激光聚焦击穿液体的爆炸气泡特性

高功率激光聚焦击穿液体介质,类似于聚焦区域发生了微爆炸.研究了激光聚焦击穿爆炸与水下微爆炸之间的相似性.采用高功率脉冲激光器、光学聚焦系统、高速摄像、测量水听器建立了激光击穿爆炸源的产生及爆炸气泡、爆炸声波特性测量系统.对不同参数激光聚焦击穿水、酒精、硅油等粘性液体,观测到了激光击穿爆炸冲击波、气泡脉动、气泡压缩冲击波...     

Numerical modeling of thermophysical processes in laser-beam welding with formation of a vapor channel

A quasi-two-dimensional mathematical model of laser welding of plates made of a binary alloy with allowance for metal vaporization and formation of a vapor channel is developed. A difference algorithm and a computer code are designed on the basis of this model. Processes of melting, vaporization, and solidification of the alloy of butt-welded aluminum plates are considered numerically as an example. The shape and depth of the vapor channel and welding bath are calculated for given conditions of the radiation power and welding velocity. Based on semi-empirical relations and numerical data obtained, the characteristic size of the dendritic structure of the solidified melt is estimated. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 88–96, September–October, 2006.     

Non-radial flow of an incompressible fluid of second grade in a contracting channel

The steady non-radial flow of an incompressible fluid of second grade in a contracting channel is studied. The dependence of the flow on the material parameters of the fluid and on the channel angle is investigated. A similarity transformation is introduced for the streamfunction which reduces the P.D.E. to a sequence of O.D.E.s. A series solution is employed to solve the problem.

---

Sommario Si studia il flusso stazionario non radiale di un fluido di secondo grado incomprimibile in un canale convergente, esaminandone la dipendenza dai parametri materiali del fluido e dall'angolo di apertura del canale. Si introduce una trasformazione di similitudine per la funzione di corrente che riduce l'equazione di moto ad una serie di equazioni differenziali ordinarie, risolte numericamente.

     

Dynamical evolution of fracture process region in ductile materials

The aim of this paper is to investigate the dynamics of evolving finite fracture process regions in ductile material taking into account the dislocation motion.     

定向断裂控制爆破的空孔效应实验分析

采用新型数字激光动态焦散线实验系统, 对爆炸荷载作用下空孔周围的动应力场分布及空孔对爆生主裂纹扩展行为的影响进行了研究。研究结果表明, 在空孔周围强应力场的影响下, 2条相向扩展的爆生主裂纹逐渐向空孔处偏转, 并在空孔处贯通; 空孔附近的主应力方向与炮孔连心线夹角基本稳定在约12°, 增大空孔尺寸对空孔附近的主应力方向影响不明显; 爆炸应力波与空孔相互作用, 产生反射拉伸波, 改变了主裂纹尖端的应力场, 降低了主裂纹的扩展速度, 且空孔直径越大, 主裂纹的扩展速度越低; 当爆生主裂纹扩展到空孔附近时, 主裂纹尖端动态应力强度因子再次出现上升的趋势。     

??????????????????????????е????

给出了混流式转轮叶片与上冠(或下环)采用部分焊透裂纹尖端应力强 度因子的分析模型. 根据给出的模型，结合某电站转轮强度分析结果，通过实际算例，明 确了预留焊缝长度的确定方法. 该方法适用于所有混流式转轮叶片与上冠(或下环)之间 采用部分焊透预留焊缝长度的确定.     

用激光三角法重构混凝土断裂面三维轮廓与断裂能分析

采用基于激光三角法的测试方法对混凝土断面表面轮廓进行了三维重构,初步的断面分析结果表明,相对于传统断面分析方法,本试验方法实现了断面的无损、快速和准确的测量分析,在测量到的断面基础上,给出了本试验所得到的混凝土材料断裂能与分维的关系。     

三维压电弹性体断裂分析的基本解

求解了在材料的特征方程有重根时，三维压电弹性体的单位集中不连续位移和不连续电势基本解。讨论了重根对断裂力学问题解的影响。     

Using geometric complexity to enhance the interfacial strength of heterogeneous structures fabricated in a multi-stage,multi-piece molding process

