Particle fluid mechanics in shear flows, acoustic waves, and shock waves

Three different categories of flow problems of a fluid containing small particles are being considered here. They are: (i) a fluid containing small, non-reacting particles (Parts I and II); (ii) a fluid containing reacting particles (Parts III and IV); and (iii) a fluid containing particles of two distinct sizes with collisions between two groups of particles (Part V).

Part I

A numerical solution is obtained for a fluid containing small particles flowing over an infinite disc rotating at a constant angular velocity. It is a boundary layer type flow, and the boundary layer thickness for the mixture is estimated. For large Reynolds number, the solution suggests the boundary layer approximation of a fluid-particle mixture by assuming W = W_p. The error introduced is consistent with the Prandtl’s boundary layer approximation. Outside the boundary layer, the flow field has to satisfy the “inviscid equation” in which the viscous stress terms are absent while the drag force between the particle cloud and the fluid is still important. Increase of particle concentration reduces the boundary layer thickness and the amount of mixture being transported outwardly is reduced. A new parameter, β = 1/Ω τ_v, is introduced which is also proportional to μ. The secondary flow of the particle cloud depends very much on β. For small values of β, the particle cloud velocity attains its maximum value on the surface of the disc, and for infinitely large values of β, both the radial and axial particle velocity components vanish on the surface of the disc.

Part II

The “inviscid” equation for a gas-particle mixture is linearized to describe the flow over a wavy wall. Corresponding to the Prandtl-Glauert equation for pure gas, a fourth order partial differential equation in terms of the velocity potential ϕ is obtained for the mixture. The solution is obtained for the flow over a periodic wavy wall. For equilibrium flows where λ_v and λ_T approach zero and frozen flows in which λ_v and λ_T become infinitely large, the flow problem is basically similar to that obtained by Ackeret for a pure gas. For finite values of λ_v and λ_T, all quantities except v are not in phase with the wavy wall. Thus the drag coefficient C_D is present even in the subsonic case, and similarly, all quantities decay exponentially for supersonic flows. The phase shift and the attenuation factor increase for increasing particle concentration.

Part III

Using the boundary layer approximation, the initial development of the combustion zone between the laminar mixing of two parallel streams of oxidizing agent and small, solid, combustible particles suspended in an inert gas is investigated. For the special case when the two streams are moving at the same speed, a Green’s function exists for the differential equations describing first order gas temperature and oxidizer concentration. Solutions in terms of error functions and exponential integrals are obtained. Reactions occur within a relatively thin region of the order of λ_D. Thus, it seems advantageous in the general study of two-dimensional laminar flame problems to introduce a chemical boundary layer of thickness λ_D within which reactions take place. Outside this chemical boundary layer, the flow field corresponds to the ordinary fluid dynamics without chemical reaction.

Part IV

The shock wave structure in a condensing medium of small liquid droplets suspended in a homogeneous gas-vapor mixture consists of the conventional compressive wave followed by a relaxation region in which the particle cloud and gas mixture attain momentum and thermal equilibrium. Immediately following the compressive wave, the partial pressure corresponding to the vapor concentration in the gas mixture is higher than the vapor pressure of the liquid droplets and condensation sets in. Farther downstream of the shock, evaporation appears when the particle temperature is raised by the hot surrounding gas mixture. The thickness of the condensation region depends very much on the latent heat. For relatively high latent heat, the condensation zone is small compared with Ʌ_D.

For solid particles suspended initially in an inert gas, the relaxation zone immediately following the compression wave consists of a region where the particle temperature is first being raised to its melting point. When the particles are totally melted as the particle temperature is further increased, evaporation of the particles also plays a role.

The equilibrium condition downstream of the shock can be calculated and is independent of the model of the particle-gas mixture interaction.

Part V

For a gas containing particles of two distinct sizes and satisfying certain conditions, momentum transfer due to collisions between the two groups of particles can be taken into consideration using the classical elastic spherical ball model. Both in the relatively simple problem of normal shock wave and the perturbation solutions for the nozzle flow, the transfer of momentum due to collisions which decreases the velocity difference between the two groups of particles is clearly demonstrated. The difference in temperature as compared with the collisionless case is quite negligible.

