Effect of compressive stress on the dispersion relation of the flexural–gravity waves in a two-layer fluid with a uniform current

The explicitly analytical solution is derived for the dispersion relation of the flexural–gravity waves in a two-layer fluid with a uniform current. The upper fluid is covered by a thin plate with the presence of the elastic, compressive and inertial forces. The density of each of the two immiscible layers is constant. The fluids of finite depth are assumed to be inviscid and incompressible and the motion be irrotational. A linear system is established within the framework of potential theory. A new representation for the dispersion relation of flexural–gravity waves in a two-layer fluid is derived. The critical value for the compressive force is analytically determined. The dispersion relation for the capillary–gravity with an inertial surface in a two-layer fluid can be obtained in parallel. Some known dispersion relations can be recovered from the present solution.     

Numerical analysis of shell-side flow-induced vibration of elastic tube bundle in heat exchanger

The responses of the flow-induced vibration of an elastic tube bundle subjected to the shell-side cross flow are investigated in this paper. The weak coupling method and the fluid solid interface are used to solve the fluid-structure interaction problem with consideration of the geometry and physical natures. The effects of the shell-side fluid flow velocity and the structural parameters on the flow-induced vibration are discussed. Numerical results demonstrate that the vibration frequency and amplitude at the monitor points increase with the increase of the shell-side water inlet velocity in all directions. The wall thickness and the external diameter of the elastic tube bundle have significant effects on the responses of the flow-induced vibration. The structural parameters affect the vibration frequency and amplitude, and the vibration equilibrium position in the water flow direction. The vibration frequency decreases with the increase of the tube external diameter. In addition, the vibration in the water flow direction has a lower equilibrium position when the elastic tube bundle has a larger wall thickness or smaller external diameter.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL TURBULENT FLOW NEAR BODY SURFACE WITH LONGITUDINAL VORTICES

A calculating model of generalized partially-parabolic approximation is adopted in the research of numericalsimulation of 3-D turbulent flow near body surface with longitudinal vortices in this paper. An analytical solutionfor Poisson equation is presented to generate the 3-D body-fitted coordinate system. The finite analytic method,which introduces 'virtual' lime variable, is used calculating examples. The partially-parabolic approximation ismade in the transformed equations. Compared with that made in the physical domain, it allows the equations to re-tain more terms. In 'zero' equation turbulence model, longitudinal eddy-viscosity and transverse eddy-viscosityare determined by using empiric formulae appropriate to inner layer and outer layer respectively. Of three examplecalculations, the results of turbulent flow past a flat plate with longitudinal vortices are compared with that ofavailable experimental data. The comparison is found to be satisfactory. The results of the others——turbulentflow along a long cylinder and that past an ellipsoid with longitudinal vortices, are qualitatively reasonable.     

Coupled hydroelastic vibrations of an elliptical cylindrical tank with an elastic bottom

An exact three-dimensional analysis based on the linear potential theory and the elaborated method of eigenfunction expansion in elliptic coordinates are presented to study the free coupled elasto-hyrodynamic characteristics of an upright non-deformable cylindrical container of elliptical planform with a flexible bottom plate, filled to an arbitrary depth with an inviscid incompressible liquid. Extensive numerical data are presented in an orderly fashion for the first few symmetric/anti-symmetric coupled hydroelastic natural frequencies as a function of fluid depth parameter for two plate aspect ratios. Also, selected hydrodynamic and structural deformation modes shapes are presented in graphical form. The effects of liquid level, bottom plate elasticity, and cross sectional aspect ratio on the sloshing frequencies and hydrodynamic pressure modes are examined. The validity of the results is examined through computations using a commercial finite element package as well as by comparison with the data available in literature.     

Numerical study of the wave-induced slamming force on the elastic plate based on MPS-FEM coupled method

Slamming is the phenomenon of structure impacting the water surface. It always results in the extremely high load on the structure. This paper is mainly concerned with the slamming force caused by the wave-plate interaction. In this paper, the process of solitary wave impacting onto the horizontal plate is simulated with the help of the moving particle semi-implicit and finite element coupled method(MPS-FEM). The MPS method is adopted to calculate the fluid domain while the structural domain is solved by FEM method. In the first series of simulations, the profiles of the solitary waves with various amplitudes, which are generated in the numerical wave tank, are compared with the theoretical results. Thereafter the interaction between the solitary waves and a rigid plate is simulated. The effects of wave amplitude, as well as the elevation of the plate above the initial water level, on the slamming force are numerically investigated. The calculated results are compared with the available experimental data. Finally, the interactions between the solitary waves and the elastic plate are also simulated. The effects of the structural flexibility on the wave-induced force are analyzed by the comparison between the cases with elastic and the rigid plate.     

