介电流变弹性体的电致变形与电流变效应研究

介电弹性体(Dielectric Elastomer,DE)具有显著的电致变形效应,刚度无法调控是制约其应用的关键瓶颈问题.本文将具有电致变模量特性的电流变弹性体(Electrorheological Elas-tomer,ERE)与商用介电弹性体VHB4910层叠复合,并涂覆柔性电板,制备介电流变弹性体(Dielec...     

骨的力–电效应研究方法与进展

骨组织在受到应力作用(正常的生理活动)变形后在骨内产生电位的现象称为骨的力–电效应,它主要包括压电效应和动电效应.研究骨在动态过程中产生的电位幅值和分布特点,不仅是了解电刺激骨生长机理的必要步骤,也是实现骨治疗和重建的生理基础.它一方面是用数学方法来描述外力作用下其电位大小与应力、应变、应变率及加载速率的关系,另一方面是考察生理环境(pH值、离子浓度、温度、湿度等)对电位的影响.首先对力–电理论进行了简单的介绍,重点总结了其研究方法,包括理论模型和分离式霍普金生杆冲击、弯曲变形及缓冲液中的动态测试等实验方法.此外,对骨替代材料和牙本质领域的力–电效应研究也进行了一定的综述.     

机翼尺度效应对等离子体分离流动控制特性的影响

等离子体流动控制技术是一种以等离子体气动激励为控制手段的主动流动控制技术. 为了进一步提高等离子体激励器可控机翼尺度, 以超临界机翼SC(2)-0714大迎角分离流为研究对象, 以对称布局介质阻挡放电等离子体为控制方式, 以测力、粒子图像测速仪为研究手段, 从等离子体激励器特性研究出发, 深入开展了机翼尺度效应对等离子体控制的影响研究, 提出了适用于分离流控制的能效比系数, 探索了分离流等离子体控制机理, 掌握了机翼尺度对分离流控制的影响规律. 结果表明: (1)随着机翼尺度的增大, 布置到机翼上的激励器电极长度会相应增加; 在本文的参数研究范围内, 激励器的平均消耗功率不会随电极长度的增加而线性增大; 当电极长度达到一定阈值时, 激励器的平均消耗功率趋于定值; (2)在固定雷诺数的情况下, 随着机翼尺度的增大, 等离子体的控制效果并未降低, 激励器能效比系数提高; (3)等离子体在主流区诱导的大尺度展向涡与在壁面附近产生的一系列拟序结构成为分离流控制的关键. 研究结果为实现真实飞机的等离子体分离流控制, 推动等离子体流动控制技术工程化应用提供了技术支撑.      

考虑挠曲电效应的压电纳米薄板力-电-热耦合特性研究

挠曲电效应是由应变梯度引起的,与尺度相关的力电耦合效应.基于Kirchhoff板假设和挠曲电理论,论文推导了温度和电压作用下的压电薄板力-电-热耦合微分控制方程,定量分析了微分控制方程中非线性项的影响,并针对四周固支压电薄板采用Ritz法求解,数值计算了压电薄板的弯曲和振动行为.在研究温度和挠曲电效应对薄板耦合特性和力学行为的影响时,论文分别考虑了材料系数不随温度变化和随温度线性变化两种情况.以PZT-5H为例,作者讨论了挠曲电和温度对压电薄板的横向位移和固有频率的影响.研究结果表明挠曲电效应对压电纳米薄板的力学行为影响很大,且具有明显的尺寸效应.此外,薄板对温度变化非常敏感.因此,可通过挠曲电效应和温度来调控压电纳米薄板的多场耦合特性和力学行为,进而优化基于压电薄板的NEMS/MEMS中传感器、作动器等电子器件的性能.     

