Muk Chen Ong

Near-Bed Flow Mechanisms Around a Circular Marine Pipeline Close to a Flat Seabed in the Subcritical Flow Regime Using a k-ɛ Model

Flow mechanisms around a two-dimensional (2D) circular marine pipeline close to a flat seabed have been investigated using the 2D unsteady Reynolds-averaged Navier–Stokes (URANS) equations with a standard high Reynolds number k-ɛ model. The Reynolds number (based on the free stream velocity and cylinder diameter) ranges from 1 × 104 to 4.8 × 104 in the subcritical flow regime. The objective of the present study is to show a thorough documentation of the applicability of the k-ɛ model for engineering design within this flow regime by means of a careful comparison with available experimental data. The inflow boundary layer thickness and the Reynolds numbers in the present simulations are set according to published experimental data, with which the simulations are compared. Detailed comparisons with the experimental data for small gap ratios are provided and discussed. The effects of the gap to diameter ratio and the inflow boundary layer thickness have been studied. Although under-predictions of the essential hydrodynamic quantities (e.g., time-averaged drag coefficient, time-averaged lift coefficient, root-mean-square fluctuating lift coefficient, and mean pressure coefficient at the back of the pipeline) are observed due to the limitation of the turbulence model, the present approach is capable of providing good qualitative agreement with the published experimental data. The vortex shedding mechanisms have been investigated, and satisfactory predictions are obtained. The mean pressure coefficient and the mean friction velocity along the flat seabed are predicted reasonably well as compared with published experimental and numerical results. The mean seabed friction velocity at the gap is much larger for small gaps than for large gaps; thus, the bedload sediment transport is much larger for small gaps than for large gaps.

Numerical Simulation of Flows Past Partially-Submerged Horizontal Circular Cylinders in Free Surface Waves

Two-dimensional (2D) numerical simulations are performed to investigate the flows past partially-submerged circular cylinders in free surface waves. The 2D simulations are carried out by solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k-ω turbulence model. The level set method is employed to model the free-surface waves. Validation studies of a numerical wave tank have been performed by comparing the numerical results with the analytical results obtained from the linear-wave theory. Wave forces on the partially-submerged cylinders have been calculated numerically and compared with the published theoretical and experimental data under regular-wave conditions. The free-surface elevations around the cylinders have been investigated and discussed.Copyright

Dynamic Analysis of Offshore Monopile Wind Turbine Including the Effects of Wind-Wave Loading and Soil Properties

An engineering approach to dynamic analysis of an offshore monopile wind turbine is presented in this paper. The wind-wave coupling for shallow water conditions is considered. Different wind, wave and current loads on the wind turbine within the cut-in and cut-off conditions are taken into account. The hydrodynamic loading is computed based on the corresponding sea-states. The interaction between the foundation and the soil is simulated by nonlinear springs, which stiffness properties are obtained from the axial load transfer (t-z) curve, the tip load-displacement (Q-z) curve and the lateral load-deflection (p-y) curve. Three types of soil conditions are considered in the simulations, i.e., 100% sand layer, 50% sand layer (top) and 50% clay layer (bottom), as well as 100% clay layer. For a given current speed, the variations of the static and the dynamic responses of the wind turbine due to the effects of different wind-wave load combinations and soil conditions have been investigated and discussed.© 2013 ASME

Dynamic Responses of a Jacket-Type Offshore Wind Turbine Using Decoupled and Coupled Models

This paper presents numerical studies of the dynamic responses of a jacket-type offshore wind turbine using both decoupled and coupled models. In the decoupled (hydroelastic) model, the wind load is included through time-dependent forces and moments at a single node on the top of the tower. The coupled model is a hydro-servo-aero-elastic representation of the system. The investigated structure is the OC4 (Offshore Code Comparison Collaboration Continuation) jacket foundation supporting the NREL 5-MW wind turbine in a water depth of 50m. Different operational wind and wave loadings at an offshore site with relatively high soil stiffness are investigated. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Good agreement was obtained in the eigen-frequency analysis, decay tests, and wave-only simulations. In order to obtain good results in the combined wind and wave simulations, two different strategies were applied in the decoupled model, which are 1) Wind loads obtained from the coupled model were applied directly as time-dependent point loads in the decoupled model; and 2) The thrust and torque from an isolated rotor model were used as wind loads on the decoupled model together with a linear aerodynamic damper. It was found that, by applying the thrust force from an isolated rotor model in combination with linear damping, reasonable agreement could be obtained between the decoupled and coupled models in combined wind and wave simulations.Copyright

