基于面元法回转体定长局部空泡的绕流计算

基于面元法 ,通过在回转体和空泡壁面放置源汇 ,对回转体定长局部空泡的绕流问题进行计算和分析 ,并讨论了空泡尾部速度过渡闭合模型对绕流计算的影响。计算结果表明 :本文的方法具有快速收敛的特征 ,第 1次叠代和最终收敛时空泡壁面切向速度的误差不超过 5 % ;随着回转体面元总数N的增加 ,局部空泡的空泡数趋于稳定 ;当回转体线型一定时 ,空泡数将随着局部空泡长度的增大而减小     

Time reversal super resolution in randomly layered media

In this paper we extend previous work on time reversal in randomly layered media [J.-P. Fouque, J. Garnier, A. Nachbin, K. Sølna, Time reversal refocusing for point source in randomly layered media, Wave Motion 42 (2005) 238–260]. We consider first the case of an active source embedded below the surface in a finely layered random medium. We carry out time reversal with a time reversal mirror placed at the surface and we consider here the case where this mirror is larger than the carrier wavelength. In contrast with the situation addressed in our previous paper, where the size of the mirror was comparable to the wavelength, we show that multi-pathing dramatically enhances the effective aperture of the mirror so that super resolution at the location of the source can be obtained. In other words, the focal spot radius of the refocused field obtained in the case of a multiply scattering medium is much smaller than the spot size obtained in the case of a homogeneous medium. This super resolution effect is obtained by time-reversing the long incoherent waves generated by the multiple scattering due to the thin layers. We also give an application to the problem of focusing on a passive scatterer buried in the random medium and illuminated by a source at the surface.     

Chaos in Acoustic Subspace Raised by the Sommerfeld–Kononenko Effect

This paper deals with vibrations of an infinite plate in contact with an acoustic medium where the plate is subjected to a point excitation by an electric motor of limited power-supply. The whole system is divided into two “exciter - foundation” and “foundation-plate-medium”. In the system “motor-foundation” three classes of steady state regimes are determined: stationary, periodic and chaotic. The vibrations of the plate and the pressure in the acoustic fluid are described for each of these regimes of excitation. For the first class they are periodic functions of time, for the second they are modulated periodic functions, in general with an infinite number of carrying frequencies, the difference between which is constant. For the last class they correspond to chaotic functions. In another mathematical model where the exciter stands directly on an infinite plate (without foundation) it was shown that chaos might occur in the system due to the feedback influence of waves in the infinite hydro-elastic subsystem in the regime of motor shaft rotation. In this case the process of rotation can be approximately described as a solution of the fourth order nonlinear differential equation and may have the same three classes of steady state regimes as the first model. That is the electric motor may generate periodic acoustic waves, modulated waves with an infinite number of frequencies or chaotic acoustic waves in a fluid.     

A Brief Summary of L. van Wijngaarden's Work Up Till His Retirement

This paper attempts to provide an overview of Professor Leen van Wijngaarden's scientific work by briefly summarizing a number of his papers. The review is organized by topic and covers his work on pressure waves in bubbly liquids, bubble dynamics, two-phase flow, standing waves in resonant systems, and flow cavitation noise. A list of publications up till his retirement in March 1997 is provided in the Appendix.     

Research progress on hydrodynamics of high speed vehicles in the underwater launching process

高速航行体水下发射水动力学研究, 是具有重大工程应用背景的前沿基础问题.与之紧密相关的非定常空化流动, 特别是空泡稳定性、溃灭等问题, 是影响发射载荷及安全性的关键.本文首先简述了这一领域的主要科学问题, 归纳了主要控制参数和影响方式; 之后针对非定常空化流动问题, 综述了已有的实验观测手段及数值模拟方法; 总结了空泡发展、稳定性、溃灭及流动控制等重要物理机制、模型及各因素相互作用规律; 最后展望了该领域仍存在的主要科学问题与未来发展趋势.     

