The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier–Stokes (URANS) based on the realizable k-epsilon (RKE) turbulence model, URANS based on the k–omega shear stress transport (SST) turbulence model and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate prediction of all. Moreover, by comparing the instantaneous vorticity contours and 3D turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF) based simulation of the free surface, it is demonstrated that the free surface has no significant effect on mean drag coefficient and Strouhal number.

Open ocean aquaculture cages became a promising alternative to traditional fish cage designs recently. The offshore environment implies larger loads on the structure and thus higher risk for fish loss. Floating rigid aquaculture cages with stiff nets are considered as a possible solution to cope with these new challenges. Their design process requires more advanced tools to account for the non-linear fluid–structure interaction. This paper presents a suitable numerical approach for analyzing the interaction of offshore aquaculture cages and waves using computational fluid dynamics. Here, a numerical wave tank accounts for the accurate propagation of the waves, and dynamic solutions are utilized for the cage system. Two-way coupling is enabled by accounting for the influence of the net on the fluid. The numerical model is validated against measurements for the loads on and the responses of a mobile floating fish farm in waves and current.

An analytical study is presented here to investigate the scattering of oblique flexural gravity waves by a pair of totally submerged vertically placed porous barriers, located at some distance from each other, for a homogenous fluid flowing over a porous sea-bed. A thin ice-sheet, replacing the usual free surface, is considered as the upper surface where it is treated as a thin elastic plate by following Euler–Bernoulli beam equation. The complete analytical solution, under the assumption of small-amplitude theory and structural response, is acquired by employing eigenfunction expansion and least square method for the problem of flexural gravity waves interacting with the submerged porous barriers. Subsequently, computation for the reflection and transmission coefficients, energy loss and wave forces are carried out and discussed for different parameter values corresponding to the ice-sheet, porous sea-bed, and porous barriers. This study establishes that the oscillatory behavior exhibited by the reflection of the waves. It further shows that when the inertial effect of the porous-effect parameter of the barriers is increased, the minima in wave reflection occur. The vertical porous barriers are found to dissipate a significant portion of the wave energy when an increase in the inertial effect of the porous barriers is affected. The hydrodynamic force on the barriers also follows an oscillatory pattern, and it increases when the length of the barrier is increased. Furthermore, wave transmission decreases significantly due to the energy dissipation by the porous sea-bed. It is demonstrated that corresponding to various structural parameters, almost no reflection and full transmission take place for an impermeable sea-bed and also when only real porosity parameter of the porous sea-bed is considered. The effect of the ice-sheet on the propagation of the flexural waves is also examined by obtaining a number of results for variation of various parameters. Variation in the elastic parameter of the floating ice-sheet is observed to command a considerable influence when the wave impinges upon the submerged vertical porous barriers.

The significant difference in length scales between the flow around a moving fish net and the flow around each twine of the net prevents the resolution of the complete structure within a discrete fluid domain. In this article, this issue is overcome by calculating the net and fluid dynamics separately and incorporate their interaction implicitly. The forces on the net are approximated using a screen force model, and the motion of the net is computed with a lumped mass method. Here, a linear system of equations is derived from the dynamic equilibria and kinematic relations. The net model is coupled to the computational fluid dynamics (CFD) solver reef3d , which solves the incompressible Navier–Stokes equations using high-order finite differences in space and time. Several numerical calculations are provided, and the comparison of loads and velocity reduction with available measurements indicates the good performance of the proposed model.

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom-mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain and frequency-domain nonlinear exciting forces in a seastate with a significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The higher-order moments of the nonlinear load are evaluated from simulated force records, and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed.

Production and deep-sea mining risers should have sufficient flexibility to avoid severe loading due to vessel motions. Free hanging catenary is the simplest and cheapest configuration because of the ease of installation and the minimal requirements for subsea infrastructure. The interaction of the inner flow with the moving riser dominates the dynamic stability of the system. In the present work, the motion equation for a long, multilayered, fiber-reinforced polymeric (FRP) flexible riser is formulated, and a numerical method for critical inner flow velocities causing buckling in risers is proposed. Taking into account the motion equation, the physics of the inner flow-induced buckling phenomena is analyzed and a detailed interpretation of the terms of the motion equation is carried out. With the aid of transfer matrices and finite elements, the dynamic buckling modes and the natural frequencies are determined. It is proved that the dynamic stability of the riser is affected by the balance of the elastic flexural restoring force, the centrifugal force of the fluid flow in the curved portions, the Coriolis force of the fluid, the inertial force of the fluid and pipe mass, and the damping due to the effect of the surrounding water.