Large Eddy Simulations of Three-Dimensional Flow Around a Pipeline in a Uniform Current

Three-dimensional (3D) modeling of the flow around a circular cylinder in a steady, uniform current subjected to low and intermediate Reynolds numbers (Re) is considered. Large eddy simulations (LES) are used to simulate the flow due to their ability to study fine structures in the turbulent wake of the cylinder. The turbulence is resolved by the Smagorinsky model. The open-source code OpenFOAM is used in the simulations. Influences of various numerical parameters, such as the grid resolution close to the cylinder, domain width and spanwise resolution are investigated through mean values of the drag and the lift coefficients, the Strouhal number, as well as through the details of the flow field in the near wake of the cylinder.The main purpose of the present paper is to study the numerical influences on the 3D LES simulations of the flow around the cylinder in the free stream at Re = 13100. However, first the flow around a cylinder in the free stream at Re = 3900 is investigated. The latter case has previously been thoroughly investigated, both with LES and direct numerical simulations (DNS); hence these results will be used for verification of the OpenFoam solver.At Re = 13100 the drag and lift coefficients and the Strouhal number are calculated for the comparable Re. Details of the flow in the intermediate wake of the cylinder are also investigated. So far, this has been investigated experimentally and by two-dimensional (2D) numerical simulations. Previous studies were conducted for moderate Re in the range 103–105.Copyright

Numerical Study of Seabed Boundary Layer Flow around Monopile and Gravity-based Wind Turbine Foundations

Computational fluid dynamics (CFD) has been used to study the boundary layer flow around three different bottom-fixed offshore wind turbine foundation designs. Two of the designs are gravity-based foundations, where one has a hexagonal bottom slab and one a circular bottom slab (bottom part). The third design is a monopile. Three-dimensional analyses have been performed with Spalart-Allmaras Delayed Detached Eddy Simulation using a Reynolds number 4 × 10 based on the free stream velocity and the diameter of the monopile, D. The boundary layer size is D in all the analyses. Time averaged results for velocities, pressure and bed shear stress were obtained. The dependence of the results on the mesh resolution was investigated and comparisons with published data were made. The results were found to be reasonably accurate. A distinct horseshoe vortex was found in front (upstream side) of the monopile foundation. Vortex shedding was present in the wake of all the foundations. Two smaller horseshoe vorticies were found in front of the hexagonal gravity-based foundation, were one was on the top of the bottom slab and one was near the seabed in front of the bottom slab. Three horseshoe vortices in total were found in front of the circular gravity-based foundation, due to the presence of two horseshoe vortices near the seabed in front of the bottom slab. A large region of downflow exists in front of the monopile, reaching all the way down to the seabed. This causes a backflow in front of the foundation near the seabed due to conservation of mass. The gravity-based foundations were found to have two main regions of downflow, one in front of the cylindrical shaft (upper part) on top of the bottom slab and a smaller region in front of the bottom slab near the seabed. The gravity-based designs are found to limit the downflow near the seabed. Pressure distributions around the foundations were studied. A positive vertical pressure gradient was found in front of the monopile foundation. It was also found in front of the cylindrical shaft and in front of the bottom slab near the seabed on the gravity-based foundations. A larger volume of increased pressure exists in front of the monopile foundation than in front of the gravity-based foundations due to its geometry. The bed shear stress in the flow direction along the upstream symmetryline on the seabed was investigated. The horseshoe vortex size, measured as the distance from the separation point to the foundation surface along this line, was found to be 0.40D for the monopile foundation, 0.125D for the hexagonal and 0.22D for the circular gravity-based foundation. Bed shear stress distributions near the foundations were obtained. The magnitude of the bed shear stress, normalized by the far field bed shear stress, was used. A maximum value of 4.89 was found near the surface of the monopile foundation at φ = ±66.5 degrees, where φ is the angle measured from the stagnation point in front of the foundation. Similarly, 2.86 at φ = ±60.1 was found for the hexagonal gravity-based foundation. The larger values of the hexagonal foundation are concentrated around the corners at φ = ±60 degrees, and the rest of the seabed has shear stresses close to the far field shear stress. The results of the circular foundations were found to be slightly asymmetric, with a maximum value of 2.59 at φ = 68.9 degrees for the upper distribution (for positive φ) and 2.72 at φ = −85.4 degrees for the lower (for negative φ).

