(RBR) of steel catenary riser(SCR) is a problem which cannot be ignored in the caculation of transverse flow displacement and other velocity and acceleration. For the chapter, on basis of the RBR, wave force model, the large-deflection slender beam model(LDSB) and the suspension point’s z direction motion, the influence of RBR on the transverse direction displacement of the SCR is explored. The results of analysis indicate that the structure is large affected by the wave and cross flow motion relative to RBR. With water depth augment, the maximum amplitude reduces from 10th to 200th.The displacement of RBR on the structure increases with the increase of vector S, and decreases with the diminution of vector S. At 140 nodes, the effect is greatest There exists phase difference between rigid body oscillation and linear motion superposition the wave load, In working conditions 2 and 3, it is more significant. And the amplitude increased, offset or even decreased.

Numerical simulation of vortex-induced vibration of long flexible risers using a SDVM-FEM coupled method

This paper presents a grid-independent numerical methodology that couples the strip theory based discrete vortex method (SDVM) with the finite element method (FEM) to simulate the vortex-induced vibration (VIV) of a long flexible vertical riser. Based on the strip theory, a three-dimensional flow filed is approximately simulated by a series of computational ‘flow strips’. A Lagrangian discrete vortex method is employed to numerically solve the unsteady vorticity transport equations of each ‘flow strip’. The flexible riser is modelled as a tensioned Bernoulli-Euler beam with the dynamical equation solved by the finite element method in time domain. The two-dimensional DVM code is firstly validated for the VIV simulation of a rigid cylinder that was experimentally studied by Khalak and Williamson (1996). Referring to two typical experimental configurations of Lehn (2003) and Chaplin et al. (2005), the VIV of a long flexible riser immersed in a uniform and stepped incoming flow are numerically simulated, respectively. A good agreement was achieved through detailed comparisons between the present numerical prediction and the experimental data, including the structural in-line and cross-flow VIV response modes, root mean square amplitudes and the dominant frequency. The occurrence of standing and travelling wave responses, dual resonance between in-line and cross-flow motions and figure-eight trajectory are reported. The wake patterns corresponding to two response waves are also presented and investigated. Related to structural local vibration amplitude, two principle vortex patterns resembling the ‘2S’ and ‘2P’ modes are identified in the wake. The standing wave component of structural response determines the vortex shedding pattern. The travelling wave component affects the spanwise vortices shedding at different phases.

Numerical simulation of vortex-induced motion of a deep-draft paired-column semi-submersible offshore platform

Understanding and predicting vortex-induced motion (VIM) of offshore systems for deep seawater applications is crucial to improve the system safety and integrity. We report on experimental tow-tank measurements and numerical simulations of VIM of a deep-draft offshore platform, specifically Paired-Column Semisubmersible (PC-Semi). The study is carried out in model scale (1:54), at subcritical flow regime with Re∼104. Motion of the floating structure has three degrees of freedom: in-line, cross-flow, and yaw. Large periodic cross-flow motion is measured for headings 0°, 11.25°, and 22.5°, for reduced velocities (Ur) between 5 and 10. Considerably smaller cross-flow amplitude is recorded at 45° heading across the Ur range considered.An extensive sensitivity study is performed using computational fluid dynamics (CFD) to capture the transient displacement history of VIM (in-line, cross-flow, and yaw motion components). Amplitude and period of cross-flow (transverse) motion are obtained from statistical analysis of VIM time history and subsequently used as the validation criterion between the CFD simulation and the model tests. Satisfactory agreement between the CFD results and tow-tank measurements is achieved with a Delayed Detached Eddy Simulation – Shear Stress Transport (DDES-SST) formulation. This work provides experimental results and serves as a practical starting point to set up a CFD problem to estimate amplitude and period of cross-flow VIM motion for offshore engineering applications.

Multivariate Analysis Of Vortex-Induced Vibrations In A Tensioned Cylinder Reveal Nonlinear Modal Interactions

The modal structure of a tension-dominated flexible cylinder undergoing vortex-induced vibrations in a uniform current is investigated through multivariate analysis. Experiments are performed in a recirculating flow channel where a flexible cylinder is mounted across the flow channel and subjected to a uniform current. The cylinder response is measured using motion tracking with two high speed cameras, allowing for the measurement of both in-line and cross-flow motions of the cylinder. The recorded motion of the cylinder is analyzed in its measured phase space using smooth orthogonal decomposition (SOD) and proper orthogonal decomposition (POD). Both decompositions show that most of the motion energy is captured in a six-dimensional subspace; however, the corresponding modal structures are different between the decompositions. While SOD-based modes are persistent regardless of whether the flow speed is increasing or decreasing, the POD modes do not maintain this consistency. For example, the dominant (i.e., most energetic) POD mode changes shape depending on whether the flow speed is undergoing an increase or decrease. This inconsistency leads to a variability in the modal energy that is affected by hysteresis if the POD method is used. Modal frequency response curves show the corresponding variance of the modal response versus the reduced flow velocity. SOD-based plots show hardening spring behavior in the first mode oscillations, which are dominant at low reduced velocities and have negligible amplitude at high reduced velocities. Second mode oscillations are negligible at low reduced velocities but undergo subcritical Hopf bifurcation at higher reduced velocities. The associated hysteresis behaviors are observed over the same range of reduced velocity values for both modes. POD analysis also detects this behavior, but it is less pronounced and leaks into higher-order modes.

CFD prediction of vortex induced vibrations and fatigue assessment for deepwater marine risers

Using 3D computational fluid dynamics techniques in recent years have shed significant light on the Vortex Induced Vibrations (VIV) encountered by deep-water marine risers. The fatigue damage accumulated due to these vibrations has posed a great concern to the offshore industry. This paper aims to present an algorithm to predict the crossflow and inline fatigue damage for very long (L/D > 103) marine risers using a Finite-Analytical Navier-Stokes (FANS) technique coupled with a tensioned beam motion solver and rainflow counting fatigue module. Large Eddy Simulation (LES) method has been used to simulate the turbulence in the flow. An overset grid system is employed to mesh the riser geometry and the wake field around the riser. Risers from NDP (2003) and Miami (2006) experiments are used for simulation with uniform, linearly sheared and non-uniform (non-linearly sheared) current profiles. The simulation results including inline and crossflow motion, modal decomposition, spectral densities and fatigue damage rate are compared to the experimental data and useful conclusions are drawn.