共查询到20条相似文献，搜索用时 93 毫秒
1.
The main characteristic of the bubble dynamics near a rigid wall is the development of a high speed liquid jet, generating highly localized pressure on the wall. In present study, the bubble dynamic behaviors and the pressure impulses are investigated through experimental and numerical methods. In the experiment, the dynamics of a spark-generated bubble near a steel plate are captured by a high-speed camera with up to 650,000 frames per second. Numerical studies are conducted using a boundary integral method with incompressible assumption, and the vortex ring model is introduced to handle the discontinued potential of the toroidal bubble. Meanwhile, the pressure on the rigid wall is calculated by an auxiliary function. Calculated results with two different stand-off parameters show excellent agreement with experimental observations. A double-peaked or multiple-peaked structure occurs in the pressure profile during the collapse and rebounding phase. Generally, the pressure at the wall center reaches the first peak soon after the jet impact, and the second peak is caused by the rapid migration of the bubble toward the wall, and the subsequent peaks may be caused by the splashing effect and the rebounding of the toroidal bubble. At last, both agreements and differences are found in the comparison between the present model and a hybrid incompressible–compressible method in Hsiao et al. (2014). The differences show that the compressibility of the flow is another influence factor of the jet impact. However, the main features of the jet impact could be simulated using the present model. 相似文献
2.
In this paper, a numerical model based on the potential flow theory is established to simulate the interaction of a gas bubble with a nearby wall. The time-integration boundary integral method is used to solve the dynamics of a gas bubble. With this method the numerical calculations show an excellent agreement with the experimental data. Employing the numerical code based on the presented algorithm, the dynamics of a gas bubble close to a rigid wall is investigated systematically, especially the relationship between various characteristic parameters and the Bjerknes effect due to the presence of a nearby wall. It is found that Blake's criterion, which is usually used to predict the direction of the bubble jet, has a great degree of accuracy for the bubble relatively far away from the wall and bubble near a wall, there is a significant error, attributed to its simplifications and assumptions. Further studies show that an oblique jet will be formed when a bubble close to an inclined wall collapses, direction and width of which have a close relationship with the characteristic parameters used to characterize the bubble. For the bubble near a horizontal wall, a liquid jet pointing directly to the wall is developed generally when the Bjerknes attraction and buoyancy are in the same direction; and at the same time, if the Bjerknes attraction is in the opposite direction of buoyancy, the direction of the jet will depend on a criterion. Then the interaction of gas bubble between complicated walls of some a submarine is also studied, which shows the most dangerous induced loading condition of structure in water, and the evidently effects of bubble jet on loading. The special phenomena mentioned above have a great significance for the further study on the interaction of the bubble with its boundaries. 相似文献
3.
The dynamics of cavitation bubble is analyzed in the compressible fluid by use of the boundary integral equation considering the compressibility.After the vertical incidence of plane wave to the rigid wall,the motion characteristics of single cavitation bubble near the rigid wall with initial equilibrium state are researched with different parameters.The results show that after the driving of acoustic wave,the cavitation bubble near the rigid wall will expand or contract,and generate the jet pointing to the wall.Also,the existence of the wall will elongate time for one oscillation.With the compressible model,the oscillation amplitude is reduced,as well as the peak value of inner pressure and jet tip velocity.The effect of the wall on oscillation amplitude is limited.However with the increment of initial vertical distance,the effect of wall on the jet velocity is from acceleration to limitation,and finally to acceleration again. 相似文献
4.
The pulse features of a bubble have a close connection with the boundary condition. When a bubble moves near a rigid wall, it will be attracted by the Bjerknes force of the wall, and a jet pointing at the wall will be generated. In real application, the bubble may move under the combined action of walls in different directions when it forms at the corner of a pipe or at the bottom of a dam. The motion of the bubble shows complex and nonlinear characteristics under these conditions. In order to investigate the bubble pulse features near complex walls, a horizontal wall and a vertical wall are put into the experimental water tank synchronously, and an electric circuit with 200 voltages is designed to generate discharge bubbles, and then experimental study on the bubble pulse features under the combined action of horizontal and vertical walls is carried out. The influences of the combined action of two walls on the bubble shape, pulse period, moving trace and inside jet are obtained by changing the distances from bubble center to the two walls. It aims at providing references for the relevant theoretical and numerical research. 相似文献
5.
