Acoustic topology optimization of sound power using mapped acoustic radiation modes

An efficient approach for acoustic topology optimization to minimize the radiated sound power from a vibrating structure is described. The topology optimization is implemented by modifying the local stiffness at discrete locations on the surface of the structure. The radiated sound power level from the structure is directly chosen as the objective function to be minimized. A sensitivity analysis is then implemented to further optimize the layout of the locations of the modified local stiffness. To speed up the computational process, the radiated sound power is computed based on mapped acoustic radiation modes. To demonstrate the acoustic topology optimization using mapped acoustic radiation modes, the radiated sound power of a compressor housing is examined. Based on results from the numerical model, the local stiffness of a compressor housing was experimentally modified. Good agreement in sound power reduction obtained both numerically and experimentally was observed for the overall trend for the sound power levels as a function of one-third octave frequency bands.     

Shear‐induced structuring kinetics in thermoplastic segmented polyurethanes monitored by rheological tools

Thermoplastic segmented polyurethanes (TPUs) are an important class of thermoplastic elastomers with a two‐phase microstructure arising from the thermodynamic incompatibility between hard (HSs) and soft segments. This microphase separation observed on cooling from a homogeneous state is often combined with the solidification of either or both types of segments. In this study, the structuring mechanism of two TPUs with HSs based on 4,4′‐diphenylmethane diisocyanate and 1,4‐butanediol was investigated from rheological measurements. Hence, in addition to the structuring temperature influence, the effect of an applied preshear flow in the melt polymer was analyzed, in particular. The results clearly show an enhancement of the solidification kinetics by the preshear. Indeed, the measured structuring time can be reduced by more than 1 decade. Rheo‐optical microscopy observations coupled with a shearing hot stage corroborated these results and showed the modification of the microstructure by the shear. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 190–201, 2010     

Wall shear stress and near-wall convective transport: Comparisons with vascular remodelling in a peripheral graft anastomosis

Fluid dynamic properties of blood flow are implicated in cardiovascular diseases. The interaction between the blood flow and the wall occurs through the direct transmission of forces, and through the dominating influence of the flow on convective transport processes. Controlled, in vitro testing in simple geometric configurations has provided much data on the cellular-level responses of the vascular walls to flow, but a complete, mechanistic explanation of the pathogenic process is lacking. In the interim, mapping the association between local haemodynamics and the vascular response is important to improve understanding of the disease process and may be of use for prognosis. Moreover, establishing the haemodynamic environment in the regions of disease provides data on flow conditions to guide investigations of cellular-level responses.     

X-ray diffraction in AT-cut quartz single crystal at acoustic excitation at the fundamental frequency

X-ray diffraction on different atomic planes of an AT-cut quartz crystal is studied experimentally in the Laue geometry in case of excitation by acoustic waves at the first resonant (fundamental) frequency. Acoustic waves lead to an increase in the integral intensity of the reflection-diffracted beam. The amplification coefficients in reflection are measured in dependence on the amplitude of a.c. voltage applied to the crystal at the resonant frequency. The frontal distributions of the intensity of the beam diffracted in the reflection direction are obtained for different atomic planes.     

Comparison of shear banding in BMGs due to thermal-softening and free volume creation

This paper reports a comparative study of shear banding in BMGs resulting from thermal softening and free volume creation. Firstly, the effects of thermal softening and free volume creation on shear instability are discussed. It is known that thermal softening governs thermal shear banding, hence it is essentially energy related. However, compound free volume creation is the key factor to the other instability, though void-induced softening seems to be the counterpart of thermal softening. So, the driving force for shear instability owing to free volume creation is very different from the thermally assisted one. In particular, long wave perturbations are always unstable owing to compound free volume creation. Therefore, the shear instability resulting from coupled compound free volume creation and thermal softening may start more like that due to free volume creation. Also, the compound free volume creation implies a specific and intrinsic characteristic growth time of shear instability. Finally, the mature shear band width is governed by the corresponding diffusions (thermal or void diffusion) within the band. As a rough guide, the dimensionless numbers: Thermal softening related number B, Deborah number (denoting the relation of instability growth rate owing to compound free volume and loading time) and Lewis number (denoting the competition of different diffusions) show us their relative importance of thermal softening and free volume creation in shear banding. All these results are of particular significance in understanding the mechanism of shear banding in bulk metallic glasses (BMGs). Supported by the Chinese Academy of Sciences under the project “Multi-Scale Complex System” (Grant No. KJCX-SW-L08), the National Natural Science Foundation of China (Grant Nos. 10725211 and 10721202), and the Doctorial Start-up Fund of Hunan University of Science and Technology (Grant No. E50840)     