Laboratory investigation of the hydroelastic effect on liquid sloshing in rectan- gular tanks

A sloshing experiment is conducted to study the hydroelastic effect in an elastic tank. For this purpose, a translational harmonic excitation is applied to a 2-D rectangular tank model. The lowest-order natural frequencies of the liquid in the tank are determined through the sweep test. The wave elevation and the sloshing pressure are obtained by changing the excitation frequency and the liquid depth. Then the characteristics and the variation of the elevation and the pressure are discussed. The results are compared with the experimental results and the theoretical calculations in a rigid tank. Our analysis indicates that, in the non- resonant cases, the elastic results, the rigid experimental results and the theoretical values are all close to each other. In contrast, under the resonant condition, the elastic experimental result is slightly smaller than the rigid one. Also, the theoretical values are smaller than the experimental results at the resonant frequency.     

Flow and heat transfer characteristics around egg-shaped tube

This paper studies the flow and heat transfer characteristics around a new type of egg-shaped tubes made up of a semicircle upstream and a semi-ellipse downstream, numerically and experimentally, for Reynolds numbers varying from 4 000 to 5 0000 based on the hydraulic diameter. These tubes have axis ratios, ε= 1, 1.5, 2, 3, 4, and 5 with the major axis parallel to the mainstream. When ε= 1, the tube is a circular one and when 1/ ε= 0, it is a flat plate. Numerical results show that the minimum value of p C occurs at an angular position, which decreases as ε decreases, and the maximum value of f C gradually increases with the increase of ε. Simulated results agree very well with those available in the existing literature. The egg-shaped tube has a higher favorable pressure gradient at its front and a lower adverse pressure gradient at its back, that helps the flow delay a separation from the tubes' surface. Empirical correlations for each tube are obtained by numerical simulations relating the dimensionless heat transfer coefficient with the Reynolds Number and the Prandtl Number. The performance evaluation criterion(PEC) is used to analyze the level of the heat transfer enhancement for egg-shaped tubes. It is found that an egg-shaped tube with = 2 has the best overall heat transfer performance when Re11 952.     

Numerical Simulation and Analysis on the Hemodynamics of An Elastic Aneurysm

Intracranial aneurysms are pathological dilatations which endanger people＇s health. Hemodynamics is thought to be an important factor in the pathogenesis and treatment of aneurysms. To date, the bulk of investigations into hemodynamics have been conducted by making use of mathematically idealized models for rigid aneurysms and associated arteries. However the walls of aneurysms and associated arteries are elastic in vivo. This study shows the differences of the simulation between elastic and rigid wall models. The numerical simulation of elastic aneurysm model is made fi＇om a representative Digital Subtraction Angiography （DSA） image and calculated with CFD software to get the wall deformation and the velocity field. Then the results are analyzed. By comparing the simulation results of the two models from their velocity vectors and shear stress distribution, many differences can be noted. The main difference exists in the distribution of velocity magnitude at some sections, with one outlet having obviously off-center distribution for the elastic wall model. The currents of the distribution of wall shear stress along the wall of aneurysm simulated in rigid and elastic wall models were similar. But there were apparent differences between the two models on the values of wall shear stress especially at the neck of aneurysm. The off-center distribution of velocity magnitude affects the distribution of wall shear stress and the exchange of substance through the wall. The analysis demonstrated clearly that the results of 2-D elastic numerical simulation were in good agreement with the clinical and pathological practice. The results of this study play an important role in the formation, growth, rupture and prognosis of an aneurysm on clinic application.     

An experimental study of impact loading on deck of shore-connecting jetties exposed to oblique waves and current

The impact pressure from waves is an important issue to be considered in the design of coastal structures. In this paper, the waves acting on the deck of a shore-connecting jetty on a slope exposed to oblique waves and in the presence of current are examined based on laboratory experiments. The impact pressures are measured on a 1：50 scale model of a jetty head with down-standing beams and berthing members. The relations of the impact pressure with the incident wave angle and the current velocity are examined. It is shown that the impact pressure is sensitive to the wave angle and the current velocity. A computational model for the impact load on the deck of shore-connecting jetties exposed to oblique waves and current is developed.     