In this paper, the propulsive performance of a caudal peduncle-fin swimmer mimicking a bio-inspired robotic fish model is numerically studied using a fully coupled FSI solver. The model consists of a rigid peduncle and a flexible fin which pitches in a uniform flow. The flexible fin is modeled as a thin plate assigned with non-uniformly distributed stiffness. A finite volume method based in-house Navier–Stokes solver is used to solve the fluid equations, while the fin deformation is resolved using a finite element code. The effect of the fin flexibility on the propulsive performance is investigated. The numerical results indicate that compliance has a significant influence on performance. Under the parameters studied in this paper, the medium flexible fin exhibits remarkable efficiency improvement, as well as thrust augment, while the least flexible fin shows no obvious difference from the rigid one. However, for the most flexible fin, although the thrust production decreases sharply, the efficiency reaches the maximum value. It should be noted that by non-uniformly distributing the rigidity across the caudal fin, our model is able to replicate some fin deformation patterns observed in both the live fish and the experimental robotic fish.

The increasing of deepwater oil field developments brings a growing need for thermal management to prevent hydrate and wax formations in the subsea production system, due to the low environment temperature and long-distance transportation. Pipeline insulation coating is a typical strategy for thermal management. In a subsea production system, pressure, temperature, flowrate, and length of each flowline vary, leading to different thermal performances of the transported fluid. Therefore, the insulation coating should be carefully designed from the overall perspective to minimize the total material volume, thus reducing the cost. In this paper, an optimization model for the insulation material volume of a rigid subsea flowline system is proposed. Then, the best insulation thickness of each subsea flowline can be determined under given flow parameters and temperature requirements. The factor that defines the temperature drop from the riser base to the top termination is introduced and analyzed. There is a proper temperature drop factor associated with the insulation material volume for subsea flowlines, as well as a proper insulation capacity for the risers. This optimization model can define the subsea system insulation and provide reliable results for cost estimation.

Two partially filled tanks fixed on rectangular barge floating in variable bathymetry is studied based on the linearized potential theory in the frequency domain. The internal sloshing motion is simulated by linearized superposition of natural sloshing modes. The motion of barge is solved by matching eigenfunctions expansion in the external fluid domain. The computational model is verified by experimental results. Coupling sway and roll motions of floating body with different filling levels are compared. The effects of both inclined bottom with different slopes and hump-shaped bottom with different heights are analyzed. Viewing from the results, coupling sway and roll motions of floating body are significantly affected by the shapes of bottoms.

A time-domain numerical model with the linear free surface condition and large amplitude body motions is developed based on the higher-order boundary element method. Five numerical formulations are used to compute the nonlinear radiation forces on a moving body in the domain. A series of submerged bodies that have sharp edges and rounded corners with small radii are considered to investigate their reliabilities. It is found that for the bodies with rounded corners the nonlinear radiation forces calculated by the five numerical formulations are close to each other, while large discrepancies exist for the body with sharp edges. Examination on the vertical nonlinear radiation forces due to the body surge motion reveals that the body edge has less influence on the method of the rate of change of the fluid momentum; however, it has a stronger effect on the other four formulations. The former applied a substitution of the spatial derivatives of the velocity potential, and the other four use a direct integration of the fluid pressure on the body surface.

The interaction between fluid and structure affects the vibration response of the structure due to the additional hydrodynamic pressure. These effects are accounted for by incorporating the so-called added mass into the vibration equation of the structure. In this paper, a containership was used to study the impact of the added mass on its free vibration response. The natural frequencies of the ship decrease after including the added mass in the vibration analysis. It is shown that the frequency-ascending sequence of the wet mode shapes, for which the added mass is accounted for, may differ from that obtained for the dry state of the ship. Also, the effects of different draft levels on the mode shapes of the ship are reported. These results provide a better insight for designing ships based on their wet-state frequencies and mode shapes, which is the typical operation condition when sailing in the open seas.