《Coastal Engineering》2002,46(2):139-157
At plunging breakers, air bubbles are entrained at the impingement of the water jet, formed at the top of the wave, with the water free surface in front. During the present study, air bubble entrainment at a pseudo-plunging breaker was investigated at near full-scale and further experimental work studied the bubble detrainment process. Experimental observations included the generation and propagation of waves downstream of the plunge point. Experimental results highlighted a number of unsteady air–water flow patterns and emphasise high levels of aeration: i.e., depth-averaged void fraction of more than 10% next to jet impact in shallow waters. Unsteady bubble injection experiments showed a strong vortical motion induced by the rising bubbles. Altogether, the results suggest that a dominant time scale is the bubble rise time d1/ur, which cannot be scaled properly with an undistorted Froude model. The study contributes to a better understanding of unsteady bubble entrainment at a pseudo-plunging breaker and the associated vortical circulation. 相似文献
6.
This paper is concerned with the interaction of bubbles, a submerged or floating structure, and free surface waves. A three-dimensional fully nonlinear model has been developed based on the coupling of the boundary integral method (BIM) for bubble dynamics and free surface waves and the finite element method for structure deformation. The present method is well validated by comparing the numerical results with the experimental data. Three structure characteristics, including fixed, rigidly moving and flexible, are investigated separately to determine their influence on bubble dynamics. For a free-floating structure, the free surface causes not only a larger reduction in peak pressure for a rigid structure compared with a fixed body but also the modification of the bubble period and structural response. The interaction between a bubble and a flexible structure, in the absence of a free surface, is simulated. Both the rigid motion and the deformation at the local structure appear in the simulation. The effect of the structural thickness on the reduction in peak pressure is also considered. 相似文献
7.
针对近岸海域极端海况的防灾减灾问题,准确模拟追踪海啸波传播过程,再现液面局部射流、崩破波等波面湍动现象。建立SPH数值水槽,边界条件基于固壁粒子法,减少海堤坡角改变对计算域精度的影响,讨论粒子间距设置对模拟精度的影响。模拟7种不同海堤坡角下波浪的爬高与衰减情况,讨论了海堤坡角变化对消波系数的影响。当粒子间距设置为0.002 m时,模型能准确地捕捉海啸波的强非线性现象。随着海堤坡角增大,海啸波峰值爬升率增大,波浪越堤后因崩破波的产生,波能衰减进一步加剧。当坡角较小时,消波系数随坡角的增大提升明显,而后趋于缓慢增长。 相似文献
8.
Highly dependent on boundary conditions, the behaviors of underwater explosion (UNDEX) bubbles would be quite unusual near boundaries that are discontinuous with abrupt changes in shape, e.g. ship structures that have already been deformed by previous attacks. The oscillation features of the UNDEX bubble near the bilge with a circular opening representing previous deformation are studied experimentally with electric-spark-generated bubbles and high-speed photographing. The bubble behaviors are found highly dependent on two non-dimensional variables, D and Φ, representing the opening-bubble distance and the opening diameter, respectively. Seven distinctive oscillation scenarios are summarized from 180 experiments, namely the ‘rim-constrained oscillation’, the ‘inward jet’, the ‘outward jet’, the ‘bump and dimple’, the ‘quasi-spherical oscillation’, the ‘spherical oscillation with jet’ and the ‘spherical oscillation without jet’. The occurrence domains of the scenarios are identified as functions of D and Φ. Significantly affected by the opening, the bubble behaviors are quite different from that near a non-opening bilge; the bubble jet might not be formed, or even develop from inside the bilge, which indicates that the bubble load on the bilge should be re-evaluated. Finally the speeds, initiation time and displacements of the jets in different scenarios are measured and noticeable variation trends are found. 相似文献
9.
采用LS-DYNA软件,对装药半径为0.15m、0.42m、0.55m的战斗部有、无壳体的爆炸特性进行数值模拟研究,分析了炸药在有无壳体的水下爆炸时的冲击波压力、气泡脉动压力等特性参数,对比总结了不同当量、不同装药半径及有无壳体的数值计算结果。结果表明:壳体对水下爆炸气泡脉动的影响是较为显著的。壳体厚度对气泡形成时间没有太多影响,但对气泡压力峰值影响较大。因此,研究战斗部水下爆炸威力时必须考虑壳体因素,不能简化。 相似文献
10.
A static experimental system based on the background of the underwater gun firing process is designed to simulate the expansion of conical-distributed multiple gas jets in a water-filled vessel. The gas is generated from the gunpowder combustion and injected into water through five orifices on a mock bullet. A high-speed digital camera is taken in experiment to record such expansion and the Euler-Euler multiphase model is utilized in simulation to describe the gas-liquid flow. Results show that the contraction of Taylor cavity in liquid and the gas-liquid entrainment both affect the shock structure in gas. The expansion zone near the orifice shrinks and the gas speed downs when the central gas jet has a contraction, but lateral jets have no contractions due to the obstruction from the wall. The contraction has a greater impact on the shock structure than the gas-liquid entrainment and occurs earlier under a higher injection pressure. 相似文献
11.
