潜体绕流及远场声特性分析

首先采用SST（shear-stress transport）k-ω两方程湍流模型求解了潜体三维非定常粘性流场,然后基于Lighthill声类比理论,对潜体流噪声的远场特性进行了数值分析,并探讨了攻角的影响。研究结果显示,基于所研究的潜体外形,潜体在舷侧及深度等正横方向上的声辐射显著高于潜体在艏、艉等轴向上的声辐射,攻角的存在使声辐射增大。     

A B-spline based higher order panel method for analysis of steady flow around marine propellers

A higher order panel method based on B-spline representation for both the geometry and the solution is developed for the analysis of steady flow around marine propellers. The self-influence functions due to the normal dipole and the source are desingularized through the quadratic transformation, and then shown to be evaluated using conventional numerical quadrature. By selecting a proper order for numerical quadrature, the accuracy of the present method can be increased to the machine limit. The far- and near-field influences are shown to be evaluated based on the same far-field approximation, but the near-field solution requires subdividing the panels into smaller subpanels continuously, which can be effectively implemented due to the B-spline representation of the geometry. A null pressure jump Kutta condition at the trailing edge is found to be effective in stabilizing the solution process and in predicting the correct solution. Numerical experiments indicate that the present method is robust and predicts the pressure distribution on the blade surface, including very close to the tip and trailing edge regions, with far fewer panels than existing low-order panel methods.     

A Bingham-plastic model for fluid mud transport under waves and currents

Simplified equations of fluid mud motion, which is described as Bingham-Plastic model under waves and currents, are presented by order analysis. The simplified equations are non-linear ordinary differential equations which are solved by hybrid numerical-analytical technique. As the computational cost is very low, the effects of wave current parameters and fluid mud properties on the transportation velocity of the fluid mud are studied systematically. It is found that the fluid mud can move toward one direction even if the shear stress acting on the fluid mud bed is much smaller than the fluid mud yield stress under the condition of wave and current coexistence. Experiments of the fluid mud motion under current with fluctuation water surface are carried out. The fluid mud transportation velocity predicted by the presented mathematical model can roughly match that measured in experiments.     

Modelling of flow around a near-bed pipeline with a spoiler

Flow around a pipeline with and without a spoiler near a smooth wall is simulated by solving the Navier–Stokes equations. Finite-difference formulation with a second-order upwind scheme in a curvilinear coordinate system is employed. The influences of the spoiler on hydrodynamic forces, pressure distribution, vortex shedding frequency, velocity profile under the pipe, as well as shear stress on the wall are investigated. The attachment of a spoiler significantly increases drag, root-mean-square (RMS) lift, flow through the gap between the pipe and the wall and shear stress on the seabed around the pipe. The spoiler also generates a non-zero mean downward force on the pipeline, which may enhance the self-burial of the pipeline.     

张力腿平台绕流数值模拟分析

在一定来流条件下,张力腿平台(tension leg platform,简称TLP)的立柱后缘出现周期性的交替旋涡脱落,致使立柱受到垂直于来流方向的升力和平行于来流方向的阻力作用,导致TLP产生大幅度往复运动,显著增加平台结构和系泊系统的负载。目前,关于单柱、多柱结构绕流问题的研究较多,但对于TLP绕流特性的研究较少,机理尚存不明确的地方。为研究TLP的绕流力变化情况和流场特征,开展了数值模拟分析。利用计算流体动力学数值模拟软件,基于雷诺平均(RANS)法和分离涡模拟(DES)法对TLP绕流场进行仿真分析,揭示了TLP的绕流特性。结果表明,在3种来流角度和多个折合速度V_r下,TLP绕流的流体力系数存在明显差异,升力系数时域曲线呈现脉动性。TLP的上、下游立柱间存在明显的相互作用,影响了旋涡的形成与发展。TLP的旋涡脱落大多集中在平台固有频率附近,且在V_r=7,来流角度为0°时,升力系数频谱峰值最大,旋涡脱落集中。     

Laboratory study of two-dimensional nonuniform surface currents over a sphere moving in a fluid column

This paper presents the results of laboratory measurements of surface currents over a sphere moving in the fluid column based on the particle tracing velocimetry. A theoretical model of the fluid flowing around the sphere is presented. The experimental data are compared with the results of theoretical calculations. It is shown that, at low tow speeds of the sphere, the experimentally measured current velocities exceed the velocities calculated from the theoretical model. At greater tow velocities when generation of surface waves by the sphere was observed in the experiment, the experimental data agree well with the results of the theoretical calculations.     

