Motion Characteristics of Cavitation Bubble near the Rigid Wall with the Driving of Acoustic Wave

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.

MOTION AND DEFORMATION OF VISCOUS DROP IN STOKES FLOW NEAR RIGID WALL

A boundary integral method was developed for simulating the motion and deformation of a viscous drop in an axisymmetric ambient Stokes flow near a rigid wall and for direct calculating the stress on the wall. Numerical experiments by the method were performed for different initial stand-off distances of the drop to the wall, viscosity ratios, combined surface tension and buoyancy parameters and ambient flow parameters. Numerical results show that due to the action of ambient flow and buoyancy the drop is compressed and stretched respectively in axial and radial directions when time goes. When the ambient flow action is weaker than that of the buoyancy the drop raises and bends upward and the stress on the wall induced by drop motion decreases when time advances. When the ambient flow action is stronger than that of the buoyancy the drop descends and becomes flatter and flatter as time goes. In this case when the initial stand-off distance is large the stress on the wall increases as the drop evolutes but when the stand-off distance is small the stress on the wall decreases as a result of combined effects of ambient flow, buoyancy and the stronger wall action to the flow. The action of the stress on the wall induced by drop motion is restricted in an area near the symmetric axis, which increases when the initial stand-off distance increases. When the initial stand-off distance increases the stress induced by drop motion decreases substantially. The surface tension effects resist the deformation and smooth the profile of the drop surfaces. The drop viscosity will reduce the deformation and migration of the drop.

Investigations of the dynamical behaviors of a millimeter-scale cavitation bubble near the rigid wall

The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet,causing cavitation damage and operation instability of the hydraulic machinery.In this paper,the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212.The morphological characteristics of the bubble during its growth phase,collapse phase and rebound phase were observed by experiment and numerical simulation,and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated.The numerical results agree well with the experimental data.The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm forγ=d/R_(max)=1.78.The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase,resulting in a higher pressure load for the rigid wall in the second collapse.The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa,and the velocity of the micro-jet reaches approximately 100 m/s.This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble.

A general method to calculate passive earth pressure on rigid retaining wall for all displacement modes

A general analytical method to calculate the passive rigid retaining wall pressure was deduced considering all displacement modes. First, the general displacement mode function was setup, then the hypotheses were made that the lateral passive pressure is linear to the corresponding horizontal displacement and the soil behind retaining wall is composed of a set of springs and ideal rigid plasticity body, the general analytical method was proposed to calculate the passive rigid retaining wall pressure based on Coulomb theory. The analytical results show that the resultant forces of the passive earth pressure are equal to those of Coulomb＇s theory, but the distribution of the passive pressure and the position of the resultant force depend on the passive displacement mode parameter, and the former is a parabolic function of the soil depth. The analytical results are also in good agreement with the experimental ones.

Non-limit passive soil pressure on rigid retaining walls

This paper aims to reveal the depth distribution law of non-limit passive soil pressure on rigid retaining wall that rotates about the top of the wall(rotation around the top(RT) model). Based on Coulomb theory, the disturbance degree theory, as well as the spring-element model, by setting the rotation angle of the wall as the disturbance parameter, we establish both a depth distribution function for sand and a nonlinear depth distribution calculation method for the non-limit passive soil pressure on a rigid retaining wall under the RT model, which is then compared with experiment. The results suggest that under the RT model: the non-limit soil pressure has a nonlinear distribution; the backfill disturbance degree and the lateral soil pressure increase with an increase in the wall rotation angle; and, the points where the resultant lateral soil pressure acts on the retaining wall are less than 2/3 of the height of the wall. The soil pressure predicted by the theoretical calculation put forward in this paper are quite similar to those obtained by the model experiment, which verifies the theoretical value, and the engineering guidance provided by the calculations are of significance.

Nonlinear Stability Analysis of Long Span Continuous Rigid Frame Bridge with Thin Wall High Piers

By utilizing the current finite element program ANSYS, two types of finite element models (FEM), the beam model (BM) and shell model (SM), are established for the nonlinear stability analysis of a practical rigid frame bridge—Longtanhe Great Bridge. In these analyses, geometrical and material nonlinearities are simultaneously taken into account. For geometrical nonlinearity, updated Lagrangian formulations are adopted to derive the tangent stiffness matrix. In order to simulate the nonlinear behavior of the plastic hinge of the piers, the multi lines spring element COMBIN39 is used in the SM while the bilinear rotational spring element COMBIN40 is employed in the BM. Numerical calculations show that satisfying results can be obtained in the stability analysis of the bridge when the double coupling nonlinearity effects are considered. In addition, the conclusion is significant for practical engineering.