A three-dimensional hydroelasticity theory for ship structures in acoustic field of shallow sea

Nowadays the development of green ship technology requires the vibration and noise control of oceangoing ships. The three-dimensional hydroelasticity theory of ships was previously extended to include the effect of fluid compressibility. This enables the dynamic responses and the acoustic radiations of a ship excited by onboard machineries or fluid fluctuation loads to be predicted. In this paper the hydroelastic analysis and sonoelastic analysis methods are fixrther incorporated with the Green＇s function in the Pekeris ocean hydro-acoustic waveguide model to work out a three-dimensional sonoelastic analysis method for ships in the ocean hydro-acoustic environment. As examples, the sound radiations of a floating elastic spherical shell excited by a concentrated force and a traveling LNG ship excited by the propeller induced pulsating forces acting on the wetted bottom plate of the stem in the shallow sea environment are predicted. The influences of the free surface and the sea bed on the generalized hydrodynamic coefficients and the acoustic pressure distributions in fluid domain are illustrated and discussed.     

Propulsive performance of a passively flapping plate in a uniform flow

Propulsive performance of a passively flapping plate in a uniform viscous flow has been studied numerically by means of a multiblock lattice Boltzmann method. The passively flapping plate is modeled by a rigid plate with a torsion spring acting about the pivot at the leading-edge of the plate, which is called a lumped-torsional-flexibility model. When the leading-edge is forced to take a vertical oscillation, the plate pitches passively due to the fluid-plate interaction. Based on our numerical simulations, various fundamental mechanisms dictating the propulsive performance, including the forces on the plate, power consumption, propulsive efficiency and vortical structures, have been studied. It is found that the torsional flexibility of the passively pitching plate can improve the propulsive performance. The results obtained in this study provide some physical insights into the understanding of the propulsive behaviors of swimming and flying animals.     

Stability analysis of the fluttering and autorotation of flow-induced rotation of a hinged flat plate

This work describes investigations performed on the interaction of uniform current and freely rotating plate about a fixed vertical axis. Fluttering and autorotation are two different motions that may ...     

开口薄壁杆与多个弹性支承偏心连接时弯扭屈曲

研究了有双轴对称截面开口薄壁压杆与多个弹性支承偏心连接时的弯扭屈曲,把作用在开口薄壁压杆上的弹性支承去掉,代之以相应的未知外力和未知扭矩,采用Laplace变换推导出了开口薄壁压杆弯扭屈曲的位移函数,求得了其弯扭屈曲的特征方程。     

既有预应力混凝土简支梁预应力损失的有限元模拟研究

应用大型有限元分析软件ANSYS建立一根已经服役20年的既有预应力混凝土后张板梁单元的有限元模型,采用弹簧单元Combin 39模拟预应力钢筋和混凝土间的粘结滑移效应,通过调整该弹簧单元的刚度来控制预应力钢筋和混凝土间的粘结程度。在模型中同时考虑了预应力瞬时损失和长期损失,较为精确的对既有预应力混凝土结构进行有限元模拟。对该模型进行构件的弹塑性变形、混凝土开裂到极限承载力破坏的全过程分析,得到的结果与试验结果有较高的吻合度,进一步表明该模拟方法的合理性和有效性,为预应力结构精细化建模分析提供一定的参考。     

模拟履带-软黏土相互作用的环形触探仪研发

海底履带机器人在软黏土海床上行走时会压陷并剪切土体。压陷反映了履带贯入深度与土体强度，剪切则决定了土体能够提供的驱动力。传统地面力学中的贝式仪将压陷和剪切过程剥离测试，忽略了两者之间的相互作用关系。提出了新的环形触探形式与工作方法，将床面行走的压陷和剪切过程统一到触探仪的压陷贯入和扭转剪切中，通过与前人室内模型试验的对比，验证了环形触探概念的有效性和可行性。针对海床常见的正常固结软黏土，采用修正剑桥模型结合有限元方法，基于环形触探仪对履齿结构参数齿高、齿距进行了敏感性分析，探究了履齿结构参数对软黏土强度发挥的影响。研究结果表明，齿高、齿距对软黏土的强度发挥影响显著，适当增大齿高、缩小齿距有助于软黏土强度的充分发挥。     