Investigation of strong earthquake ground motion

The pattern of energy release during the Imperial Valley, California, earthquake of 1940 is studied by analysing the El Centro strong motion seismograph record and records from the Tinemaha seismograph station, 546 km from the epicenter. The earthquake was a multiple event sequence with at least 4 events recorded at El Centro in the first 25 seconds, followed by 9 events recorded in the next 5 minutes. Clear P, S and surface waves were observed on the strong motion record. Although the main part of the earthquake energy was released during the first 15 seconds, some of the later events were as large as M = 5.8 and thus are important for earthquake engineering studies. The moment calculated using Fourier analysis of surface waves agrees with the moment estimated from field measurements of fault offset after the earthquake. The earthquake engineering significance of the complex pattern of energy release is discussed. It is concluded that a cumulative increase in amplitudes of building vibration resulting from the present sequence of shocks would be significant only for structures with relatively long natural period of vibration. However, progressive weakening effects may also lead to greater damage for multiple event earthquakes.

The model with surface Love waves propagating through a single layer as a surface wave guide is studied. It is expected that the derived properties for this simple model illustrate well several phenomena associated with strong earthquake ground motion. First, it is shown that a surface layer, or several layers, will cause the main part of the high frequency energy, radiated from the nearby earthquake, to be confined to the layer as a wave guide. The existence of the surface layer will thus increase the rate of the energy transfer into the man-made structures on or near the surface of the layer. Secondly, the surface amplitude of the guided SH waves will decrease if the energy of the wave is essentially confined to the layer and if the wave propagates towards an increasing layer thickness. It is also shown that the constructive interference of SH waves will cause the zeroes and the peaks in the Fourier amplitude spectrum of the surface ground motion to be continuously displaced towards the longer periods as the distance from the source of the energy release increases.

输运层厚度与迁移率对双层有机发光器件性能的影响

基于一些合理假设与近似，建立了具有欧姆接触载流子注入的双层有机电致发光器件的解析模型．模型中计算参数只有空穴和电子输运层的厚度、迁移率、外加电压及材料参数Fh和Fe，且这些参数都是可以通过测量得到的．研究了材料参数Fh，Fe对器件复合电流密度的影响及空穴输运层厚度和载流子迁移率对器件电场强度和复合电流密度的影响．结果表明：空穴输运层厚度和载流子迁移率引起电场强度的重新分布，进而对复合电流密度产生影响；接触限制电极注入少数载流子的器件可以取得比欧姆接触注入多数载流子的器件更高的复合电流．

Subsurface damage of Nd-doped phosphate glasses in optical fabrication

We present a destructive method for detecting and measuring subsurface damage of Nd-doped phosphate glasses. An instrument based on the dimple method - a destructive method - was developed. Subsurface damage depth produced in each fabrication procedure was obtained. We extend the surface roughness-subsurface damage relation to Nd-doped phosphate glasses. The constant ratio of subsurface damage and surface roughness was obtained as well. We also analyse the relation of abrasive size and subsurface damage experimentally. From a measurement of the surface roughness or abrasive size, one can obtain an accurate estimate of the damage layer thickness that must be eliminated by polishing or subsequent grinding operations. (C) 2007 Elsevier GmbH. All rights reserved.

光学表面的标量散射理论

简要论述了标量散射理论的研究进展做,着重介绍了Beckman的一维标量散射理论和几种典型的多层膜散射模型-非相关表面粗糙度模型、附加表面粗糙度模型和非相关体内不均匀模型,比较了这些模型在中心波长为632.8nm的11层高反膜的散射特性.结果表明,非相关体内的不均匀性引起反射能带边缘散射,反射能带内的散射主要由附加表面粗糙度引起.理想粗糙度对膜系反射带内的散射影响很小,对反射带边缘几乎无影响.预测了标量散射理论的应用领域及前景.

利用等效层的消偏振宽带减反膜设计

分析了倾斜入射条件下导致光学薄膜产生偏振的原因, 针对不同偏振态的等效导纳与等效相位进行了分析, 并计算了对称膜层在45°入射条件下不同偏振态的等效折射率与等效相位厚度, 采用等效层方法设计了光学性能良好的600～900 nm波段消偏振宽带减反膜。最后利用电子束蒸发技术制备了薄膜样品, 样品的光谱性能完全能够满足使用要求。其中在600～900 nm波段范围内, 平均反射率均小于1.38%, 反射率的偏振度均低于0.89％。另外, 通过对其理论及实验光学性能、角度敏感性、膜层厚度误差敏感性等方面的分析结果可