Static behavior of semi-rigid thin-walled steel-concrete composite beam-to-column joints with bolted partial-depth flush end plate:experimental study

A new type of semi-rigid thin-walled steel-concrete composite beam-to-column joint has been proposed in this paper.Five semi-rigid composite beam-to-column joint specimens subjected to hogging moments under monotonic loading were tested to study the static behavior of this new type of joint.The main variable parameters for the five joint specimens were the longitudinal reinforcement ratio and the joint type.The experimental results designated that the magnitude of extension of the longitudinal reinforcement is the most important factor that influenced the moment-rotation characteristic of the new type of joint.The concrete slabs could resist 3.8%-19.1% of the total shear load applied to the cross-sections near the beam-to-column connection.The edge stiffened elements,such as the flange of the lipped I-section thin-walled steel beam,were capable of having considerable inelastic deformation capacity although they had comparatively large width-to-thickness ratios.The shear failure of the concrete cantilever edge strip must be taken into account in practical design because it has significant influence on the anchorage of the longitudinal reinforcement in the new type of external joints.

Seismic responses of the steel-strip reinforced soil retaining wall with full-height rigid facing from shaking table test

To investigate the seismic response of the steel-strip reinforced soil retaining wall with fullheight rigid facing in terms of the acceleration in the backfill, dynamic earth pressure in the backfill, the displacements on the facing and the dynamic reinforcement strain distribution under different peak acceleration, a large 1-g shaking table test was performed on a reduced-scale reinforced-earth retaining wall model. It was observed that the acceleration response in non-strip region is greater than that in potential fracture region which is similar with the stability region under small earthquake,while the acceleration response in potential fracture region is greater than that in stability region in middle-upper of the wall under moderately strong earthquakes. The potential failure model of the rigid wall is rotating around the wall toe. It also was discovered that the Fourier spectra produced by the inputting white noises after seismic wave presents double peaks, rather than original single peak, and the frequency of the second peak trends to increase with increasing the PGA(peak ground amplitude) of the excitation which is greater than 0.4 g. Additionally,the non-liner distribution of strip strain along the strips was observed, and the distribution trend was not constant in different row. Soil pressure peak value in stability region is larger than that in potential fracture region. The wall was effective under 0.1 g-0.3 g seismic wave according to the analyses of the facing displacement and relative density. Also, it was discovered that the potential failure surface is corresponds to that in design code, but the area is larger. The results from the study can provide guidance for a more rational design of reinforced earth retaining walls with full-height rigid facing in the earthquake zone.

开洞刚性墙后土体破坏模式运动单元上限分析

超载作用下开洞地下刚性连续墙后土体稳定性问题具有广泛的工程应用背景。考虑墙体矩形开洞宽度足够大,且刚性墙与土体完全光滑接触等条件,参考构造刚性多块体上限法思路,预设墙后土体极简破坏机构并转化为初始三角网格,提出交替循环式新增节点、单元缩减、对角互换及成层加密等网格更新措施,同步开展基于刚体平动运动单元上限方法的刚性墙后土体破坏分析,通过多层级非线性规划运算获得高精度临界超载系数σ_(s)/c上限解,界定墙后土体滑移线网破坏模式及其定量范围与扩展边界。进一步依前述流程计算获得多参数影响下的临界超载系数σ_(s)/c上限解图表,结果表明:1)刚性墙后土体典型破坏特征表现为土体上方和开洞处的整体下沉刚性区域,开洞前方核心区域以密集滑移线网区隔的刚性滑块群,以及外围主要滑动面;2)临界超载系数σ_(s)/c随土体内摩擦角ϕ增大而增大,且后续增幅显著增加;3)σ_(s)/c随开洞深度比C/D的增大呈现出近似线性增大的趋势;4)相较于ϕ和C/D,土体重度参数γD/c的变化对于σ_(s)/c的影响不甚明显,σ_(s)/c随γD/c增大呈现近似线性减小的趋势;5)σ_(s)/c受剪胀角参数ψ/ϕ的影响与ϕ的增大程度密切相关。本研究结果可为开洞地下刚性连续墙后土体失稳破坏模式定量分析、破坏机理研究及工程措施制定等工作提供有力的数据支持。