Coastal disposal of waste water can be idealized as the problem of a jet under random waves. Understanding of this phenomenon is important for engineering design and environmental impact assessment. The present study aims to simulate such phenomenon by using a 3D numerical model based on the solution of the spatially filtered and σ-transformed Navier–Stokes equations with dynamic sub-grid scale model of turbulence. The numerical solution procedures are split into three steps: advection, diffusion and pressure propagation, and a Lagrange–Euler method is used to track the free surface. Cases of vertical jet in stagnant water, pure random waves and vertical jet in random waves are simulated with the same grid system for comparative study. Different methods of generating jet inflow turbulence have been tested and the method of jet azimuthal modes is found to be the optimum. The numerical results reproduce the distinct characteristics of jet in waves, including faster decay of centerline velocity, wider lateral spreading and the occurrence of wave tractive mechanism. 相似文献
12.
The shock wave and cavitation are main effects in the far-field underwater explosion, which could cause serious damage to marine structures. In this paper, the fluid mechanical behavior of blast load is described by the propagation of pressure wave. The acoustic pressure caused by far-field explosion is determined by solving the wave equation, where a strongly discontinuous axisymmetric numerical model is established with the local discontinuous Galerkin (LDG) method. The model can calculate the dynamic pressure in the fluid field and capture the high-resolution shock wave. The pressure cutoff model is employed to deal with the cavitation effect due to the reflection of the shock wave. The numerical model is verified by comparing with the analytical solution of the cavitation effect near the structure in one dimension. With the same mesh discretization, the present model shows higher precision than the results calculated by the acoustic finite element method. In addition, the propagation of shock wave in the cylindrical water column is studied. Finally, the formation, growth and collapse of the cavitation region near the free surface are simulated. The LDG model can remove the spurious oscillations behind the shock front and it’s more accurate than the results of the acoustic finite element method, in terms of capturing the sharpness of shock wave and calculating the shock and cavitation loading. And the present model can be applied to calculate the structural damage caused by shock wave in three dimensions. 相似文献
13.
The motion of a bubble near the free surface is solved by the boundary element method based on the linear wave equation, and the influence of fluid compressibility on bubble dynamics is analyzed. Based on the solution of the bubble motion, the far-field radiation noise induced by the bubble is calculated using Kirchhoff moving boundary integral equation, and the influence of free surface on far-field noise is researched. As the results, the oscillation amplitude of the bubble is weakened in compressible fluid compared with that in incompressible fluid, and the free surface amplifies the effect of fluid compressibility. When the distance between the bubble and an observer is much larger than that between the bubble and free surface, the sharp wave trough of the sound pressure at the observer occurs. With the increment of the distance between the bubble and free surface, the time of the wave trough appearing is delayed and the value of the wave trough increase. When the distance between the observer and the bubble is reduced, the sharp wave trough at the observer disappears. 相似文献
14.
Simulation of Flat-Bottom Structure Slamming 总被引：1，自引：0，他引：1
1.Introduction Inthepastdecades,theproblemofwaterentryhasbeenstudiedextensively(VonKarman,1929;AraiandMiyauchi,1998;ZhaoandFaltinsen,1993;Chuang,1966,1967;Takagiand Dobashi,2003).ThepioneerworkcanbetracedtoWagner’smomentumimpacttheory.Hecalculat edthevelocitypotentialaroundthefallingbodyandeducedaformulaofpressureactingonthefalling bodyvaryingwithtimeandspace.Butinhistheory,manyassumptionsweremadeandtheroleofair wasignoredsothatsomeerrorsoccurwhenhisformulaisusedtopredictthepressureofimp… 相似文献
15.
通过自由落体的入水方式,分别在静水和规则波中开展了两种不同横剖面的曲面楔形体入水砰击问题试验研究。使用高速摄像系统记录楔形体入水过程流场演变和运动特性,采用加速度传感器和压力传感器进行数据的动态采集。试验结果表明,在静水中入水时,外凸剖面楔形体入水砰击后模型两侧的射流飞溅比反曲剖面更剧烈,而在楔形体前端的水面以下部分形成的气腔更小;在规则波中入水时,对于相同模型,在波峰和上跨零点相位下模型入水砰击后两侧的射流飞溅比在波谷相位更剧烈。相同工况时,反曲剖面模型所受砰击的加速度峰值和压力峰值更小;在相同的入水速度下,对于相同模型,波浪载荷和砰击载荷的共同作用会使模型所受砰击压力显著增大。 相似文献
16.
