Numerical analysis of seabed dynamic response in vicinity of mono-pile under wave-current loading

Pile foundations have been widely used in offshore engineering.In this study,a three-dimensional numerical model was used to investigate the seabed response around a mono-pile under wave-current loading.Reynolds-averaged Navier-Stokes equations were used to simulate the flow field,and Biot's consolidation equations were used for simulating the response of a porous seabed.The pore water pressure within soil and the effective stress along the depth of the seabed were simulated for various current velocities,with currents traveling either along or against the wave.Results indicate that the current has a significant effect on the effective stress and the pore water pressure distributions,which increases with the current velocity,and that the current traveling against the wave increases the liquefaction depth of the porous seabed.

Numerical Model of Cnoidal Wave Flume

The volume of fluid (VOF) method is used to set up a wave flume with an absorbing wave maker of cnoidal waves. Based on the transfer function between wave surface and paddle velocity obtained by the shallow water wave theory, the velocity boundary condition of an absorbing wave maker is introduced to absorb reflected waves that reach the numerical wave maker. For Hid ranging from 0.1 to 0.59 and T root g/d from 7.9 to 18.3, the parametric studies have been carried out and compared with experiments.

A two-dimensional numerical wave flume based on SA-MPLS method

A spatially adaptive (SA) two-dimensional (2-D) numerical wave flume is presented based on the quadtree mesh system,in which a new multiple particle level set (MPLS) method is proposed to solve the problem of interface tracking,in which common intersection may be traversed by multiple interfaces.By using the adaptive mesh technique and the MPLS method,mesh resolution is updated automatically with time according to flow characteristics in the modeling process with higher resolution around the free surface and the solid boundary and lower resolution in less important area.The model has good performance in saving computer memory and CPU time and is validated by computational examples of small amplitude wave,second-order Stokes wave and cnoidal wave.Computational results also indicate that standing wave and wave overtopping are also reasonably simulated by the model.

Establishment of Numerical Wave Flume Based on the Second-Order Wave-Maker Theory

With growing computational power, the first-order wave-maker theory has become well established and is widely used for numerical wave flumes. However, existing numerical models based on the first-order wave-maker theory lose accuracy as nonlinear effects become prominent. Because spurious harmonic waves and primary waves have different propagation velocities, waves simulated by using the first-order wave-maker theory have an unstable wave profile. In this paper, a numerical wave flume with a piston-type wave-maker based on the second-order wave-maker theory has been established. Dynamic mesh technique was developed. The boundary treatment for irregular wave simulation was specially dealt with. Comparisons of the free-surface elevations using the first-order and second-order wave-maker theory prove that second-order wave-maker theory can generate stable wave profiles in both the spatial and time domains. Harmonic analysis and spectral analysis were used to prove the superiority of the second-order wave-maker theory from other two aspects. To simulate irregular waves, the numerical flume was improved to solve the problem of the water depth variation due to low-frequency motion of the wave board. In summary, the new numerical flume using the second-order wave-maker theory can guarantee the accuracy of waves by adding an extra motion of the wave board. The boundary treatment method can provide a reference for the improvement of nonlinear numerical flume.

Extreme Wave Simulation with Iterative Adaptive Approach in Numerical Wave Flume

Extreme wave is highly nonlinear and may occur due to diverse reasons unexpectedly.The simulated results of extreme wave based on wave focusing,which were generated using high order spectrum method,are presented.The influences of the steepness,frequency bandwidth as well as frequency spectrum on focusing position shift were examined,showing that they can affect the wave focusing significantly.Hence,controlled accurate generation of extreme wave at a predefined position in wave flume is a difficult but important task.In this paper,an iterative adaptive approach is applied using linear dispersion theory to optimize the control signal of the wavemaker.The performance of the proposed approach is numerically investigated for a wide variety of scenarios.The results demonstrate that this approach can reproduce accurate wave focusing effectively.

Numerical Simulation on Hydraulic Performances of Quarter Circular Breakwater

Quarter circular breakwater （QCB） is a new-type breakwater developed from senti-circular breakwater （SCB）. The superstructure of QCB is composed of a quarter circular front wall, a horizontal base slab and a vertical rear wall. The width of QCB＇ s base slab is about half that of SCB, which makes QCB suitable to be used on relatively finn soil foundation. The numerical wave flume based on the Reynolds averaged Navier-Stokes equations for impressible viscosity fluid is adopted in this paper to simulate the hydraulic performances of QCB. Since the geometry of both breakwaters is similar and SCB has been studied in depth, the hydraulic performances of QCB are given in comparison with those of SCB.