基于阻抗匹配的吸声方法

对于低中频声波入射,采用主动吸声方法可以获得较好的吸收性能,而对于高频声波入射则不能达到很好的吸收性能,为此,又提出了通过调节吸声材料后扬声器的表面阻抗来改变吸声系数的半主动吸声方法。将可控扬声器置于吸声材料后面,调节其表面阻抗,使其与空气的声阻抗达到最佳匹配,从而实现对吸声性能的有效调控。实验结果表明:半主动吸声方法可以在不断改变入射声波频率情况下对吸声系数进行有效调整,使吸声系数始终处于最大值。

Thin-Walled Tube Extension by Rigid Curved Punch

Computer model is developed for non-steady and steady-state process of thin-walled tube extension by the rigid punch with curved profile. Rigid-plastic membrane shell theory with quadratic yield criterion is used. Tube material normal anisotropy, work hardening, wall thickness variation and friction effects are considered. FORTRAN programs of the model predict distributions of the thickness, meridian stress, yield stress and pressure along curved generator of deformed tube and the tube extension force versus punch displacement relation. Model predictions are correlated with experimental data.

薄壁结构件的正向吸附辅助支撑钻削工艺研究

航空航天领域中薄壁结构件的连接装配要求加工高精度的连接孔,但此类结构件钻削加工时变形大、易振动,严重影响制孔精度和表面质量,传统装夹方法存在装夹跨距大、难以充分有效支撑等问题。本文提出通过正向吸附辅助支撑,提高薄壁结构件加工时的局部刚度,进而提高薄壁结构件的加工质量,并通过仿真和试验相结合的方式研究了正向吸附辅助支撑对薄壁结构件钻削加工的影响。结果表明,在本文仿真和试验条件下,正向吸附辅助支撑可使钻削位置刚度提升40倍以上,孔壁粗糙度值降低67%以上,圆度公差降低60%以上,孔径偏差降低50%以上,同一孔的靠近出口、入口位置的孔径偏差降低90%以上。可见,正向吸附辅助支撑可显著提高薄壁结构件的制孔精度和质量。

地下连续墙新型刚性接头设计与研究

城市建设的快速发展促使对地下空间的需求日益增加,地下连续墙作为深大基坑的支护或结构外墙的应用也越来越多。针对“两墙合一”的地下连续墙,提出了一种新型刚性接头、防绕流接头板及施工工艺,能有效预防混凝土绕流问题,进而提高地下连续墙的整体刚度、强度以及接头的止水能力。通过数值模拟计算了刚性接头的承载力与延性,并与普通工字钢接头进行了对比。结果表明,采用新型刚性接头的地下连续墙,具有较好的刚度和受力性能,施工可行性高,具有较好的应用和推广前景。总结的经验可供今后类似工程借鉴。

三水河特大桥桥墩温度场及温度效应研究

为研究温度效应对连续刚构桥空心薄壁墩受力行为的影响,以(98+5×185+98)m变截面预应力混凝土连续刚构桥——三水河特大桥主桥为背景,通过现场温度场测试,研究桥墩垂直温度场和水平温度场的分布规律,并以实测温度场为依据,选取对桥墩刚度影响较大的桥墩高度、顺桥向宽度和壁厚作为关键参数,分析整体均匀温度T和梯度温度ΔT对空心薄壁墩受力的影响。结果表明:实测桥墩垂直温度场随着墩高的变化分布差异较小,水平温度场受日光照射影响较大,桥墩夏季的梯度温度值约为冬季的2倍。墩高对桥墩温度应力的影响基本呈反比关系,顺桥向宽度对墩底温度应力的影响呈线性增长趋势,壁厚与温度应力呈反比关系。对于长联刚构桥,为减小桥墩的温度效应,建议边主墩高度不宜小于60 m,桥墩顺桥向宽度宜尽量减小,桥墩壁厚宜取80~100 cm。

双层隔震梁结构关键施工技术

以北京市城市规划设计研究院新建业务综合楼为载体,针对狭窄环境下核心筒结构隔震支墩、地下结构劲性双层隔震框架梁、超高隔震沟外墙等关键施工部位,合理地进行隔震支座与主体结构的节点深化,优化隔震支座与框架结构的连接工艺措施,调整隔震沟外墙与支护结构的施工工艺,重视韧性结构关键节点的质量与安全,取得了良好的技术经济效益,值得类似项目借鉴。