电流变液和电流变效应

电流变液是一种极有发展前景的新颖材料,通常由不导电的母液和均匀散布在其中的电介质微粒组成。对于外加电场的变化,它的力学性能可以作出迅速的响应,因而在工业上极具应用前景。本文将就电流变液的研究进行综述,涉及的内容有:电流变效应的机理、电流变液材料及其力学性能、应用和展望．      

固体电介质中的挠曲电效应

挠曲电效应通常描述为非均匀变形如应变梯度引起的电极化或者电场梯度引起的变形.应变梯度能够局部破坏晶体的反演对称从而在材料中诱导电极化,因此挠曲电效应是固体电介质材料中普遍存在的一种力电耦合效应.应变梯度和电场梯度均随材料尺寸的减小而迅速增大,在宏观尺度通常被忽略的挠曲电效应在微纳尺度反而起着非常重要的作用,会显著影响材料的物理性能.与压电效应和电致伸缩效应相比,挠曲电效应具有独特的尺寸依赖特征,其不受材料对称性和铁电材料居里相变温度的限制.论文综述了固体电介质中的挠曲电效应,并重点从理论、材料和应用方面综述了固体电介质中挠曲电效应的研究进展,对挠曲电效应的独特性能进行了详细地讨论,最后论文展望了固体电介质中挠曲电效应相关研究的开放性问题和发展方向.     

基于广义应变梯度理论的纳米梁挠曲电效应研究

挠曲电效应是应变梯度与电极化的耦合,它存在于所有的电介质材料中。在纳米电介质结构的挠曲电效应研究中,应变梯度弹性对挠曲电响应的影响一直以来被低估甚至被忽略了。根据广义应变梯度理论,应变梯度弹性中独立的尺度参数只有三个,而文献中所采用的一个或两个尺度参数的应变梯度理论只是它的简化形式。基于该理论,论文建立了考虑广义应变梯度弹性的三维电介质结构的理论模型,并以一维纳米梁为例研究了其弯曲问题的挠曲电响应及其能量俘获特性。结果表明,纳米梁的挠曲电响应存在尺寸效应,并且弹性应变梯度会影响结构挠曲电的尺寸效应,特别是当结构的特征尺寸低于尺度参数时。论文的工作为更进一步理解纳米尺度下的挠曲电机理和能量俘获特性提供理论基础和设计依据。     

电磁加速等离子体喷涂技术的原理分析

对自行研制的电磁加速等离子体喷涂技术（Electromagnetically accelerated plasma spraying,EMAPS）从原理上进行了分析,建立了等离子体的运动方程,得到了脉冲电流作用下等离子体上任一点的磁感应强度的计算公式和等离子体及它所压缩周围空气的速度表达式;并分析了粉末粒子与冲击波的相互作用,得到了粉末粒子的喷涂速度与喷枪长度、粉末粒子直径以及电流强度之间的关系;同时还对喷枪的长度进行了设计。     

基于LS-DYNA的液电效应冲击波数值模拟

液电效应机理复杂,鲜有成熟的商用数值模拟软件能够描述等离子体通道内部特性,为了将液电效应产生的冲击波运用于已有的数值模拟软件中,以满足工程需要,介绍了两种基于显式动力学软件LS-DYNA间接模拟液电效应产生冲击波的方法:水下爆炸等效（分为爆炸能量等效与冲击波能量等效）和理想气体等效,并进行了比较与改进,分析了不同沉积能量下采用不同等效方法得到的峰压计算结果的差异。结果显示,在沉积能量相同的条件下,基于爆炸能量等效方法得到的冲击波峰值压力最高,基于冲击波能量等效方法得到的冲击波峰值压力次之,基于理想气体等效方法得到的冲击波峰值压力最低,理想气体等效模拟的峰压相较于前两种等效方法小1～2个数量级;爆炸能量等效与冲击波能量等效的冲击波波速相等,且高于理想气体等效的冲击波波速;沉积能量减小会使得3种等效方法模拟的峰压均有不同程度的减小,但大小顺序不发生变化;改进后的等效爆炸方法能够适应沉积能量的变化,与Touya经验公式拟合较好;基于LS-DYNA对液电效应冲击波峰值压力进行准确模拟,除了选取适合的等效方法,还应结合具体的放电条件,建立适当的数值模型,在满足计算要求的条件下实现冲击波峰值压力的快速计算。     