Acoustic Waves in Channels with Porous and Permeable Walls

The propagation of acoustic disturbances in a porous medium crossed by numerous cracks (double porosity medium) is a complex problem that we here simplify by investigating the acoustics of a permeable channel. We consider a fluidfilled channel in two possible geometries, a slit or a cylindric pipe. The channel is surrounded by a porous medium (saturated with the same fluid) and is itself surrounded by an external medium. To simulate the average properties of the cracked rock, the external medium is either nonpermeable (few connections between cracks) or highly permeable (numerous connections). We present analytical and numerical results concerning acoustic disturbances of small amplitude generated in the channel, such as harmonic waves, step disturbanses and pulses.     

Dynamic adhesive strength of fiber-polymer systems

A new device for studying the dynamic adhesive strength is created. A procedure for determining the dynamic adhesive strength in fiber—polymer systems under impact loading (pull-out technique) is developed. The adhesive strength of the interface of polymer—steel wire joints formed by polymers of different chemical nature (epoxy resin, polysulfone, and polypropylene) is examined. It is shown that the dynamic adhesive strength grows as the loading rate increases for all the systems under investigation and that the relationship between the adhesive strength and the loading rate, , over a wide range of rates can be described by two straight lines corresponding to the quasi-static and impact loading, respectively. When passing from the quasi-static to dynamic loading, the character of scale relations of the adhesive strength does not change.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 6, pp. 689–700, November–December, 1999.     

边界吸收效应对非定常剪切弥散的影响

采用延迟扩散方程［７，９］描述了具有边界吸收条件下，非定常剪切流动中的剪切弥散特性．给出了记忆函数、中心位移函数的控制方程．特例分析结果表明：所采用的模型方程是合理的；边界吸收效应使得纵向浓度分布具有后倾的趋势．这主要是由于边界吸收使得低速强剪切区浓度减少，剪切弥散贡献减少，从而污染物对流速度高于断面的平均速度．     

Zum Fließverhalten kurzglasfasergefüllter Thermoplastschmelzen im stationären und instationären Bereich

Zusammenfassung Durch Zugabe von Glasfasern wird das rheologische Verhalten von Kunststoffschmelzen verändert. Kurzglasfasergefüllte Styrol-Acrylnitril-Copolymere mit verschiedenen Füllgraden wurden mit dem Rotations-und dem Kapillarrheometer untersucht. Sowohl im stationären Bereich als auch im instationären Bereich (Anlauf- und Abklingverhalten) sind Unterschiede zum ungefüllten Material festzustellen: Mit Zunahme des Faseranteils steigt der Schubmodul, und das viskoelastische Überschwingen wird geringer als bei ungefüllten Systemen. Erhöhung von Temperatur, Schergeschwindigkeit und Faseranteil führen zu einer Verkürzung der Relaxationszeiten, Steigerung des hydrostatischen Druckes erwartungsgemäß zu deren Verlängerung.

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Summary The rheological behaviour of polymer melts is changed by addition of glass fibres. Experiments were conducted with styrene-acrylonitrile copolymers filled with various amounts of short fibres on a rotational and a capillary rheometer, both under steadystate and transient conditions. It is observed that by adding fibres the shear modulus increases but the stress overshoot decreases. With increase of temperature, shear rate and fibre content relaxation times become shorter whereas with higher hydrostatic pressures they grow longer, as is to be expected.

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