Forces on a circular cylinder near a plane wall

The drag and lift force are measured on circular cylinders fitted with end plates in a wind tunnel. The gap between the cylinder and the wall, G, the thickness of the turbulent boundary layer along the wall, δ, and the Reynolds number, Re, are varied in the following ranges: 0 < G/D < 2, 0.12 < δ/D < 0.97 and 4.8 × 10⁴ Re 3 × 10⁵. The lift and drag coefficients are presented in terms of a new variable G/δ.

It is found that the lift coefficient is governed by the gap to diameter ratio G/D while the drag coefficient is dominated by the ratio of gap to thickness of the boundary layer, G/δ.     

Intense turbulent convection in a horizontal plane liquid layer

Direct numerical simulation of turbulent convection in a horizontal liquid layer heated from below is performed within the framework of the nonstationary Navier—Stokes equations with the use of the Bubnov—Galerkin method. The main attention is given to calculations for superhigh supercriticalities. Computational burden is reduced by the use of the splitting method at each step of integration. Previously, the smallness of the residual arising from substitution of simulated results into the initial system of equations is demonstrated and the residual’s dependence on the number of reference functions and supercriticality is considered. A good agreement of the results obtained with the use of different numerical implementations of the Bubnov—Galerkin procedure is shown, in particular, for the stochastic processes corresponding to a low supercriticality and appearing with the formation of strange attractors close to a Mobius strip. The calculations were carried out for a wide range of supercriticality (from 1 to 34000). It is shown that simulations and experiment are in good qualitative agreement.     

Wave forces on submarine pipelines near a plane boundary

Forces induced by regular waves on submarine pipelines resting on as well as near a plane boundary and aligned parallel to wave fronts of the oncoming waves are investigated experimentally. The inline hydrodynamic coefficients of drag and inertia are evaluated through the use of Morison equation and the least squares method. The transverse force is analysed in terms of maximum transverse force and transverse root mean square (r.m.s.) coefficients. The resulting inline and transverse hydrodynamic coefficients are correlated with the period parameter or Keulegan-Carpenter number and relative clearance of the pipeline from the plane boundary. The effect of depth parameter on these coefficients and the correlation between maximum transverse force and transverse r.m.s. coefficients are also reported.     

Fluid impact onto a corrugated panel with trapped gas cavity

Initial stage of incompressible liquid impact onto a corrugated elastic panel with account for compressible gas trapping between the corrugations is studied. The liquid free surface is flat and parallel to the panel before impact. The impact velocity is constant in this study. The corrugations are modelled as identical rigid short structures on the surface of the flat panel. The panel is either of infinite or finite length. There are only two corrugations which are placed symmetrically on the panel. Only a part of the panel between these two corrugations is elastic. The liquid free surface closes the gas cavity between the two corrugations at the initial instant of impact and compresses the gas before the fluid comes in contact with the elastic part of the panel. The elastic deflections of the panel are caused by gas pressure in the cavity. The elastic deflections modify both the pressure in the cavity and the hydrodynamic pressure distribution along the wetted part of the panel. The hydroelastic problem is solved within the Wagner approach. The effect of gas compressibility on the elastic behaviour of the corrugated elastic panel is investigated. It is shown that the pressure in the gas cavity and elastic deflections grow beyond all bounds for the panel of infinite length and are finite if the panel is of finite length. The present model is relevant for the strength assessment of the cargo containment system (CCS) in the tanks of LNG carriers.     

On the transport of suspended sediment by a swash event on a plane beach

We develop solutions for the transport of suspended sediment by a single swash event following the collapse of a bore on a plane beach, and we investigate the morphodynamical role that such transport may play. Although the intrinsic asymmetry between uprush and backwash velocities tends to encourage the export of sediment, we find that swash events may be effective in distributing across the swash zone much or all of the sediment mobilised by bore collapse; additionally, settling lag effects may promote a weak onshore movement of sediment. We quantify both effects in terms of the properties of the sediment and of the swash event, and comment on the relationship between our findings and recent field studies of swash zone sediment transport.     

Run-up of long solitary waves of different polarities on a plane beach

We study the run-up of long solitary waves of different polarities on a beach in the case of composite bottom topography: a plane sloping beach transforms into a region of constant depth. We confirm that nonlinear wave deformation of positive polarity (wave crest) resulting in an increase in the wave steepness leads to a significant increase in the run-up height. It is shown that nonlinear effects are most strongly pronounced for the run-up of a wave with negative polarity (wave trough). In the latter case, the run-up height of such waves increases with their steepness and can exceed the amplitude of the incident wave.     