Simulation of free surface flow around a VLCC hull using viscous and potential flow methods

Numerical simulations have been carried out to determine the incompressible free surface flow around a VLCC hull form for which experimental results are available. A commercial viscous flow finite volume code using the two-phase Eulerian–Eulerian fluid approach and a potential flow code based on the Rankine source method have been used in this study. The simulation conditions are the ones for which experimental results exist. The shear stress transport (SST) turbulence model has been used in the viscous flow code. A tetrahedral unstructured grid was used with the viscous flow code for meshing the computational domain, while quadrilateral structural patches were used with the potential flow code for meshing the VLCC hull surface and the water surface around it. The results compare well with the available experimental data and they allow an understanding of the differences that can be expected from viscous and potential flow methods as a result of their different mathematical formulations, which make their complementary application useful for determining the total ship resistance.     

Simulation of a 3D submerged jet flow around a pile

The submerged 3D turbulent jet flow behavior around a pile on a rigid bed and on a scoured bed was studied experimentally and numerically. ADV was used to obtain the jet velocity distributions and Realizable k–ε turbulence model was used for the prediction of flow field around a pile. The jet flow area was three-dimensional and thus numerical model was a three-dimensional model. The numerical results were used to predict the velocity close to the pile and bed shear stress on the bed. In general, the results indicated that the flow field was also in agreement with the findings of previous experiments in literature and the related principles in the subject area. The experimental results demonstrated that Acoustic Doppler Velocimeter (ADV) measurements were almost identical with the Realizable k–ε turbulence model results for turbulence intensity I=10%.     

非对称歪斜波引起的片流输沙数值研究

本文用了一个可考虑相位差作用和波浪边界层非对称性的瞬态理论模型和一个两相紊流模型共同研究非对称歪斜波引起的片流输沙现象。为了解速度偏度和加速度偏度对输沙通量和输沙率的贡献,两相流模型为理论模型提供了必要的相位超前、瞬时侵蚀深度和边界层的发展过程。理论模型研究显示了由速度偏度和加速度偏度引起的向岸阶段和离岸阶段的泥沙运动非对称性,解释了净输沙的产生原因。在以往的非对称歪斜波片流输沙研究中,净输沙的产生主要被归结于相位差作用。本文的研究则表明了非对称的边界层发展所产生的净流量和动床面效应在净输沙产生过程中的比相位差作用更为重要。     

Nearshore tidal currents and sand transport in a macrotidal environment

Benthic nearshore currents were measured continuously for a week over the subtidal zone fronting an open coast, macrotidal beach (spring tide range 9.5 meters) in Northwestern Australia. The shore-parallel currents were dominated by the semidiurnal tide; however pronounced asymmetries expressed the contributions of higher harmonics. Northerly flows at high tide were considerably stronger and of longer duration than southerly flows at low tide. Considering the combined effects of sediment agitation by waves and net transport by the tidal currents, it is shown that a mechanism may exist which could produce net northerly transport of “bed load” and southerly transport of suspended load.     

Numerical simulation of viscous cavitating flow around a ship propeller

In the present study, cavitation and a ship propeller wake are reported by computed fluid dynamics based on viscous multiphase flow theory. Some recent validation results with a hybrid grid based on unsteady Navier-Stokes (N-S) and bubble dynamics equations are presented to predict velocity, pressure and vapor volume fraction in propeller wake in a uniform inflow. Numerical predictions of sheet cavitation, tip vortex cavitation and hub vortex cavitation are in agreement with the experimental data, same as numerical predictions of longitudinal and transversal evolution of the axial velocity. Blade and shaft rate frequency of propeller is well predicted by the computed results of pressure, and tip vortex is the most important to generate the pressure field within the near wake. The overall results indicate that the present approach is reliable for prediction of cavitation and propeller wake on the condition of uniform inflow.     

桩周局部冲刷三维数值模拟

用数值模拟手段研究桩周冲刷越来越引起人们的重视.基于OpenFOAM开源CFD软件,采用VOF方法处理自由水面,通过动网格技术考虑床面地形随时间的变化,构建起桩周局部冲刷的动态三维数学模型.其中,通过改进沙滑模型以修正当生成的剖面坡度超过泥沙休止角时造成的模拟失真,同时克服数值计算的不稳定性.计算结果和水槽实验资料对比分析,二者符合较好.     

船舶螺旋桨尾流场的数值分析

利用基于速度势的低阶面元法计算船舶螺旋桨的尾流场。采用计算较为简捷的关于扰动速度势的基本积分微分方程，并采用双曲面形状的面凶以消除面元间的缝隙。Newton-Raphson迭代过程被用来在桨叶随边满足压力Kutta条件，使桨叶面上表面的压力在随边有良好的一致性。在计算面元的影响系数时，应用了Morino导出的解析计算公式，加快了数值计算的速度。从解面元法的基本积分方程得到的偶极强度和源汇强度，直接求得尾流场的速度分布。     

A numerical study of the summertime flow around the Luzon Strait

Luzon Strait, a wide channel between Taiwan and Luzon islands, connects the northern South China Sea and the Philippine Sea. The Kuroshio, South China Sea gyre, monsoon and local topography influence circulation in the Luzon Strait area. In addition, the fact that the South China Sea is a fairly isolated basin accounts for why its water property differs markedly from the Kuroshio water east of Luzon. This work applies a numerical model to examine the influence of the difference in the vertical stratification between the South China Sea and Kuroshio waters on the loop current of Kuroshio in the Luzon Strait during summer. According to model results, the loop current’s strength in the strait reduces as the strongly stratified South China Sea water is driven northward by the southwest winds. Numerical results also indicate that Kuroshio is separated by a nearly meridional ridge east of Luzon Strait. The two velocity core structures of Kuroshio can also be observed in eastern Taiwan. Moreover, the water flowing from the South China Sea contributes primarily to the near shore core of Kuroshio.     