带PC填充墙的装配式RC联肢剪力墙受力性能数值分析

为研究带预制混凝土(PC)填充墙的装配式RC联肢剪力墙在水平荷载作用下的受力性能,基于试验建立了无填充墙试件(CSW-N)和带PC填充墙试件(CSW-F和CSW-R)的有限元模型,并考察了轴压比、PC填充墙混凝土强度等级和刚性连接试件锚固钢筋布置等对装配式RC联肢剪力墙受力性能的影响。结果表明:轴压比由0.1增加至0.6时,试件CSW-F峰值承载力增加了约69.92%,延性系数下降了约40.68%;试件CSW-R的峰值承载力增加了约47.00%,延性系数下降了约42.06%。对于试件CSW-F,由于PC填充墙对剪力墙的约束作用较小,随PC填充墙混凝土强度等级增大,其峰值承载力增幅较小;对于试件CSW-R,当实心墙和夹心墙混凝土强度等级由C15增加至C30时,试件的峰值承载力分别提高了7.20%和4.32%,在相同混凝土强度下,实心墙的承载力高于夹心墙的承载力。对于试件CSW-R,锚固钢筋直径变化对剪力墙受力性能影响较小,但增加锚固钢筋数量可提高剪力墙峰值承载力。

不等高双薄壁墩结构参数对连续刚构桥力学行为影响分析

对于因地形和地貌原因而使双薄壁墩连续刚构桥相邻桥墩高度相差较大的情况,设计常采用不等高双薄壁墩。相比常规双薄壁墩,不等高双薄壁墩结构设计及优化可供借鉴和参考的成果不多。以某不等高、不等厚双薄壁墩预应力混凝土连续刚构桥为工程依托,基于有限元数值分析方法,改变低墩壁厚和墩间距,针对结构恒载作用、最大悬臂状态稳定及结构自振,采用单因素分析方法研究薄壁墩结构参数对连续刚构桥结构力学行为影响。结果表明:恒载作用下,低墩壁厚对主梁根部弯矩影响很小,对跨中弯矩影响相对较大;增大双肢间距可明显降低主跨跨中、根部弯矩。低墩稳定安全系数对壁厚更敏感;墩参数变化不改变失稳模态。随着低墩壁厚和间距增加,结构自振频率增大,壁厚、双肢间距影响结构一阶振动模态,且壁厚对一阶自振影响最显著。分析结论可供不等高双薄壁墩连续刚构桥结构设计和优化参考。

近壁面深水爆炸气泡射流演化特性的研究

近壁面气泡载荷是水中战斗部对障碍物毁伤作用的重要载荷之一。以近壁面深水爆炸气泡脉动及射流载荷特征问题为中心,开展了模拟300 m水深环境下的爆炸实验。通过高速摄影仪得到了气泡射流的演化过程;利用Autodyn轴对称模型对气泡射流的演化过程进行了计算,分析了比例距离对气泡脉动周期、射流演化形成时刻及射流强度特征的影响;探讨了壁面近场范围内的压力特征,总结得到射流载荷的演化规律。可为深水环境下水中战斗部对障碍物的毁伤效应研究提供理论参考。

装配式冷弯厚壁型钢半刚性梁-柱节点抗震性能试验研究

冷弯型钢在装配式结构中应用具有天然的优势,为了扩展冷弯厚壁型钢结构的使用范围,克服冷弯厚壁钢的缺点,设计6个新型分层装配式“十字型”梁-柱节点,进行循环加载试验,对梁-柱节点的破坏模式、初始刚度、骨架曲线、等效黏滞阻尼系数、能量耗散系数、割线刚度、滞回曲线和关键部位应变分布进行研究,主要得到以下结论:通过螺栓和节点板的滑移,半刚性节点具有一定的耗能能力。增设高强螺栓可以提高梁柱节点的承载力,初始刚度和耗能能力也得到提高,避免梁端与连接板孔壁承压。增加可更换节点翼缘板,对梁-柱节点的破坏模式没有明显影响,承载能力、耗能能力、初始刚度分别得到较大提高。

钢筋混凝土薄壁空心高墩塑性铰长度计算方法

针对我国现行抗震设计规范尚未明确给出钢筋混凝土(RC)薄壁空心高墩塑性铰长度计算方法的问题,首先收集整理了既有RC桥墩塑性铰长度计算方法以及试验数据,分析影响塑性铰计算长度的主要因素;同时对实际桥梁工程采用的RC薄壁空心墩构造参数进行统计归纳.在验证有限元建模方法准确性后,采用ABAQUS软件建立了高度为40~120 m的8种典型RC薄壁空心高墩的精细化有限元模型,并开展非线性推覆分析,得到不同高度桥墩的等效塑性铰长度.最后讨论不同塑性铰长度计算方法的适用性,发现Eurocode 8规范的计算公式精度最高,可用于计算RC薄壁空心高墩的塑性铰长度.