Numerical study of resonance induced by wave action on multiple rectangular boxes with narrow gaps

By introducing a source term into the Laplace equation, a two-dimensional fully nonlinear time-domain numerical wave flume （NWF） is developed to investigate the resonance induced by the interaction between waves and multiple objects with narrow gaps. In the numerical model, the fully nonlinear kinematic and dynamic boundary conditions are satisfied on the instantaneous free surface and the constant artificial damping is employed in the gaps to approximate the viscous dissipation due to vortex motion and flow separation. The computational domain is discretized using a higher-order boundary element method （HOBEM）. The proposed model is firstly validated against the published experimental data and numerical results of the wave height in the narrow gap between two boxes, the wave heights in the two gaps of three boxes, and wave loads on the boxes. Then, the extensive numerical experiments are performed to study the influences of the number of the boxes and the gap spacing on the resonant frequency, reflected and transmitted wave heights and wave loads on the boxes.

Numerical Simulation of Irregular Wave Overtopping Against A Smooth Sea Dike

Based on the filtered Navier-Stokes equations and Smagorinsky turbulence model, a numerical wave flume is developed to investigate the overtopping process of irregular waves over smooth sea dikes. Simulations of fully nonlinear standing wave and regular wave＇s run-up on a sea dike are carried out to validate the implementation of the numerical wave flume with wave generation and absorbing modules. To model stationary ergodic stochastic processes, several cases with different random seeds are computed for each specified irregular wave spectrum. It turns out that the statistical mean overtopping discharge shows good agreement with empirical formulas, other numerical results and experimental data.

Numerical investigation of solitary wave action on tworectangular boxes with a narrow gap

Based on the time-domain higher-order boundary element method(HOBEM), a two-dimensional numerical wave flume is developed to investigate solitary wave interaction with two rectangular boxes with a narrow gap.In the numerical model, the fully nonlinear boundary conditions are satisfied on the free surface, the mixed Eulerian-Lagrangian method is adopted to track the transient water surface and the fourth-order Runga-Kutta method is used to predict the velocity potential and wave elevation on the free surface. The acceleration potential technique is used to compute the transient wave forces along the wetted object surface. A piston-type wavemaker is used to generate solitary waves. The proposed model is validated by comparing the simulated wave run-up and the wave loads with the published experimental and numerical results of the reflection of a solitary wave from a vertical wall. Then, numerical experiments are performed to study the effects of the narrow gap and the size of each box on the wave run-ups at the two sides of the two-box system and in the narrow gap between two boxes, and the wave loads on the two boxes. The interaction between double solitary waves with a time interval between them with a two-box system is also investigated.

A Numerical Model for Nonlinear Wave Propagation on Non-uniform Current

On the basis of the new type Boussinesq equations （Madsen et al., 2002）, a set of equations explicitly including the effects of currents on waves are derived. A numerical implementation of the present equations in one dimension is described. The numerical model is tested for wave propagation in a wave flume of uniform depth with current present. The present numerical results are compared with those of other researchers. It is validated that the present numerical model can reasonably reflect the nonlinear influences of currents on waves. Moreover, the effects of inputting different incident boundary conditions on the calculated results are studied.

Numerical modelling of nonlinear extreme waves in presence of wind

A numerical wave flume with fully nonlinear free surface boundary conditions is adopted to investigate the temporal characteristics of extreme waves in the presence of wind at various speeds. Incident wave trains are numerically generated by a piston-type wave maker, and the wind-excited pressure is introduced into dynamic boundary conditions using a pressure distribution over steep crests, as defined by Jeffreys＇ sheltering mechanism.A boundary value problem is solved by a higher-order boundary element method（HOBEM） and a mixed Eulerian-Lagrangian time marching scheme. The proposed model is validated through comparison with published experimental data from a focused wave group. The influence of wind on extreme wave properties,including maximum extreme wave crest, focal position shift, and spectrum evolution, is also studied. To consider the effects of the wind-driven currents on a wave evolution, the simulations assume a uniform current over varying water depth. The results show that wind causes weak increases in the extreme wave crest, and makes the nonlinear energy transfer non-reversible in the focusing and defocusing processes. The numerical results also provide a comparison to demonstrate the shifts at focal points, considering the combined effects of the winds and the wind-driven currents.