考虑挠曲电效应的压电纳米薄板力-电-热耦合特性研究

摘要：挠曲电效应是由应变梯度引起的,与尺度相关的力电耦合效应。基于Kirchhoff板假设和挠曲电理论,本文推导了温度和电压作用下的压电薄板力-电-热耦合微分控制方程,定量分析了微分控制方程中非线性项的影响,并针对四周固支压电薄板采用Ritz法求解,数值计算了压电薄板的弯曲和振动行为。在研究温度和挠曲电效应对薄板耦合特性和力学行为的影响时,本文分别考虑了材料系数不随温度变化和随温度线性变化两种情况。以PZT-5H为例,我们讨论了挠曲电和温度对压电薄板的横向位移和固有频率的影响。研究结果表明挠曲电效应对压电纳米薄板的力学行为影响很大,且具有明显的尺寸效应。此外,薄板对温度变化非常敏感。因此,可通过挠曲电效应和温度来调控压电纳米薄板的多场耦合特性和力学行为,进而优化基于压电薄板的NEMS/MEMS中传感器、作动器等电子器件的性能。     

Experimental investigation of convective heat transfer enhancement from 3D-shape heat sources by EHD actuator in duct flow

Heat transfer enhancement from cylindrical heat sources as electronic components established at the bottom of duct with in-line arrangement and also from the bottom by electrohydrodynamic (EHD) actuator has been investigated experimentally. Air flow is drawn to the duct with various Reynolds numbers based on hydraulic diameter of inlet of the test section (Re = 0, 500, 1100, 2500 and 3870) that include natural convection (confined and unconfined cases) and forced convection (laminar and turbulent flows). Wire electrodes are arranged in transverse direction and perpendicular to the main flow with two various arrangements and high voltages are applied up to 30 kV in the wires. The results revealed that the second electrode arrangement (three wires over the ribs) is more effective due to more enhancement of heat transfer and less corona power consumption in comparison with the first one (four wires between the ribs). Also the electric field is obviously more effective for low Reynolds numbers.     

A numerical investigation of electrohydrodynamic (EHD) effects on bubble deformation under pseudo-nucleate boiling conditions

In this article, the electrohydrodynamic (EHD) effects on nucleate boiling are studied by developing a numerical modelling of EHD effect on bubble deformation in pseudo-nucleate boiling conditions. The volume of fluid (VOF) method is employed to track the interface between the gas–liquid two phases; the user-defined code is written and added to the commercial software FLUENT to solve the electric field and the corresponding electric body force. On this basis, the model is applied to study the EHD effects on heat transfer and fluid flows. An initial air bubble surrounded by liquid CCl₄ and attached to a horizontal superheated wall under the action of electric field is studied. The results of the EHD effect on bubble shape evolution are compared with those of available experiments showing good agreement. The mechanism of EHD enhancement of heat transfer and the EHD induced phenomena including bubble elongation and detachment are analyzed in detail.     

圆柱绕流电磁控制影响因素的实验研究

实验与数值模拟表明,利用电介质溶液中圆柱体侧表面附近分布的电磁场产生电磁力可有效改变圆柱流体边界层,控制圆柱绕流.本文对圆柱绕流电磁控制过程中的主要影响因素进行了实验研究,电磁包覆在圆柱表面后部时,其控制效果与全部包覆相当,而包覆在其它位置时,消涡效果较差;电磁极板窄的圆柱绕流控制效果较极板宽的消涡与减阻效果好.另外,电磁作用参数N愈大,消涡减阻的效果愈好.     

