多体海洋结构间窄缝内流体共振的试验和数值研究

发布时间：2018-04-01 15:14

本文选题：物理试验　切入点：流体共振　出处：《大连理工大学》2014年硕士论文

【摘要】：海上多浮体结构间往往存在相对于结构自身尺寸很小的窄缝。在特定频率的波浪作用下,这些窄缝内会出现流体大幅度振荡现象,称为窄缝流体共振。此时,结构物所受波浪力将显著增大,严重威胁工程安全作业。为了深入了解和认识窄缝流体共振的物理特性和作用机制,本文通过物理试验和数值分析方法对该问题开展研究。传统的势流模型因无法考虑流动中的机械能耗散,所以会严重高估窄缝内流体的共振波高。粘性流模型能够给出较可靠的模拟结果,但非常耗时。在势流理论框架内,通过引入合适的人工阻尼项,修改自由表面边界条件或边界匹配条件,可得到较可靠的共振幅值的预测结果,然而经验性的人工阻尼系数需要结合试验结果或CFD结果进行率定。本文通过二维水槽物理试验,对存在窄缝的箱体-箱体结构以及方箱-直墙结构,在正向线性入射波作用下的流体共振特性,以及窄缝流体共振中,入射波波高和结构尖角引起的涡流对机械能耗散的影响作用开展了研究。在双箱窄缝流体共振试验中,通过设置具有不同半径的圆角底角的箱体,观测对应的窄缝内流体共振特性的变化规律。试验结果表明,首先,窄缝内流体共振波高随入射波波高的增大呈幂函数趋势增大,这体现了入射波波高对水体流动过程中的机械能耗散的影响作用。而共振波高与入射波波高之间的依赖关系,与模型的几何参数及设置相关。其次,当箱体的底角形状由直角变为具有较小半径的圆角后,窄缝内流体共振频率变化很小,而共振波高则显著增大,这意味着由方箱尖角引起的涡流对机械能耗散的影响十分重要。而随着圆角半径增大,窄缝内流体的共振频率不断增大,共振波高则逐渐减小,但仍明显高于箱体底角为直角的情况,这反映了结构尖角对窄缝流体共振过程中的机械能耗散的影响机制。而窄缝流体共振频率的变化则与窄缝内流体质量的变化有关,随着箱体底角半径的增大,窄缝内参与垂向振荡的流体质量减小,对应的共振频率因此增大。此外,随着箱体底角形状的变化,窄缝内流体共振波高与入射波波高之间的依赖关系也随之改变。本文在方箱-直墙系统窄缝流体共振试验中,分别研究了不同的方箱吃水深度、窄缝间隙以及入射波频率和波高的影响作用,为数值模型的验证和人工阻尼(力)系数的率定提供了依据。此外,通过分析水动力系数随入射波频率、波高的变化规律,对水体流动过程中的机械能耗散进行了研究。试验结果表明,方箱与直墙间窄缝内共振波高可达到入射波波高的7倍以上；直墙-方箱系统的反射系数对方箱吃水深度、窄缝宽度和入射波波高存在复杂的非线性依赖关系,这体现了共振响应过程中的机械能损耗机制。在数值研究方面,本文在势流理论框架内建立控制方程,通过引入人工阻尼改善共振波高的预测精度,分别采用匹配特征函数展开法和边界元法求解方法分别建立了半解析模型和边界元模型。首先,根据物理试验结果对数值模型的阻尼(力)系数进行率定。然后,采用数值分析方法对窄缝流体共振特性与结构几何参数间的依赖关系进行了分析,并对结构所受波浪力进行了研究。数值结果表明,双箱(圆角底角)间窄缝流体共振中,双箱所受水平方向波浪力可达2.5倍pgha,其中ρ为流体密度,g为重力加速度,h为水深,a为入射波波幅。方箱-直墙间窄缝内流体共振中,方箱吃水深度、窄缝宽度及方箱宽度的增大,均会导致窄缝内水体共振频率单调减小。在共振频率附近,方箱及直墙所受水平方向波浪力可达4倍pgha,且随着方箱吃水深度的增大而逐渐增大。
[Abstract]:The multi body floating structure often exist with respect to narrow their very small size. In the wave of specific frequency, the narrow gap in large amplitude oscillation will appear fluid, called narrow fluid structure resonance. At this time, the wave pressure will significantly increase, Yan Zhongwei threatened the safe operation in order to project. A deeper understanding of the physical characteristics and mechanism of narrow resonance fluid, this paper carried out research on this problem through physical experiments and numerical analysis method.
The traditional potential flow model cannot consider the flow of mechanical energy dissipation, so it will seriously overestimate the height of fluid in narrow resonance. Simulation results are given to a reliable viscous flow model, but is very time-consuming. In potential theory, by introducing a suitable artificial damping, modify the free surface boundary condition or boundary the matching condition, the predicted results can be more reliable than the resonance amplitude, however artificial damping coefficient requires a combination of empirical test results or CFD results were calibrated. Through 2D physical test, the structure of box - slot structure of box and box - straight wall, fluid in the role of positive linear resonance characteristics of incident wave the narrow and fluid resonance, wave structure and high angle caused by the eddy current research on effect of mechanical energy dissipation.