具有粘弹性支撑的平板上势流的稳定性

本文研究了具有粘弹性支撑的平板上均匀势流中的稳定性问题。在研究中采用基于空间－时间稳定性理论的映射法搜寻绝对不稳定特征模态。研究结果表明，当支撑刚度足够小时总量会有不稳定发生。对于纯弹性支撑，尽管有不稳定，但它并不是绝对不稳定；而对于粘弹性支撑，所发生的不稳定则一定是绝对不稳定。     

Hydrodynamics of the interceptor on a 2-D flat plate by CFD and experiments

Nowadays, the use of interceptor by both partial and total dynamic lift crafts is quite common. In this article, a lot of evidence is given regarding the effectiveness of interceptor. The interceptor, when placed at the stern region, changes the pressure distribution around the craft. Its presence affects drag force, lifting force and the position of pressure's center leading to a new trim. This study focuses on hydrodynamic effects of interceptors on a 2-D flat plate based on both computational fluid dynamic(CFD) and experimental approaches. The Reynolds average Navier-Stokes(RANS) equations are used to model the flow around a fixed flat plate with an interceptor at different heights and attack angles. Based on finite volume method and SIMPLE algorithm which uses static structures, this model can be analyzed and the RANS results can be compared with the experimental data obtained in the current channel of the laboratory of waves and current of COPPE/UFRJ(LOC in Portuguese acronym). According to the results, the increase of pressure at the end of the flat plate was proportional to the interceptor height. In addition, the existence of interceptors can significantly increase the lift force coefficient at high angles of attack also proportional to the interceptor height. The presence of interceptor at the end of the flat plate increased both the lift coefficient and the drag coefficient but hydrodynamic drag did not grow as fast as the lift coefficient did. The lift coefficient increased much more. Furthermore, the results showed that the interceptor effectiveness is proportional to the boundary layer thickness at the end of the flat plate. As the interceptor was inside the boundary layer alterations of flow speed led to changes in boundary layer thickness, directly affecting interceptor's efficiency. Optimum choice of interceptor height had a great effect on its efficiency, and in choosing it the flow speed and length of the boat must be taken into consideration.     

扁平管袋裸坝基于波浪作用的稳定性公式推求

为获得扁平管袋裸坝结构在波浪作用下的变形稳定特性,分析了扁平管袋在波浪作用下的受力情况,通过引入管袋变形因子,分别推导出考虑管袋变形情况的裸坝结构滑动失稳和倾覆失稳稳定性公式。结合管袋裸坝结构模型试验得到的管袋失稳方式和相应的变形参数,探讨了变形对管袋裸坝结构发生滑动失稳时所受波浪力和自重力及倾覆失稳时相应力矩的影响,计算出相应的变形因子,进一步完善了考虑管袋变形情况下管袋裸坝结构稳定性公式。该公式充分考虑波浪引起的管袋变形特征,具有普遍适用性。     

斜置单层折板钢板剪力墙的屈曲分析

通过对以往钢板剪力墙资料的研究,在竖直折板钢板剪力墙的基础上提出一种将整片折板斜置的钢板剪力墙,并对其进行了有限元弹性屈曲分析。通过改变截面几何形状,以期望改善薄钢板剪力墙的出平面弹性失稳问题,这样就可以更好的发挥钢板材料的强度和延性。本文为这种新型钢板剪力墙的研究奠定了基础,并对实际工程中钢板剪力墙的设计与应用提出建...     

平底型与反拱型水垫塘动水荷载对比试验研究

水垫塘结构是影响高坝泄洪消能的关键,优化水垫塘底板型式对有效地增加底板稳定性和减小动水荷载至关重要。平底型和反拱型底板作为两种常见的水垫塘结构型式被业界广泛采用,但关于两者的动水荷载特性对比研究尚不多见。基于白鹤滩水电站1∶50整体模型,采用物理模型试验的方法对比了平底型和反拱型水垫塘在时均压强和脉动压强方面的特性。结果表明:反拱型水垫塘底板的时均压强峰值小于平底型,来流方向上反拱型水垫塘底板的时均压强低谷值小于平底型;两种型式水垫塘脉动压强峰值差异较小,反拱型底板整体脉动压强大于平底型,但反拱型脉动压强梯度小于平底型。