Vortex shedding suppression for flow over a circular cylinder near a plane boundary

In this study, the Navier-Stokes equations and the pressure Poisson equation for two-dimensional time-dependent viscous flows are solved with a finite difference method in a curvilinear coordinate system. With this numerical procedure, the vortex shedding flow past a circular cylinder near a wall is investigated. The flow is calculated for a broad range of gap ratios for different Reynolds numbers ranging from 80 to 1000. Based on the numerical solutions, the vortex shedding is observed using various methods, and the mechanism for the vortex shedding suppression at small gap ratios is analyzed. The critical gap ratio at which the vortex shedding is suppressed is identified at different Reynolds numbers.     

Global buckling of pipelines in the vertical plane with a soft seabed

Offshore pipelines are usually buried to avoid damage from fishing activities and to provide thermal insulation. Provided that the buried pipelines are sufficiently confined in the lateral direction by the passive resistance of the trench walls, they may be subject to vertical buckling caused by a rise in temperature. Vertical buckling is usually called upheaval buckling because the heated pipeline is assumed to move upwards conventionally. However, the seabed may be very soft, especially where a pockmark or abyssal ooze appears. Consequently, under thermal compressive force, the pipeline may buckle downward and penetrate into the seabed because the downward soil resistance is small. In this study, we extended an analytical solution for vertical pipeline buckling on a rigid seabed to a soft seabed, and the effects of soil resistance on pipeline stability, buckling mode and amplitude are illustrated and analyzed.     

Three-dimensional wave patterns in long air cavities on a horizontal plane

Significant drag reduction of large displacement vessels can be achieved by applying multi-wave air cavities arranged on the hull bottom. Waves generated on the air–water interface of air cavities impose requirements on the dimensions of a hull recess that accommodates the air cavity. An approximate model for calculating wave patterns in the critical upstream part of long air cavities in a simplified, horizontal-plane geometry is presented in this paper. The influence of the recess planform boundaries and other factors on the wave patterns is studied parametrically. Some hydrodynamic aspects of multi-wave air cavities are discussed.     

Effect of the tangential component of a force field on convection in a rotating plane layer

Thermal convection in an inclined plane layer having boundaries with different temperatures and rotating around an axis perpendicular to its plane is studied experimentally. It is shown that the convection and heat transfer are determined by two different convective mechanisms—gravitational and thermovibrational ones; they manifest themselves in the threshold excitation of cellular convective structures with different sizes. The thermovibrational convection is caused by the period-averaged mass force arising as a result of tidal oscillations of a nonisothermal fluid with respect to the cavity under the action of the external force-field component tangential to the layer. The map of convection regimes on the plane of governing parameters—the gravitational Rayleigh number and its vibrational analog—has been constructed. It has been found that the thermovibrational convection can develop even in a layer heated from above. The role of the dimensionless velocity of rotation, which has a stabilizing effect on the excitation of both gravitational and thermovibrational convection, has been studied.     

近壁圆柱绕流水动力特性数值模拟与实验研究

通过数值模拟和物理模型实验,对距壁面一定高度的圆柱绕流水动力特性进行了研究。数值模拟采用有限体积法对标准k-ε模式方程进行离散,采用SIMPLE算法进行求解,模拟绕流流场。在物理模型实验中,将PVC圆管制作的实验模型安放在水槽内,在圆管的跨中沿表面周向均匀布置水下压力传感器,用于测量绕流圆柱体表面动水压力分布。通过改变Re数和间隙比来分析它们对近壁圆柱绕流水动力特性的影响。基于数值流动显示技术,给出了近壁绕流流场的尾流流态分析。通过数值结果与实验结果的对比,对近壁绕流圆柱体的升力系数及其表面动水压力分布进行了研究,对比结果显示了较好的一致性。     

Regular waves onto a truncated circular column: A comparison of experiments and simulations

Accurate prediction of hydrodynamic forces on offshore structures is critical for safe and cost effective design of fixed and floating offshore structures exposed to a harsh environment. In the present paper, nonlinear interactions between regular waves and a single surface-piercing truncated circular column have been investigated using a frequency domain potential flow solver (DIFFRACT) and a full CFD solver in OpenFOAM for direct comparisons. Both the predicted free surface elevation around the column and the total force acting on the column have been analysed and compared with experimental data from MOERI. The degree of non-linearity and the contribution of each harmonic to the free surface run-up and wave forces have been examined, and evaluations of the accuracy and computational efficiency of the potential flow solver and the full CFD solver are provided and compared in the paper. Also of note are the local forms of the scattered waves around the column in numerical simulations, which are consistent with the Type-1 and Type-2 patterns identified in physical experiments at Imperial College.     