The development of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines is presented. Explicit expressions are derived for the time-domain nonlinear exciting forces in a sea state with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory (FIT). The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using support vector machine learning algorithms is discussed.

Presented is the performance analysis of annular blowout preventer (BOP) reciprocating elastomer hydraulic seals operating in subsea environments. The method is based on a systems-level model that combines the effects of friction, material mechanical properties of the seal, installation compression, subsea hydrostatic pressure, and control system dynamics into one model. The model is calibrated using data from tests conducted on the surface and then validated on subsea operational data. Through model simulations, it will be shown that insufficient installation squeeze of the seal in combination with low elasticity seal material results in cases where the seal does not leak at the surface but show substantial internal leakage in subsea conditions. Leakage is also observed under dynamic operation when the walls of the seal groove do not energize the seal. The proposed model-based analysis method in conjunction with surface level testing offers a new paradigm in evaluating reciprocating seal subsea performance a priori of subsea operation thereby avoiding costly downtimes and subsea failures.

Ship roll motion is critical for offshore operations due to its lack of damping mechanism. This paper demonstrates a dynamic simulation scheme of an active roll reduction system using free-flooding tanks controlled by vacuum pumps. A tank is installed on each side of a catamaran. Both the tank hatches are opened to the sea and the air chambers of both tanks are connected by an air duct. Vacuum pumps and air valve stabilized the wave-induced roll motion by controlling the water levels in the tanks through a feedback controller. The catamaran is a dynamic model with single degree-of-freedom (DOF) in roll, and its hydrodynamic behavior is calculated using potential theory by SHIPX. The air chambers are modeled as isothermal processes of ideal gas. The behavior of the liquid flow in a tank is simulated by incompressible Reynolds-averaged Navier–Stokes solver with the volume of fluid model, then summarized as a response function for the vessel model. A simplified control plant model for the vacuum pumps is proposed where higher order behaviors are neglected and the external wave-induced load is unknown. A parameter-dependent observer and a backstepping controller are adopted to estimate the external load and to reduce the roll motion. The system stability is proved by Lyapunov's direct method. The performance of the entire system is evaluated in terms of roll reduction capability and power cost. The system is more suitable for roll reduction in low-speed or resting conditions.

This study illustrates a comparison of two numerical methods under a unified computational platform for solving fluid–structure interaction (FSI) problems. The first is an arbitrary Lagrangian–Eulerian (ALE)-based fluid model coupled to a structural finite element (FE) method (ALE-FE/FE), and the second is a smoothed particle hydrodynamics (SPH) method coupled to the same structural FE code (SPH/FE). The predictive capabilities and computational efficiency of both the numerical methods are evaluated and validated against a canonical problem of a rapidly varying flow past an elastic gate for which experimental data are available. In both numerical solutions, the fluid flow is governed by the Navier–Stokes equation, and the elastic gate is modeled as a flexible structure. Numerical simulation results show that the ALE-FE/FE continuum approach not only captures the dynamic behavior properly but also predicts the water-free surface profiles and the elastic gate deformations accurately. On the other hand, the coupled purely Lagrangian approach of the SPH/FE under an identical computational platform is found to be less accurate and efficient in predicting the dynamics of the elastic gate motion and the water-free surface profiles.

The study presents a one-dimensional (1D) numerical model of wave propagation as well as transmission/reflection phenomena in Newtonian and non-Newtonian drilling mud flow associated with oil/gas drilling activities. Propagation of wave formed due to back pressure changes by means of a choke is investigated. In general, the reflection and transmission of pressure waves at intersection of conduits with different cross sections or in case of partial blockage typical of drilling practices is multidimensional and caused by nonuniform boundary conditions over the cross section. The 1D approach is investigated to approximate the multidimensional reflection and transmission of pressure pulse at areal discontinuity in conduit. The approach is facilitated by introduction of a local force exerted by solid wall on the fluid at the intersection of the conduits into conservative form of the equation for momentum conservation. In addition, nonconservative formulation of momentum equation was explored. To solve the differential equations, MacCormack numerical scheme with second-order accuracy is applied to the nonlinear Euler and 1D viscous conservation equations. A grid refinement study is performed. It is shown that nonconservative form of the conservation laws results in more accurate prediction of transmission and reflection in case of areal discontinuity. The results of the numerical modeling are presented in terms of pressure wave propagation and attenuation upon reflection and transmission at consequent interfaces.