Numerical Investigation of the Effect of Current on Wave Focusing

In the present study, a numerical wave tank is developed to simulate the nonlinear wave-current interactions based on High Order Spectral （HOS） method. The influences of current on wave focusing are investigated by use of numerical model. The current is assumed to be constant in space. Focused waves with different amplitudes and frequency spectra are simulated with and without current. The focused wave characteristics, such as surface elevation, the maximum crest and frequency spectrum, with different current are compared. The results show that the opposing current increases the maximum crest and the energy transform during wave focusing process, and vice versa for the following current.

Numerical Investigations on Hydrodynamic Performance of An Open Comb-Type Breakwater Under Medium Water Levels

The comb-type breakwater(CTB)has been proposed and investigated in recent years due to its advantages in terms of deep-water adaptability,material saving and water exchanges.All existing empirical formulae for CTBs have been so far restricted to the water level above the bottom of the superstructure,which mainly occurs under the high tides or storm tides.However,based on recent engineering applications and experimental observations,the most severe conditions for CTBs are more likely to occur under a medium water level,because impulsive wave pressure may occur due to interactions between waves and the special chamber in CTBs.Meanwhile,during the most of construction and operation periods,the CTBs are mainly working under the medium water levels,i.e.,water levels below the bottom of the superstructure.In this study,the effects of main influence parameters on the horizontal wave force coefficient and wave transmission coefficient for open CTBs(with partially immersed side plates)under medium water levels were investigated based on a 3D numerical wave flume and corresponding empirical formulae were proposed.It is indicated that the location of the side plate related to the main caisson has significant influence on the hydrodynamic performance of CTBs.In engineering applications,the location of the side plate can be designed at b/L≤0.15 or b/L≥0.3(where b is the distance between the side plate and the front face of the main caisson and L is the incident wave length)for efficiently lowering the horizontal wave force and wave transmission.The flow mechanism of impulsive wave force on CTBs was revealed based on synchronous analyses of flow fields and pressure distribution.Through appropriate design of the height of the superstructure according to H/hD≤1.0 or H/hD≥1.5(where H is the incident wave height and hD is the distance between the still water level and the bottom of the superstructure),the likely impulsive wave pressure on the side plate can also be diminished.

履齿结构对深海沉积物扰动分析

为探究集矿车在行进过程中履带运动对深海沉积物的扰动机理,并建立适用于机械结构与沉积物作用的仿真模型,结合水槽实验和模拟分析,设定对照试验,研究机械结构运动对沉积物的扰动作用和羽状流扩散现象。实验与仿真结果表明:偏置一字形履齿低扰动性能较好,扰动最大速度在1.5 m/s以下;扰动源的下陷深度和转速对水槽中的浊度变化有显著影响,浊度计最大和最小测量值为4 381.3 mg/L和3 073.3 mg/L,相差29.9%,峰值时间相差超过50%。搭建欧拉多相流模型,设定参数和边界条件,仿真与试验结果接近,可用于后续开展小范围内的机械扰动研究。

基于SSA-BP算法的仿鸮翼形量水槽水力性能

为了设计出构造简单、水力性能优良,同时能满足灌区量水需求的量水槽,该研究根据鸮翼曲线提出了一种仿鸮翼形渠道量水槽。首先,利用基于麻雀搜索算法的BP神经网络(sparrow search algorithm,SSA-BP)优化鸮翼展向比,得到最小水头损失的量水槽线形,并探究了优化后的量水槽在输水渠道的适用性。进一步地,在8种流量工况下,选用6组收缩比开展测流试验,结合FLOW-3D数值模拟分析仿鸮翼形量水槽水头损失、弗劳德数、壅水高度、临界淹没度等水力参数,建立测流公式。试验表明,基于SSA-BP优化的仿鸮翼形量水槽具有良好的测流性能,在建议收缩比0.60~0.66范围内,其水头损失为7.75%~14.21%,临界淹没度为0.81~0.90,上游壅水高度为0.84~3.16 cm,上游弗劳德数均小于0.42,符合灌区量水要求。建立的测流公式,平均测流误差为1.49%,测流精度较高。仿鸮翼形量水槽各项水力性能指标优良,能满足灌区测量精度,针对灌区优化水资源配置、精细化用水具有一定的推广价值。

