Downstream subsonic jet noise: link with vortical structures intruding into the jet coreRayonnement acoustique aval des jets subsoniques : lien avec l'intrusion de structures tourbillonnaires dans le cône potentiel

A subsonic circular jet with a Mach number of 0.9 and a Reynolds number of 65 000 is computed by a compressible Large Eddy Simulation (LES) to determine both the flow field and the sound field in the same calculation. The noise radiated by the jet, provided by LES, is in good agreement with experimental data of the literature in terms of sound pressure spectra, levels and directivity, showing the feasibility of this direct noise calculation. The dominant sound generation mechanism is also investigated, by presenting a correlation between its radiation, observed for an angle of 30° from the downstream direction, and the intrusion of vortical structures into the jet core. To cite this article: C. Bogey, C. Bailly, C. R. Mecanique 330 (2002) 527–533.     

基于叙词表的海洋领域本体构建研究

为有效地进行海洋管理,将本体思想引入该领域,研究构建了海洋领域本体.从海洋领域的实际情况入手,并在专家构建的《海洋主题词表》的基础上,提出一种基于叙词表的本体构建方法.在此方法的指导下,构建了海洋领域本体.该本体能效解决我国各部门协调和信息共享问题,促进我国海洋信息管理的发展.     

OSMAR2000海洋表面径向流场的生成及后处理

为了径向流图的双站合成,在OSMAR2000超分辨率海洋表面流算法提取径后向流后,再经过均匀网格径向流图的生成、多幅径向流图的组合、海流实际探测张角和有效探测距离的截取,以及径向流空缺区域的局域插值,最终生成了所需的单站径向流图,经双站合成,形成海洋表面矢量流图,现场对比验证实验证明,本文的后处理方法是有效的。     

用于振动、冲击和噪声控制的SMA智能复合材料结构

本文简要叙述SMA智能复合材料结构的基本概念,介绍其分析和求解的基本力学原理和方法,重点展示其在三个主要应用研究领域,包括振动控制、噪声控制及抗低速冲击的最新进展,并对存在问题和未来的发展给予评述．     

Closed crack imaging using time reversal method based on fundamental and second harmonic scattering

A recent variant of time reversal imaging is employed for reconstructing images of a closed crack, based on both the fundamental and the second harmonic components of the longitudinal scattered field due to an incident longitudinal wave. The scattered field data are generated by a finite element model that includes unilateral contact with Coulomb friction between the crack faces to account for the Contact Acoustic Nonlinearity. The closure state of the crack is controlled by specifying a pre-stress between the crack faces. The knowledge of the scattered field at the fundamental (incident) frequency and the second harmonic frequency for multiple incident angles provides the required inputs for the imaging algorithm. It is shown that the image reconstructed from the fundamental harmonic closely matches the image that is obtained from scattering data in the absence of contact, although contact between the crack faces reduces the amplitude of the scattered field in the former case. The fundamental harmonic image is shown to provide very accurate estimates of crack length for low to moderate levels of pre-stress. The second harmonic image is also shown to provide acceptable estimates of crack length and the image is shown to correlate with the source location of second harmonic along the crack, which becomes increasingly localized near the crack tips for decreasing levels of pre-stress. The influence of the number of sensors on the image quality is also discussed in order to identify the minimum sensors number requirement. Finally, multiple frequency imaging is performed over a fixed bandwidth to assess the potential improvement of the imaging algorithm when considering broadband information.     

Offshore floating wind turbine and its dynamic problems

Green energy sources and ocean wind power are plentiful in deep sea. More and more offshore wind power plants are constructed in the deep water over hundred meters below the surface. While offshore floating wind turbine system is working, wind turbine, floating foundation, and mooring system affect each other with wind, waves, and currents acting on them. Various offshore floating wind turbine systems and the encountered environmental loads are briefly reviewed and discussed. It is difficult and crucial to comprehensively analyze the aerodynamic-hydrodynamic-service system-structure under the coupling effect of offshore floating wind turbine system. The environmental flow field, structure scale, and rational applications of theories and approaches should be well considered in advance.     