Three-Dimensional Effects of the Flow Normal to a Flat Plate at a High Reynolds Number

Plate components are often found in offshore and marine structures, such as heave damping plates in spar platform and bilge keels in ships. Two-dimensional (2D) and three-dimensional (3D) numerical simulations are performed to investigate the 3D effects of the flow normal to a flat plate at a high Reynolds number (Re = 1:5×105, based on the height of the plate and the free stream velocity). The ratio of the plate thickness to the plate height is 0.02. The 2D simulations are carried out by solving the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k-ω Shear Stress Transport (SST) turbulence model, while the 3D simulations are carried out with the large-eddy simulation (LES) method. The hydrodynamic results (such as time-averaged drag coefficient, Strouhal number and mean recirculation length) are compared with the published experimental data. The near-wake flow structures are also discussed. The 3D simulation results are in good agreement with the published experimental data; however, the 2D simulations show a poor comparison with the experimental data. This shows that the 3D effects are important for the high Reynolds number flow normal to a flat plate.Copyright

Scour Around Vertical Pile Foundations for Offshore Wind Turbines due to Long-Crested and Short-Crested Nonlinear Random Waves

This paper provides a practical stochastic method by which the maximum scour depth around vertical piles exposed to long-crested (2D) and short-crested (3D) nonlinear random waves can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall (2000) wave crest height distribution representing both 2D and 3D nonlinear random waves, and using the regular wave formulas for scour depth by Sumer et al. (1992b). An example of calculation is provided. Tentative approaches to related random wave-induced scour cases are also suggested.Copyright

Numerical study on the water entry of curved wedges

ABSTRACT The water entry problem of wedges has attracted a great deal of research, as the V-shaped cross-section is a typical section form of the bottom of high-speed crafts. While most investigations focus on flat wedges, the effect of geometric curvature on the water entry of rigid sections is yet to fully be analysed. In this paper, the water entry of curved wedges is numerically investigated using the explicit finite element method. A convergence study is conducted for two typical curved wedges. The numerical method is validated by comparing the impact velocity and the penetration depth with the experimental data. Moreover, the water entry of different curved wedges is simulated with different impact velocities and motion states. And the influences of the curvature and the impact velocity on the impact force, the slamming pressure distribution and the wetted width are investigated and discussed.

Numerical Simulations of Regular and Irregular Wave Forces on a Horizontal Semi-Submerged Cylinder

Two-dimensional (2D) numerical simulations have been performed using OpenFOAM (an open source CFD software package [1]) and waves2Foam (an OpenFOAM based add-on library for wave generations and absorption [2]) to investigate free surface waves past one fixed horizontally semi-submerged cylinder. The 2-D simulations are carried out by solving NavierStokes equations which are discretized based on finite volume method (FVM). Volume of Fluid (VOF) method is employed to capture the free surface in the numerical wave tank. Validation studies have been performed by comparing the numerical results of Stokes first-order wave past a semi-submerged circular cylinder with the published experimental data at different incident wave properties. The numerical results are in good agreement with the experimental data. Subsequently, regular and irregular waves past semi-submerged cylinder at different wave heights and the wave lengths are computed numerically to investigate the effect of the wave height and wave length on wave-structure interaction. The numerical results for irregular ∗Address all correspondence to this author. waves are compared with those induced by regular waves.