Hardening evolution of AZ31B Mg sheet

The monotonic and cyclic mechanical behavior of O-temper AZ31B Mg sheet was measured in large-strain tension/compression and simple shear. Metallography, acoustic emission (AE), and texture measurements revealed twinning during in-plane compression and untwinning upon subsequent tension, producing asymmetric yield and hardening evolution. A working model of deformation mechanisms consistent with the results and with the literature was constructed on the basis of predominantly basal slip for initial tension, twinning for initial compression, and untwinning for tension following compression. The activation stress for twinning is larger than that for untwinning, presumably because of the need for nucleation. Increased accumulated hardening increases the twin nucleation stress, but has little effect on the untwinning stress. Multiple-cycle deformation tends to saturate, with larger strain cycles saturating more slowly. A novel analysis based on saturated cycling was used to estimate the relative magnitude of hardening effects related to twinning. For a 4% strain range, the obstacle strength of twins to slip is 3 MPa, approximately 1/3 the magnitude of textural hardening caused by twin formation (10 MPa). The difference in activation stress of twinning versus untwinning (11 MPa) is of the same magnitude as textural hardening.     

不连续岩体变形破坏过程中AE的空间演化与意义

通过一些典型节理化岩体模型变形破坏过程中的AE事件的定位分析和破裂面形态的研究,探讨了不连续岩体变形破坏过程的空间演化特征。结果表明：单节理岩体变形破坏的空间动态特征受节理角控制,随着节理角从小变大,岩体的变形破坏方式从AE在局部集中的突发式失稳向AE弥漫于整个节理层面的类似渐进式的破坏形式发展；失稳破裂面从简单的整体状岩-膏界面变成形态复杂的层内复合破裂面；复杂节理岩体的变形破坏受一条（组）主导构造控制,强AE事件集中发生在不连续构造交汇部位,在包含多条主构造的岩体模型中最终失稳破裂面只与最后阶段AE的位置一致。     

体外冲击波碎石技术的力学机理的研究

体外冲击波粉碎结石术(extracorporeal shock wave lithotripsy, ESWL)是冲击波和聚焦原理在医疗领域的最成功的应用, 因其无手术介入, 使用方便, 碎石率高, 治疗费用低, 已成为尿路结石的首选治疗方法.但是这种技术也有尚待完善之处, 例如: 对于大结石ESWL的治疗效率较低; 对人体组织有短期或长期的副作用,甚至可能造成 器官功能的不可逆性丧失等.ESWL通过一系列作用力粉碎结石和损伤人体组织的过程颇为复 杂.从ESWL技术诞生之时起, 人们就同步开展了关于ESWL的实验室的和临床的基础研究, 以期达到最好的碎石效果和最小的副作用.至今ESWL的破坏机制逐渐被人们所认识,但仍然 存在争论.冲击波直接的破坏机制已经比较清楚, 而它诱发的空化破坏机制是当前ESWL研究 的一个具有挑战性的热点.评定碎石机冲击波以及碎石效率的标准还在不断的完善之中.     

Jet-Cavitation Flow Past “Fluid Cylinders”

A new class of plane steady-state flows of an inviscid incompressible weightless fluid in the presence of point singularities inside the flow and constant-pressure regions is studied. Solutions of the problems of jet and cavitation flow past the atmospheres of these singularities are constructed. At positive cavitation numbers, the singular-point method of Chaplygin and the Efros scheme are used for cavity closure. The case of negative cavitation numbers is also considered. A parametric and numerical analysis of the solutions obtained is carried out. The studied flows can be treated as either jet or circulation flow past curvilinear contours of special shape. They can also be used for constructing new schemes for the closure of developed cavitation zones.     

New expressions for the radiation force function of spherical targets in stationary and quasi-stationary waves

A new expression for the radiation force function – which is the radiation force per unit energy density and unit cross-sectional surface area – for spheres in a stationary (or standing) and quasi-stationary wave is obtained based on the far-field acoustic scattering field. The radiation force function formulation has been simplified mathematically and improved into a more general form. Numerical results are presented for rigid and elastic spheres, air bubbles in water as well as liquid drops in air to illustrate the theory. It is demonstrated that expressions for the radiation force functions obtained from the far-field derivation approach are equivalent to those obtained from the near-field-based derivation.