缩尺模型试验中早期侧向反射声测量方法研究

文章利用半波长干涉的简单原理制作成高频∞字形指向性传声器,同时应用信号处理技术,形成简易的早期侧向反射声测试系统,在缩尺模型试验中实现早期侧向反射声系数的测量,扩大了缩尺模型试验的功能。     

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.     

A more general investigation for the longitudinal stress waves in microrods with initial stress

In the present work, the propagation of longitudinal stress waves along a nanoscale bar with initial stress is investigated by using a unified nonlocal model with two length scale parameters. In principle, the analysis of wave motion is based on Love rod theory including the effects of lateral deformation. However, here are not ignored the contribution of shear stress components due to lateral deformations in the calculation of total elastic strain energy. By applying Hamilton’s principle, the explicit general solution is obtained, and comparative results containing the different effects are presented and discussed.     

粘弹性复合材料的有效三维阻尼矩阵预报与能耗计算

提出了一种计算纤维增强复合材料粘弹性阻尼和能耗的新方法。在层片的层次上,基于各向异性粘弹性理论导出以阻尼矩阵表示的材料能耗计算公式。在层板的层次上由能量等效原理得到有效阻尼矩阵。由此,可由层板的有效应力和有效应变计算出其能量耗散进而得到比阻尼。其中的关键点是有效阻尼矩阵的推导,最终得到的有效阻尼矩阵是各单层阻尼系数、体积分数、刚度系数以及层合板有效刚度系数的函数。用有效阻尼矩阵计算能耗可以适用于任意复合材料的应力应变状态,这对于粘弹性复合材料结构能耗的数值分析尤其重要。最后将该理论应用于几种情况下的复合材料的能耗和比阻尼分析,算例表明与其它的理论预测和试验相比吻合良好,证实了该方法的合理性。     

Modeling 1-3 composite piezoelectrics: hydrostatic response

A simple physical model of 1-3 composite piezoelectrics that was advanced for the material properties relevant to thickness-mode oscillations is extended to address the hydrostatic response. The model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions are derived for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer. The results are similar to those derived by Haun and Newnham (1983, 1986) using a parallel-series connectivity model. The model is illustrated by analyzing composites made from conventional PZT5 and anisotropic modified lead titanate piezoelectric ceramics. For PZT5, the composite structure enhances its hydrostatic charge coefficient, hydrostatic voltage coefficient, hydrophone figure of merit, and hydrostatic coupling coefficient, while three of these quantities fall short of their pure ceramic values in the modified lead titanate composites. The shortfall is due to an enhanced composite that arises from lateral stress on the polymer being transferred to a longitudinal stress along the ceramic rods by the Poisson effect in the polymer, thus producing a charge through the ceramic's d(33).     

A theory for the formation of tetrahedral amorphous carbon including deposition rate effects

A theoretical model is presented to describe the effect of ion beam bombardment rate on the formation of tetrahedral amorphous carbon (ta-C) films. The critical ion energy, E_c, corresponding to a 50% sp³ content in the films, is found to be dependent on both the effective thermal resistance and the ion beam bombardment rate. In the model, the ‘window’ width in the ion energy scale for the formation of ta-C material increases with decreasing deposition rate and with a reduction in the effective thermal resistance, until limited by lower and upper boundary thresholds. Experimental data are reproduced by the model. The plasmon energy, which correlates with sp³ fraction, is found experimentally of be higher for lower deposition rate and smaller effective thermal resistance. Data points for high sp³ content ta-C films deposited on silicon substrates at room temperature occupy a region in the ion energy-deposition rate (E-r) diagram similar to that predicated from the theory.     

Simulated pulsed flow of gas and particles in a horizontal oppose-pulsed gas jets of bubbling fluidized bed

Flow behavior of gas and particles with a horizontal oppose-pulsed gas jets are simulated by means of a three dimensional Computational Fluid Dynamics (CFD) model with the kinetic theory of granular flow in a gas-particles bubbling fluidized bed. The effects of amplitudes and frequencies on the hydrodynamics of gas and particles are analyzed. The simulation results are presented in terms of phase velocity vector plot, volume fraction of phases, granular temperature, power spectrum and Reynolds stresses in the bed. Results show that the impingement caused by the oppose-pulsed gas jets oscillates with the variation of pulsed gas velocity. The impingement zone with the high solid volume fraction reciprocates from the left side to the right side through the bed center with the variation of pulsed jet gas velocities. The lateral velocity and gas turbulent kinetic energy, granular temperature and Reynolds stresses of gas and particles are larger near the pulsed gas jets than that at the center of the bed. The large dispersion coefficients of particles using the horizontal oppose-pulsed gas jets enhance the mixing of particles in gas-solid fluidized bed.     

大跨度球面网壳结构表面风压间歇特性研究

在同济大学TJ-3风洞试验室对大跨度球面网壳结构进行刚性模型测压试验。通过对结构表面风压信号进行连续及离散小波变换,得到各尺度小波系数时程。对结构表面测点信号的尺度能量比率,间歇能量比率及间歇因子三个参数进行比较性分析,得到以下结论：（1）尺度11处,环向测点尺度能量随位置的变化很小,而在尺度12处侧向测点1－7和1－19的尺度能量明显大于其他区域,约为迎风及背风区域测点1－1和1－13的2倍;（2）测点1－7和1－19的间歇能量主要集中在尺度11处,而测点1－1和1－13的间歇能量在各尺度分布较为均匀;（3）测点1－1,1－7,1－13和1－19的间歇因子随尺度的变化趋势相似,在低尺度区变化平缓而在高尺度区域迅速增大。上述说明,结构表面不同区域的风压信号间歇特性不尽相同,可以进一步为结构风压基本特性的精细化分析提供帮助。     

Modelling of microstructure evolution during hot rolling of AA5083 using an internal state variable approach integrated into an FE model

Hot rolling, a critical process in the manufacturing of aluminum sheet products, can significantly impact the final properties of the cold rolled sheet. In this research, a mathematical model was developed to predict the through-thickness thermal and deformation history of a sheet undergoing single stand hot rolling using the commercial finite element (FE) package, ABAQUS^™. A physically based internal state variable microstructure model has been incorporated into the FE simulation for an AA5083 aluminum alloy to predict the evolution of the material stored energy and the subsequent recrystallization after deformation is complete. The microstructure predictions were validated against experimental measurements conducted using the Corus pilot scale rolling facility in IJmuiden, the Netherlands for an AA5083 aluminum alloy. The model was able to predict the fraction recrystallized as well as the recrystallized grain size reasonably well under a range of industrially relevant hot deformation conditions. A sensitivity analysis was carried out to determine the influence of changing the material constants in the microstructure model and deformation conditions on the predicted recrystallization behaviour. The analysis showed that the entry temperature was the most sensitive process parameter causing significant changes in the predicted driving force for recrystallization, nucleation density, fraction recrystallized, and recrystallized grain size.     

Dense inclined flows of inelastic spheres

We outline an extension of the hydrodynamic equations for dense flows of identical, inelastic spheres that incorporates an additional length scale in the expression for the collisional rate of dissipation. This length scale is identified with the length of a particle chain. In steady, fully developed inclined flows, the resulting theory predicts that at a given angle of inclination a range of flow depths is possible, that such flows possess a region of uniform volume fraction, and that this volume fraction decreases as the angle of inclination increases. The balance of particle fluctuation energy, integrated through the depth of a flow, results in a relation between the mean flow velocity, the depth, and the angle of inclination that collapses experimental data taken over a range of inclination angle.