Free vibration analysis of functionally graded (FG) rectangular plates on two-parameter elastic foundation and vertically coupled with fluid is the objective of this work. The fluid domain is considered to be infinite in length, but it is bounded in depth and width directions, and the effects of hydrostatic pressure and free surface waves are not taken into account. The mechanical properties of the FG plates are assumed to vary continuously through the thickness direction according to a power-law distribution of the volume fraction of the constituents. The accuracy and applicability of the formulation is illustrated by comparison studies with those reported in the open literature. At the end, parametric studies are carried out to examine the impact of different parameters on the natural frequencies.

Near field flow characteristics around catamarans close to resonant conditions involve violent viscous flow such as energetic vortex shedding and steep wave making. This paper presents a systematic and comprehensive numerical investigation of these phenomena at various oscillating frequencies and separation distances of twin sections. The numerical model is based on the solution of Navier–Stokes equations assuming laminar-flow conditions with a volume of fluid (VOF) approach which has proven to be particularly effective in predicting strongly nonlinear radiated waves which directly affect the magnitude of the hydrodynamic forces around resonant frequencies. Considered nonlinear effects include wave breaking, vortex shedding and wave-body wave-wave interactions. The method is first validated using available experiments on twin circular sections: the agreement in a very wide frequency range is improved over traditional linear potential flow based solutions. Particular attention is given to the prediction of added mass and damping coefficients at resonant conditions where linear potential flow methods fail, if empirical viscous corrections are not included. The results of the systematic investigation show for the first time how the so-called piston-mode motion characteristics are nonlinearly dependent on the gap width and motion amplitude. At low oscillation amplitudes, flow velocity reduces and so does the energy lost for viscous effects. On the other hand for higher oscillation amplitude, the internal free surface breaks dissipating energy hence reducing the piston mode amplitude. These effects cannot be numerically demonstrated without a computational technique able to capture free surface nonlinearity and viscous effects.

The flow around an oscillating cylinder close to a horizontal solid boundary is studied to gather information about the load acting on pipelines, while they are laid on the sea bottom. The problem is simplified assuming that the pipeline section is rigid and oscillates harmonically only in the normal-to-seabed direction so that the problem can be tackled in two dimensions. A computational fluid dynamics (CFD) solver is used to take into account viscous effects in the hypothesis of laminar flow conditions. This best suits the conditions of pipeline layering when the Reynolds number, Re = U m · D / ν , ranges in order of 450–120,000, while the Keulegan–Carpenter number, KC = U m · D / T , ranges in order of 0.45–2. Nonetheless, boundary layer separation and vortex shedding are considered. Focus is on the determination of the lift force for which a novel analytical approximate expression is proposed. Such an analytical result can provide useful support to the studies related with the structural analysis of the pipe laying.

Sloping structures are widely used in ice-infested waters because of their ability to reduce ice loading by inducing a bending failure in ice sheets. From model test data, a significant velocity effect on the breaking load of ice sheets has been reported. In this paper, the ice–fluid interaction process is investigated by adopting the Euler–Bernoulli beam theory for the ice sheet and the potential theory for the underlying fluid domain. Accounting for the inertia effect of the ice sheet and the hydrodynamics of sea water beneath the ice sheet, the results demonstrate a velocity effect on the ice breaking loads in-plane deformation, which compare well with the available model test data. Moreover, our model formulation and implementation is such that the solutions for different ice velocities can be obtained rapidly from the reference solution, which facilitates the development of a real-time simulator. It is also shown that the velocity effect depends on the ice compressive strength and the angle of sloping structure.