Interfaces in heterogeneous structures are typically engineered for optimal strength through the control of surface roughness and the choice of adhesives. Advances in manufacturing technologies are now making it possible to also tailor the geometries of interfaces from the nanoscale to the macroscale to create geometrically complex interfaces that exhibit enhanced performance characteristics. However, the impact of geometric complexity on the mechanical behavior of interfaces has not yet been ascertained. In this investigation, the first step is taken towards understanding the effects of geometric complexity on interfacial strength. A new multi-stage, multi-piece molding process is used to create heterogeneous polymer structures with geometrically complex interfaces consisting of rectangular and circular interlocking features. The structural integrity of these heterogeneous structures is characterized through interfacial tension testing. The full-field deformation measurement technique known as digital image correlation is also used during the testing to visualize the deformation fields around the geometrically complex features. Through this characterization, it is determined that the complex geometries increase the interfacial strength by approximately 20–25%, while reducing the statistical variation by 50%. These effects are attributed to a transition in the failure mechanism from interfacial fracture to homogeneous ligament failure. Results also indicate that geometrically complexity can be used on completely debonded interfaces to increase the strength to at least 25–35% of the bonded interface. Based on these results, some simple design rules have been proposed that enable geometrically complex interfaces to be engineered with enhanced strengths approaching the weaker of the two base materials. These design rules can also be used in the engineering of interfaces to facilitate the development of heterogeneous structures using new design paradigms, such as design for recyclability and the design of products based on bio-inspired concepts.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.     

激光光通量位移计在Hopkinson杆中的应用

简要介绍了激光位移测试技术的基本原理及实现方案,准静态和动态标定结果表明该系统可以用 于动态测试。给出了激光光通量位移计在Hopkinson杆实验中的3种应用:(1)用激光光通量位移计可以监 测试样的横向变形,从而得出试样的动态泊松比;(2)在动态断裂实验中,激光光通量位移计可以监测裂尖张 开位移,结合应变片的信息可以得出试样的动态断裂能;(3)将激光光通量位移计和石英晶体应力计相结合可 以直接在试样两端得到应力应变信息,用来测量超软材料在中应变率下的力学响应。     

Natural convection induced in a fluid by the hot inner walls of a “submerged” channel

The jet flows induced around a submerged channel due to the hot inner channel walls and the flow inside the channel are calculated. The formation of high-and low-density regions at the inlet and outlet of the channel is detected. The dependence of the flow rate on the channel orientation relative to the gravity force is analyzed. The onset of coherent flow structures results in the development of unsteady oscillating flows. Natural convection in the fluid is studied using the JoinCAD/FEM program package. The regularized Oberbeck-Boussinesq equations are solved using a finite-element method with the same order of the approximating functions.     

DNS of heat transfer in turbulent and transitional channel flow obstructed by rectangular prisms

Direct numerical simulation (DNS) of heat transfer in a channel flow obstructed by rectangular prisms has been performed for Re_τ = 80–20, where Re_τ is based on the friction velocity, the channel half width and the kinematic viscosity. The molecular Prandtl number is set to be 0.71. The flow remains unsteady down to Re_τ = 40 owing to the disturbance induced by the prism. For Re_τ = 30 and 20, the flow results in a steady laminar flow. In the vicinity of the prism, the three-dimensional complex vortices are generated and heat transfer is enhanced. The Reynolds number effect on the time-averaged vortex structure and the local Nusselt number are investigated. The mechanism of the heat transfer enhancement is discussed. In addition, the mean flow parameters such as the friction factor and the Nusselt number are examined in comparison with existing DNS and experimental data.     

利用格子Boltzmann方法模拟单个气泡在复杂流道内的运动特性

利用格子Boltzmann方法模拟了单个气泡在具有三个半圆形喉部的复杂流道内的上升过程.通过分析气泡运动过程中的形态、运动轨迹及运动速度的变化,研究复杂流道对气泡运动特性的影响.研究结果表明,在上升过程中,由于壁面的影响,气泡的形状发生严重的变形,运动轨迹也发生相应的偏转.通过实验结果的对比,验证了模拟结果的正确性.结果表明格子Boltzmann方法可以用于模拟具有复杂边界的两相流问题.     

Numerical investigation of a water flow in a rib-roughened channel by using Reynolds stress model

In this paper, water flow in a rib-roughened channel is investigated numerically by using Reynolds stress turbulence models (RSM) on a three-dimensional (3-D) domain. Computational results for mean streamwise velocity component and turbulent kinetic energy show good agreements with available experimental data. Five rib pitch-to-height ratios (p/h) of 1, 5, 10, 15 and 20 are analysed for six different Reynolds numbers (Re) of 3000, 7000, 12,000, 20,000, 40,000 and 65,000. Velocity vectors, streamlines and Reynolds stresses are showed for these ratios and Re numbers. Streamlines revealed that Reynolds numbers do not affect flowfield but play an important role in the Reynolds stresses.     