Unit-pockmarks and their potential significance for predicting fluid flow

Unit-pockmarks were recognized as more-or-less insignificant features on the seafloor in the early 1980s. However, this investigation, at four different regions in Norwegian waters, suggests they are more significant for seep detection than previously believed. They occur as circular depressions in the seafloor (diameter < 5 m) either as singular features, as strings, or as clusters. One of our main conclusions is that they are widespread and represent the most recent and most active local seep locations. This is based on their areal density distribution, the finding of relatively high hydrocarbon concentrations inside sampled unit-pockmarks and at locations where they are abundant, and on theoretical considerations. When unit-pockmarks occur together with ‘normal-sized’ pockmarks, they often form to the side of the normal-pockmark centre. Our study also suggests that (1) the driving force behind seafloor hydraulic activity, i.e., the formation of unit-pockmarks, normal-pockmarks, and many other fluid flow features, is pockets of buried free gas, and (2) whereas unit-pockmarks likely manifest cyclic pore-water seepage, their larger related, normal-pockmarks, likely manifest periodic or intermittent gas bursts (eruptions), with extended intervening periods of slow, diffusive, and cyclic pore-water seepage. Our findings suggest that seep detection is most efficiently performed by mapping the seafloor with high-resolution bathymetry (at least 1 m × 1 m gridding), and acquiring geochemical samples where the density of unit-pockmarks is locally highest.     

Pore water geochemistry of eastern Mediterranean mud volcanoes: Implications for fluid transport and fluid origin

The pore water chemistry of mud volcanoes from the Olimpi Mud Volcano Field and the Anaximander Mountains in the eastern Mediterranean Sea have been studied for three major purposes: (1) modes and velocities of fluid transport were derived to assess the role of (upward) advection, and bioirrigation for benthic fluxes. (2) Differences in the fluid chemistry at sites of Milano mud volcano (Olimpi area) were compiled in a map to illustrate the spatial heterogeneity reflecting differences in fluid origin and transport in discrete conduits in near proximity. (3) Formation water temperatures of seeping fluids were calculated from theoretical geothermometers to predict the depth of fluid origin and geochemical reactions in the deeper subsurface.No indications for downward advection as required for convection cells have been found. Instead, measured pore water profiles have been simulated successfully by accounting for upward advection and bioirrigation. Advective flow velocities are found to be generally moderate (3–50 cm y^− 1) compared to other cold seep areas. Depth-integrated rates of bioirrigation are 1–2 orders of magnitude higher than advective flow velocities documenting the importance of bioirrigation for flux considerations in surface sediments. Calculated formation water temperatures from the Anaximander Mountains are in the range of 80 to 145 °C suggesting a fluid origin from a depth zone associated with the seismic decollement. It is proposed that at that depth clay mineral dehydration leads to the formation and advection of fluids reduced in salinity relative to sea water. This explains the ubiquitous pore water freshening observed in surface sediments of the Anaximander Mountain area. Multiple fluid sources and formation water temperatures of 55 to 80 °C were derived for expelled fluids of the Olimpi area.     

Numerical flow and performance analysis of a water-jet axial flow pump

The purpose of the present study is to investigate the performance and three-dimensional flow fields in a water-jet pump. TASCflow is employed to simulate the rotator-stator coupling flow field. A standard k-ε turbulence model combined with standard wall functions is used. In order to investigate the effect of a rear stator on flow fields, the flows in two water-jet pumps with and without a rear stator are studied. Computational fluid dynamics (CFD)-predicted overall performances are in good agreement with the experimental results. Then the flow fields, such as the pressure distribution on the blade surfaces, and the axial and tangential velocity distribution, especially the radial loading distribution, are investigated at different flow rates. In addition, the effects of a rear stator and different spacings between the rotor and the stator on the overall performance and the flow fields of the water-jet pump are also investigated.     

Source models of flow through and around screens and gauzes

The source panel method is used to investigate the singularity models of flow through and around screens that have been developed by various authors. These studies are extended to complete a framework of models in terms of the level of complexity, the nature of the flow in the wake and the form of pressure drop boundary condition. The models that best represent the limited experimental data that is available are identified and the need for more experimental data, in order to carry out a thorough assessment, is noted.