多孔线性控制套筒致裂效果影响因素分析

针对坚硬难垮落顶板弱化问题,在分析多孔线性控制套筒致裂机理的基础上,建立700mm×300mm×400mm试块。通过套筒致裂压力及钻孔孔壁周围应变变化情况,分析孔壁周围应力大小、分布及应力集中现象,研究钻孔直径、孔间距、套筒致裂压力对应力集中现象的影响,同时分析这三者对裂纹扩展方向的影响,得出套筒致裂最优法。试验结果表明:(1)套筒致裂效果受钻孔直径、孔间距和套筒压力等综合因素影响,采用大直径、大间距的布孔方式可获得较好的致裂效果;(2)多孔线性控制致裂试验的张性裂缝扩展方向主要沿着各钻孔中心连线方向;(3)钻孔直径30mm、间距300mm的试块致裂压力为18.6MPa,远大于顶板岩体抗拉强度,将套筒致裂法应用于顶板弱化是可行的。本研究对解决采空区坚硬顶板大面积悬顶具有理论意义和应用价值。     

Experimental study on the wake behind tapered circular cylinders

The flow around tapered cylinders can act as basic models for numerous bluff body flows with a spanwise variation of either the body shape or the inflow conditions. The well-known vortex street is influenced by strong three-dimensional effects from the spanwise variation of the shedding frequency, namely oblique vortex shedding and vortex dislocations. Stereo-PIV was chosen to study these phenomena, since it allows analyzing planes with the full three-component, instantaneous velocity fields and local, time-dependent variations in the same setting. Hence, detailed aspects of the vortex dislocation phenomenon are presented. Single vortex dislocation events are presented through the local variation of the three measured velocity components u, v and w. Longer time-series reveal both period and location of these dislocation events, as well as quantity and sizes of the cells of constant shedding velocity in between them. The influence of the Reynolds number and the cylinder aspect ratio on the vortex cells could be shown. The analysis of the vortex shedding behavior shows good agreement with previously published results. At the same time, the applied PIV technique provides more spatial information than point-based measurements and offers insight into a Reynolds number range that is currently out of reach of Direct Numerical Simulations.     

Experimental study on wake flow structures of screen cylinders using PIV

Experiments were conducted in a water flume using Particle Image Velocimetry (PIV) to study the evolution of the vortical structures in the wakes of four types of screen cylinders at a Reynolds number of about 3200. The results were compared with that of a bare cylinder. The screen cylinders were made of stainless steel screen meshes of various porosities (37%, 48%, 61% and 67%) rolled into cylindrical shapes. Smoke wire flow visualisations in a wind tunnel were also conducted in support of the PIV tests. Depending on the porosity of the screen mesh, two vortex formation mechanisms for the screen cylinder wakes were identified. One was associated with wake instability and the other was associated with shear-layer (Kelvin-Helmholtz) convective instability which involved merging through pairing and tripling of small-scale vortices within the shear layers. The former was responsible for the formation of large-scale vortices in the bare cylinder and the screen cylinder wakes with 37% and 48% porosities, while the latter was responsible for the screen cylinder wakes with 61% and 67% porosities. The results also showed that with increasing porosity, the vortex formation region was extended farther downstream and the Reynolds shear stress, the Turbulent Kinetic Energy (TKE) and vortex intensity were decreased constantly.     

湍流能量耗散率标度律实验研究

应用粒子图像测速(PIV)系统对平板湍流边界层内流向和法向的瞬时速度进行了测量。湍流的能量耗散率由轴对称假设得到,同时在研究湍流动能耗散率标度律的过程中采用传统的统计学方法。实验结果显示,对于不同尺度上和不同法向位置湍流耗散率标度律来说,湍流耗散主要发生在小尺度上,也就是说湍动能耗散率标度律在小尺度上具有普适性。另外,根据层次结构理论假设,通过PIV实验数据对最高激发态的标度指数进行了研究,结果发现,最高激发态存在绝对标度指数,并且绝对标度律是由信号中最强耗散涨落的局部结构产生的。     

Experimental techniques for mechanical characterization of hydrogels at the microscale