Analysis of the near‐wall behaviour of some self‐adaptive subgrid‐scale models in finite‐differenced simulations of channel flow

Self‐adaptive subgrid‐scale models are proposed and assessed. They are based on the use of the Germano–Lilly dynamic procedure and the use of a selection function. These models, which do not incorporate any information related to the location of the solid walls, are well suited for the simulation of turbulent flows in complex geometries. Their reliability, when used together with a second‐order non‐dissipative numerical method, is assessed on the plane channel configuration for two values of the Reynolds number (Reτ = 180 and 395) for two grid resolutions. The selection function approach for deriving self‐adaptive subgrid models is found to yield results very similar to those obtained using a dynamic model, without requiring any numerical stabilization procedure. The use of the selection function is shown to be the only one which is able to capture the backscatter process in the buffer layer, while producing a strictly positive subgrid viscosity. This is demonstrated to be linked to the capability of the selection function to permit a decorrelation between the mean strain and the fluctuations of the subgrid stresses. That point is illustrated thanks to the introduction of a new decomposition of the fluctuating strain subgrid dissipation. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical analysis of turbulent structure in compound meandering open channel by algebraic Reynolds stress model

The turbulent flow in a compound meandering channel with a rectangular cross section is one of the most complicated turbulent flows, because the flow behaviour is influenced by several kinds of forces, including centrifugal forces, pressure‐driven forces and shear stresses generated by momentum transfer between the main channel and the flood plain. Numerical analysis has been performed for the fully developed turbulent flow in a compound meandering open‐channel flow using an algebraic Reynolds stress model. The boundary‐fitted coordinate system is introduced as a method for coordinate transformation in order to set the boundary conditions along the complicated shape of the meandering open channel. The turbulence model consists of transport equations for turbulent energy and dissipation, in conjunction with an algebraic stress model based on the Reynolds stress transport equations. With reference to the pressure–strain term, we have made use of a modified pressure–strain term. The boundary condition of the fluctuating vertical velocity is set to zero not only for the free surface, but also for computational grid points next to the free surface, because experimental results have shown that the fluctuating vertical velocity approaches zero near the free surface. In order to examine the validity of the present numerical method and the turbulent model, the calculated results are compared with experimental data measured by laser Doppler anemometer. In addition, the compound meandering open channel is clarified somewhat based on the calculated results. As a result of the analysis, the present algebraic Reynolds stress model is shown to be able to reasonably predict the turbulent flow in a compound meandering open channel. Copyright © 2005 John Wiley & Sons, Ltd.     

激光辐照下多层圆柱体中三维瞬态温度场的解析解

考虑外表面的气流影响和层间温度与传热的协调关系,建立了激光辐照下,层合圆柱体中的三维瞬态热传导解析模型。利用特征值法和Bessel函数,导出了各层柱体中三维瞬态温度场的封闭解析解。以一维轴对称问题为例计算了柱体中的瞬态温度场,给出了柱体内部温度随时间的变化和柱体表面换热系数对温度场的影响规律。本文的理论解可进一步用于分析层合圆柱体中的三维瞬态热-力效应,并可作为相应问题的数值模拟中数值模型的修正依据。     

Instantaneous structure and statistical feature of unsteady flow in a channel obstructed by a square rod

A two-dimensional numerical computation has been made for an unsteady flow in a channel obstructed by an inserted square rod. The results of the computation made for the flow with a parabolic inlet velocity profile at a specific value of channel Reynolds number are analyzed in detail. The obtained results reveal that momentum transfer is enhanced due to the apparent shear stress resulting from the nonzero value of cross-correlation between the streamwise and normal components of fluctuating velocity, , just as in turbulent shear flows, although the studied flow is quite different from turbulent flows in the sense that it is highly periodical and therefore free from randomness. This periodicity leads to a quick recovery of the velocity defect in some region of the wake of the rod. Special attention is paid to the time variation of flow structure. The crisscross motion of the Karman vortex previously found to occur is discussed again, and how it appears is explained in terms of the interaction between the Karman vortex and the disturbed wall shear layer. In the discussion of this relationship, wavering motion of the separation vorticity layers formed on both sides of the rod and the periodic formation of an isolated vortex island from the lifted tip of the wall vorticity layer are analyzed. The vortex island is found to play an important role not only for the occurrence of the crisscross motion of Karman vortex but also for the generation of the nonzero value of .