Numerical simulation of wave-current interaction using the SPH method

In this paper, the smoothed particle hydrodynamics（SPH） method is used to build a numerical wave-current tank（NWCT）. The wave is generated by using a piston-type wave generator and is absorbed by using a sponge layer. The uniform current field is generated by simultaneously imposing the directional velocity and hydrostatic pressure in both inflow and outflow regions set below the NWCT. Particle cyclic boundaries are also implemented for recycling the Lagrangian fluid particles. Furthermore, to shorten the time to reach a steady state, a temporary rigid-lid treatment for the water surface is proposed. It turns out to be very effective for weakening the undesired oscillatory flow at the beginning stage of the current generation. The calculated water surface elevation and horizontal-velocity profile are validated against the available experimental data. Satisfactory agreements are obtained, demonstrating the good capability of the NWCT.

Wave-current impacts on surface-piercing structure based on a fully nonlinear numerical tank

Fully nonlinear wave-body interactions for a surface-piercing structure with combined wave-current flow in different water depths are studied by using a 2-D numerical tank. The model is based on Reynolds averaged Navier-Stokes （RANS） equations and renormalization group （RNG） k- E model. The mean and maximum wave-current impacts, including forces in two directions and rotational moment, are calculated and discussed. The effects of U/C and water depth condition on forces and moment have been investigated and the results for combined irregular waves and waves and currents. currents are compared with those induced by combined regular

波浪侵蚀诱发碎石土岸坡变形的模拟试验研究

波浪对岸坡的侵蚀不仅造成水土流失,甚至诱发塌岸和滑坡灾害发生。为探究波浪侵蚀对碎石土岸坡变形的影响,开展了波浪侵蚀岸坡的水槽试验,采用PIV技术对不同侵蚀阶段的岸坡位移进行定量处理,并利用ABAQUS中的“生死单元”功能模拟实现不同阶段的岸坡前缘侵蚀,分析侵蚀进程对岸坡应力场和位移场的影响。研究结果表明:岸坡坡度越大、碎石含量越少,岸坡侵蚀速率越快;水槽试验和数值模拟均表明岸坡后部变形相较于前部变形小,且存在滞后现象,呈现牵引式变形的特征;侵蚀改变了岸坡前缘形态,导致上部土体产生卸荷效应,是岸坡变形的内在原因。

大型波流水槽整流区间长度的优化研究

本文利用OpenFOAM程序基于物理水槽结构设计建立了波浪-水流水槽模型。由于造流出口位于造波推板正前方的水槽底部,在造流出口的下游存在一个波浪-水流相互作用的整流区间,直到波浪-水流场趋于稳定。本文的主要目的是研究各主要影响参数对于整流区间长度的影响,通过结构参数的优化获得最小整流区间长度,以使有效试验段达到最大化。计算分析了波浪-水流场在水槽中各个位置处的时间平均水平速度的垂向分布,并将平均速度与试验段稳定值的偏差为10%的位置点作为整流区间与试验段的分界点。基于数值模拟结果,确定了最优的水流出口角度、水流出口宽度及其与造波推板的距离,同时研究了工作水深、波浪周期和波高以及水流速度对于整流区间长度的影响。

海洋平台桩腿周围局部冲刷及其对基础承载力的影响

在海洋平台桩靴基础安装施工过程中,容易发生穿透现象而导致破坏。业界建议可通过在桩靴周边加设向上的袖结构来抑制贯入时周围土体的回流,从而缓解穿透破坏。但这种袖结构的存在会导致局部冲刷,形成的冲刷坑会减小桩靴有效埋深,进而影响其使用期的承载力。通过一系列水槽试验,探讨了不同形式桩腿的面积比、开孔率、来流角度等因素对于冲刷坑形状的影响,进而开展了考虑冲刷坑影响的桩靴基础承载力数值计算,并与忽略最大冲刷深度以上土体的传统模型、假设冲刷坑为倒圆台状的简化模型计算结果进行了对比。主要研究结果如下:(1)加设圆形袖结构的桩腿引起的冲刷深度要比方形袖结构桩腿小,建议工程中采用面积比较小的圆形袖结构来缓解穿透破坏;(2)桩腿的面积比、开孔率、来流角度等因素对冲刷坑形状都存在影响。最大冲刷深度随着开孔率的增大而减小,当开孔率较大时,冲刷现象近似于群桩冲刷,出现随着冲刷角度的增大冲刷深度先增大后减小的现象;(3)当考虑冲刷坑对于桩靴承载力的影响时,简化模型能够合理反映冲刷坑对于桩靴基础承载力的影响。