Experiments and numerical methods are developed to investigate the water entry of a freefall wedge with a focus on the evolution of the pressure on the impact sides (the side contacting water) and the top side (the dry side on the top of the wedge), evolution of the global hydrodynamic loads, evolution of the air–water interface, and wedge motion. It is found that a typical water entry of a freefall wedge can be divided into slamming, transition, collapse and post-closure stages. A single-fluid numerical model is presented to simulate the first three stages. The results are compared to experiments and good agreements are obtained. A two-fluid BEM is proposed to investigate the influence of the air flow before the closure of the cavity created on the top of the wedge. It is found that for the closure of the 2D cavity, the air flow starts to play an important role just before closure but due to the short duration, the influence of air flow on the body velocity and configuration of the air–water interface is limited. 相似文献
17.
楔形体在波浪中自由入水的数值模拟 总被引：1，自引：0，他引：1
物体入水时波浪的影响不可忽略,基于流体力学模型采用VOF法,并利用自定义函数,模拟了楔形体的自由入水过程;同时结合推波板原理及海绵层消波理论实现了数值水槽的造消波,完成了波浪中楔形体自由入水的模拟,计算了楔形体入水时所受的水作用力、自由液面变化及物面压强分布等,研究了不同波高、周期以及在波浪不同位置入水时对楔形体的影响。结果表明:本文建立的数值模型可很好地模拟楔形体入水造成的射流及空泡的形成发展过程,波浪对楔形体入水的影响主要由波浪内部流场变化及表面波形决定,在波浪不同位置处入水对楔形体受力及入水形态均有较大影响。 相似文献
18.
The hydroacoustic method for the quantification of the gas flux from a submersed bubble plume 总被引：2，自引：0，他引：2
This article presents an inverse hydroacoustic method for the remote quantification of the total gas flux transported from
an underwater bubble plume. The method includes the surveying of the bubble plume by a vertically looking echo sounder and
the calculation of the flux using the spatial distribution of the ultrasound backscattering at a fixed depth. A simplified
parameterization containing only a few parameters is introduced to describe the empirical bubble size distribution. The linear
correlation between the backscattering cross section of the bubble stream and the vertical gas flux is found. The calculation
procedure takes into account the occurrence of a gas hydrate film at the bubble’s surface. The influence of different parameters
on the accuracy of the method is investigated. The resolution volume of the echo sounder corresponding to the fixed distance
is considered as a two-dimensional spatial window. The method was applied to quantify the total convective methane flux at
the Haakon-Mosby mud volcano (HMMV) depth 1280 m. The calculated values of the total flux near the bottom (100–400 t/year)
are in good agreement with the independently estimated flux for the single bubble jet observed from the ROV (70 t/year). These
calculations also show significant temporal variability of the flux at the HMMV. The total flux was found to vary by about
a factor of 2–3 within time scales of days. 相似文献
19.
This paper considers steep wave impact on seawalls of various geometries. A simple analytical model for the pressure impulse due to a wave of idealized geometry and dynamics is developed and applied to the following geometries: (a) vertical seawall with a berm, (b) vertical seawall with a ditch at its base and (c) vertical seawall with a block missing (damaged condition).The method uses eigenfunction expansions in each of the rectangular regions that satisfy some of the rigid surface conditions and a simplified free-surface condition. Their unknown coefficients are determined from the impact boundary condition, rigid wall conditions and by matching the values and the horizontal derivatives of the solutions in each rectangular region at their mutual boundary. The method yields the pressure impulse throughout the entire region. The overall impulse and moment impulse on the seawall and a simple model for the uprush of the spray jet after the impact are also presented. The effects of different impact regions and different geometries can therefore be quickly estimated and used to show trends in the results. It is shown that berms generally have a beneficial effect on reducing the impulse, moment impulse and uprush, but not the maximum pressure impulse on the seawall, whereas ditches are generally and sometimes strongly detrimental for all effects except uprush. A missing block in the seawall gives an almost constant or linearly decreasing value inside the gap (depending on the boundary condition applied at the rear of the gap being hard or soft respectively); the soft case can affect the pressure impulse on the front face of the seawall, thereby affecting the impulse and moment impulse. 相似文献
20.
M Salih Kirkgz 《Ocean Engineering》1991,18(1-2)
Laboratory tests are conducted to measure the impact pressures of breaking waves on vertical, 5° forward, and 5, 10, 20, 30, and 45° backward sloping walls. The base structure of the wall has a foreshore slope of
. Regular waves are used throughout the experiments for all wall angles. The maximum impact pressures on the wall are shown to satisfy the log-normal probability distribution. It is found from the present experiments that the impact pressures and resulting forces on sloping walls can be greater than those on a vertical wall. On the seven different walls tested, the maximum impact pressures occur most frequently slightly below the still-water level. The pattern of the impact pressure history does not change with the slope of the wall, and as the probability of maximum impact pressure decreases, the pressures around the peak pressure region of the impact pressure histories remain longer. 相似文献