In the double slit fluid resonance test box, box by setting with different radii of the rounded corners, variation of the observations corresponding to narrow the fluid in the resonance characteristics. The experiment results show that, firstly, the fluid in the narrow resonance wave with wave height increases exponentially increasing trend, which reflects the incident wave high impact on water flow in the process of mechanical energy dissipation effect. And the resonance between wave height and incident wave height dependence associated with the geometric parameters and the set model. Secondly, when the box bottom shape by angle varying with smaller radius fillet, narrow fluid in the resonance frequency change is very small, and the resonance wave height increases obviously, which means that the important influence of eddy current by square box on the cusp of mechanical energy dissipation. As the radius increases, increasing the resonance frequency of the fluid in the narrow resonance wave. Then gradually decreased, but still higher than the bottom of a rectangular box, which reflects the structure of narrow angle influence mechanism of fluid resonance in the process of mechanical energy dissipation. And narrow resonance frequency changes of fluid changes and narrow fluid quality, with the increase of the radius of the bottom of the box body angle, narrow in the vertical mass of fluid oscillation decreases, thus increasing the corresponding resonance frequencies. In addition, with the change of the shape of the box bottom, the dependence between the fluid in the narrow resonance wave height and wave height change.
In this paper, square box - straight wall system narrow fluid resonance test, were studied in different depth box, narrow gap and the influence of the frequency of the incident wave and wave function verification for the numerical model and artificial damping coefficient (force) was provided. In addition, through the analysis of water power the coefficient of variation with the incident wave frequency, wave height, the water flow in the process of mechanical energy dissipation are studied. The experimental results show that the square box and the straight wall between the narrow resonance wave high Gao can reach more than 7 times; the reflection coefficient of square box box system of straight wall - depth, narrow the width and the incident wave height are complex nonlinear dependence, which reflects the resonance response process of the mechanical energy loss mechanism.
In the numerical study, the theoretical framework established within the control equation in potential flow, improve the prediction accuracy of resonance wave by introducing artificial damping, respectively using a semi analytical model and boundary element model is established and the matched eigenfunction expansion method and boundary element method respectively. First, according to the physical test results of the numerical model (damping force) coefficient was set. Then, analyzes the narrow dependence of fluid resonance characteristic and structure of geometric parameters between using the numerical analysis method, and the structure of the wave force are studied. The numerical results show that the double box (rounded corner) between the narrow resonance fluid in the horizontal double box the direction of the wave is 2.5 times pgha, where p for fluid density, G for the acceleration of gravity, H depth, a wave amplitude. The square box - straight wall slot between the fluid in the resonance, square box depth, narrow width and square box The increase of width, which leads to narrow gap in water resonance frequency decreases monotonically. Near the resonance frequency, the square box and straight wall under horizontal direction of the wave is 4 times of pgha, and with the increasing of square box draft and increased gradually.

【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：U661.1;O353

【参考文献】