Hydrogel actuators in microfluidic devices must endure the forces of aqueous flow, the constraint of device walls, and the restoring force of elastic membranes. In order to assess the capabilities of hydrogels, three experimental techniques for determining the uniaxial tensile properties and functional swelling properties of microscale hydrogel structures have been developed. Tensile tests were conducted to determine Young's modulus and Poisson's ratio at varying degrees of swelling equilibrium. Force response tests were performed to determine the force exerted by cylindrical hydrogel structures on compression platens held at fixed displacement. Particle image velocimetry, a method originally developed to measure velocity fields in fluid flows, was adapted to investigate the deformation rates at various times within hydrogel structures during volumetric swelling. The techniques and sample fabrication methods outlined are applicable to a variety of hydrogel chemistries.     

Experimental investigation on turbulence modification in a horizontal channel flow at relatively low mass loading

Particle-laden flows in a horizontal channel were investigated by means of a two-phase particle image velocimetry (PIV) technique. Experiments were performed at a Reynolds number of 6 826 and the flow is seeded with polythene beads of two sizes, 60 μm and 110 μm. One was slightly smaller than and the other was larger than the Kolmogorov length scale. The particle loadings were relatively low, with mass loading ratio ranging from 5×10⁻⁴ to 4×10⁻² and volume fractions from 6×10⁻⁷ to 4.8×10⁻⁵, respectively. The results show that the presence of particles can dramatically modify the turbulence even under the lowest mass loading ratio of 5×10⁻⁴. The mean flow is attenuated and decreased with increasing particle size and mass loading. The turbulence intensities are enhanced in all the cases concerned. With the increase of the mass loading, the intensities vary in a complicated manner in the case of small particles, indicating complicated particle-turbulence interactions; whereas they increase monotonously in the case of large particles. The particle velocities and concentrations are also given. The particles lag behind the fluid in the center region but lead in the wall region, and this trend is more prominent for the large particles. The streamwise particle fluctuations are larger than the gas fluctuations for both sizes of particles, however their varying trend with the mass loadings is not so clear. The wall-normal fluctuations increase with increasing mass loadings. They are smaller in the 60 μm particle case but larger in the 110 μm particle case than those of the gas phase. It seems that the small particles follow the fluid motion to certain extent while the larger particles are more likely dominated by their own inertia. Finally, remarkable non-uniform distributions of particle concentration are observed, especially for the large particles. The inertia of particles is proved to be very important for the turbulence modification and particles behaviors and thus should be considered in horizontal channels. The project supported by the National Natural Science Foundation of China (50276021), and Program for New Century Excellent Talents in University, Ministry of Education (NCET-04-0708) The English text was polished by Yunming Chen.     

Experimental study of the three-dimensional flow field in cross-flow fans

High-resolution PIV measurements of the flow field inside cross-flow fans have been performed in planes normal and parallel to the fan axis, both outside and inside the impeller. The well known difficulties in obtaining the optical access inside the impeller have been overcome by allowing the internal flow planes to be illuminated by the laser light sheet or shot by the CCD camera through the moving blade vanes. Measurements have been performed in two cross-flow fans having the same two-module impeller but casing geometries based on very different design concepts. PIV data in planes normal to the rotor axis show a strong correlation between vorticity distribution and turbulent shear stresses inside the eccentric vortex of each fan. Furthermore, they provide useful elements to explain the very different performance of the two fans evidenced by their characteristic curves. Measurements in planes parallel to the impeller axis show that wide three-dimensional recirculation structures develop near the casing end walls at the discharge of the fans. These mean flow structures are responsible for the backflow into the end portions of the impeller of part of the discharged fluid, which is then transported axially by the eccentric vortex towards the rotor central disc before being discharged once again outside the impeller. In the case of cross-flow fans including few rotor modules, the existence of significant axial velocity components inside the eccentric vortex can alter substantially the flow picture, common in the current literature, resulting from 2-D numerical models or measurements performed in a single transverse plane of the fan.