Bulk and surface acoustic wave phenomena in crystals: Observation and interpretation

Crystal acoustics is a field that has engaged the attention of theoreticians and experimentalists alike for decades and more. Many striking effects have been revealed, and elegant analytical techniques applied to their interpretation. This article is oriented towards the experimental aspects of the field and the interpretation of the phenomena that have been observed. Particular attention is given to reviewing the techniques that have probed the intricacy of acoustic wave propagation in crystals, including phonon imaging, laser- and capillary-fracture-generated ultrasound, transmission acoustic microscopy and surface Brillouin scattering, and a selection of results obtained with these techniques is presented. Some of these studies pertain to bulk waves and others to surface acoustic waves. The interpretation of far-field observations is carried out within the ray approximation, and elastodynamic Green’s functions are invoked in the interpretation of near-field results. Extensive use is made of the acoustic-slowness and wave surfaces, in particular features such as acoustic axes, with their attendant polarization singularities, and folds in the wave surface.     

Stability and consistency of nonhydrostatic free‐surface models using the semi‐implicit θ‐method

The θ‐method is a popular semi‐implicit finite‐difference method for simulating free‐surface flows. Problem stiffness, arising because of the presence of both fast and slow timescale processes, is easily handled by the θ‐method. In most ocean, coastal, and estuary modeling applications, stiffness is caused by fast surface gravity wave timescales imposed on slower timescales of baroclinic variability. The method is well known to be unconditionally stable for shallow water (hydrostatic) models when , where θ is the implicitness parameter. In this paper, we demonstrate that the method is also unconditionally stable for nonhydrostatic models, when for both pressure projection and pressure correction methods. However, the methods result in artificial damping of the barotropic mode. In addition to investigating stability, we also estimate the form of artificial damping induced by both the free surface and nonhydrostatic pressure solution methods. Finally, this analysis may be used to estimate the damping or growth associated with a particular wavenumber and the overall order of accuracy of the discretization. Copyright © 2012 John Wiley & Sons, Ltd.     

Uniform Decay properties of a model in structural acoustics

We investigate decay properties for a system of coupled partial differential equations which model the interaction between acoustic waves in a cavity and the walls of the cavity. In this system a wave equation is coupled to a structurally damped plate or beam equation. The underlying semigroup for this system is not uniformly stable, but when the system is appropriately restricted we obtain some uniform stability. We present two results of this type. For the first result, we assume that the initial wave data is zero, and the initial plate or beam data is in the natural energy space; then the corresponding solution to system decays uniformly to zero. For the second result, we assume that the initial condition is in the natural energy space and the control function is L²(0,∞) (in time) into the control space; then the beam displacement and velocity are both L²(0,∞) into a space with two spatial derivatives.     

Ultrasound and light scattering from a suspension of reversible fractal clusters in shear flow

Shear break-up of reversible fractal clusters is investigated by ultrasound and multiple light scattering in the low shear regime. We consider a dense suspension of Rayleigh scatterers (particles or clusters) with acoustic properties close to those of the surrounding liquid so that the attenuation of the ultrasonic coherent field is weak and multiple scattering is negligible. The concept of variance in local particle volume fraction is used to derive an original expression of the ultrasound scattering cross-section per unit volume for Rayleigh fractal clusters. On the basis of a scaling law for the shear break-up of aggregates, then we derive the shear stress dependence of the ultrasound scattered intensity from a suspension of reversible fractal clusters. In a second part, we present rheo-acoustical experiments to study the shear break-up of hardened red cell aggregates in plane-plane flow geometry and we examine both the self consistent field approximation and the scaling laws used in microrheological models. We further compare the ability of acoustical backscattering and optical reflectometry techniques to estimate the critical disaggregation shear stress and the particle surface adhesive energy. Finally, the microrheological model from Snabre and Mills [#!ref5!#] based on a fractal approach is shown to describe the non Newtonian behavior of a dense distribution of hardened red cell aggregates. Received 12 November 1998 and Received in final form 17 May 1999