长江口邻近海域沉积物对磷的吸附作用初步研究

选取长江口邻近海域的沉积物风干研磨后筛分为4个不同粒度的样品(40~80目,80~100目,120~200目及200目以下),考察粒度及磷浓度对动力学吸附的影响,比较了处理方式、介质pH及所含腐殖酸HA对等温吸附的影响。结果发现:(1)沉积物对磷的吸附可用伪二级动力学方程描述,沉积物粒度越小、初始磷浓度越大,对磷的吸附速率越大;(2)等温吸附曲线可用Langmuir-交叉型等温式拟合,磷的吸附量随粒度减小而增加。沉积物经HCl处理之后,对磷的吸附能力增强,沉积物对磷的吸附量与介质中HA的浓度正相关,pH对沉积物吸附磷的影响较为复杂,"稳定pH范围"约为7~9。     

Control performance in the horizontal plane of a fish robot withmechanical pectoral fins

The mechanism of locomotion of aquatic animals can provide us with new insight into the maneuverability and stabilization of underwater robots. This paper focuses on biomimesis in the maneuvering performance of aquatic animals to develop a new device for maneuvering underwater robots. In this paper, guidance and control in the horizontal plane of a fish robot equipped with a pair of two-motor-driven mechanical pectoral fins on both sides of the robot in water currents is presented. The fish robot demonstrates high performance in terms of maneuverability in such activities as lateral swimming. The use of fuzzy control enables the fish robot to perform rendezvous and docking with an underwater post in water currents     

The effect of frequency dispersion on plane waves generated as a result of bed motions

Using a linear statement, the paper studies surface waves occurring due to minor shifts of the bottom sections. A plane case is considered. An analytical solution to the problem has been derived using Fourier transforms. Asymptotic laws for the degeneration of waves propagating over finite bottom deformations have been defined. Numerical analysis of the integrals is applied to study the effect of the horizontal extent of a wave generation area and bottom irregularities on the shape of waves and their amplitudinal and energetic parameters. Attention is focused on the manifestation of frequency dispersion at the stage of wave generation as a developed wave process. Translated by Vladimir A. Puchkin.     

Comparison of turbulence schemes for prediction of wave-induced near-bed sediment suspension above a plane bed

Based on a wave bottom boundary layer model and a sediment advection-diffusion model, seven turbulence schemes are compared regarding their performances in prediction of near-bed sediment suspension beneath waves above a plane bed. These turbulence algorithms include six empirical eddy viscosity schemes and one standard two-equation k-ε model. In particular, different combinations of typical empirical formulas for the eddy viscosity profile and for the wave friction factor are examined. Numerical results are compared with four laboratory data sets, consisting of one wave boundary layer hydrodynamics experiment and three sediment suspension experiments under linear waves and the Stokes second-order waves. It is shown that predictions of near-bed sediment suspension are very sensitive to the choices of the empirical formulas in turbulence schemes. Simple empirical turbulence schemes are possible to perform equally well as the two-equation k-ε model. Among the empirical schemes, the turbulence scheme, combining the exponential formula for eddy viscosity and Swart formula for wave friction factor, is the most accurate. It maintains the simplicity and yields identically good predictions as the k-ε model does in terms of the wave-averaged sediment concentration.     

Comparison of flow patterns in the near wake of a circular cylinder and a square cylinder placed near a plane wall

The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25 mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height (S) between the cylinder bottom and the wall surface, over the gap ratio range S/D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/D, as well as the trend of suppression of vortex shedding at small S/D and onset of vortex shedding at large S/D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/D.     

A field evaluation of plate and screen microlayer sampling techniques

Field studies of surface microlayer sampling methods involving simultaneous use of glass plates and two depths of screens have been conducted. Intercomparisons of the two methods are based upon thicknesses of sample and amounts of dissolved organic carbon, dissolved UV-absorbing phenolic materials, chlorophyll a, ATP and particulate organic carbon and nitrogen collected. Plate samples are always thinnest, and are influenced by wave state and bulkwater temperature. Amounts of materials collected generally follow sample thicknesses. Biases toward fractions of microlayer materials are not evident in either method. It was concluded that intercomparisons of data collected separately by the two methods is inadvisable.     

Dynamics of localized vortices on the beta plane

This paper studies the joint influence that rotation and the earth’s sphericity have on the dynamics of localized synoptic scale vortices within the quasi-geostrophic barotropic model in the beta-plane approximation. Rossby solitons (two-dimensional vortices exponentially localized in space which propagate without changing their form along the latitude circles) are considered in the first part of the article. The general properties of such solutions are discussed. The simplest examples are presented, and a brief review of the main results is given. The second part is dedicated to the theory of nonstationary monopoles. The physical mechanisms governing the evolution of such vortices are described; different stages of this evolution are determined for intense vortices. Analytical and numerical results are used to confirm the qualitative explanations.