4H-SiC基底Al_2O_3/SiO_2双层减反射膜的设计和制备

在4H-SiC基底上设计并制备了Al2O3/SiO2紫外双层减反射膜,通过扫描电镜(SEM)和实测反射率谱来验证理论设计的正确性。利用编程计算得到Al2O3和SiO2的最优物理膜厚分别为42.0nm和96.1nm以及参考波长λ=280nm处最小反射率为0.09%。由误差分析可知,实际镀膜时保持双层膜厚度之和与理论值一致有利于降低膜系反射率。实验中应当准确控制SiO2折射率并使Al2O3折射率接近1.715。用电子束蒸发法在4H-SiC基底上淀积Al2O3/SiO2双层膜,厚度分别为42nm和96nm。SEM截面图表明淀积的薄膜和基底间具有较强的附着力。实测反射率极小值为0.33%,对应λ=276nm,与理论结果吻合较好。与传统SiO2单层膜相比,Al2O3/SiO2双层膜具有反射率小,波长选择性好等优点,从而论证了其在4H-SiC基紫外光电器件减反射膜上具有较好的应用前景。

渐变反射率镜的设计与制备

按照高斯型渐变反射率镜(GRM)的参数要求,采用了中间层厚度渐变的方案对膜系和掩模板形状进行设计.根据薄膜的实际需求和具体的沉积设备,设计了掩模和掩模切换装置.在一次高真空环境下镀制了渐变反射率镜的所有膜系.采用直接测量的方法,测量了高斯型渐变反射率镜反射率的径向分布.测试结果表明,用这种技术制备的样品,与设计要求基本一致.分析得出,掩模板形状与精度对镀制结果有影响.随着设计尺寸减小,掩模板对膜料分子的散射作用增强,使样品中心反射率小于设计要求,边缘出现旁瓣.提出了减小基片与掩模板之间的距离和提高膜厚监控的精度的改善方案.

Techniques for fabrication of large-size graded reflectivity mirrors

According to the parameter requirements of a graded reflectivity mirror with a Gaussian profile, the layer structure and the mask pattern are designed using a graded-thickness middle layer. The mask and the automatic mask-switchover equipment are designed considering the actual requirement of the thin films and the specific deposit facility. The uniformity of the layer thickness is analyzed. The measurement results indicate that samples prepared with this technique are basically in accordance with the design parameter. The scattering effect between the material molecules and the mask, thickness errors, and the alignment error between the mask and the substrate are the main factors that influence the deposit result. (c) 2008 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3027595]

Computational study of the vortex path variation with the VG height

An extensive range of conventional, vane-type, passive vortex generators (VGs) are in use for successful applications of flow separation control. In most cases, the VG height is designed with the same thickness as the local boundary layer at the VG position. However, in some applications, these conventional VGs may produce excess residual drag. The so-called low-profile VGs can reduce the parasitic drag associated to this kind of passive control devices. As suggested by many authors, low-profile VGs can provide enough momentum transfer over a region several times their own height for effective flow-separation control with much lower drag. The main objective of this work is to study the variation of the path and the development of the primary vortex generated by a rectangular VG mounted on a flat plate with five different device heights h = delta, h(1) = 0.8 delta, h(2) = 0.6 delta, h(3) = 0.4 delta and h(4) = 0.2 delta, where delta is the local boundary layer thickness. For this purpose, computational simulations have been carried out at Reynolds number Re = 1350 based on the height of the conventional VG h = 0.25m with the angle of attack of the vane to the oncoming flow beta = 18.5 degrees. The results show that the VG scaling significantly affects the vortex trajectory and the peak vorticity generated by the primary vortex.

Previsão do comportamento da argila mole da Baixada de Jacarepaguá: o efeito da submersão do aterro e do adensamento secundário.

A argila encontrada na Barra da Tijuca, Rio de Janeiro, é conhecida por ser muito mole e de peso específico bastante baixo. Passados 17 anos após a execução de um aterro em um dos depósitos argilosos da região, foram realizados ensaios de piezocone e SPT para aferir a espessura atual da camada mole. Tendo-se as espessuras originais da camada, determinadas a partir de ensaios de palheta e SPT na época da realização da obra, foi possível,assim, determinar o recalque que realmente ocorreu. O recalque primário foi calculado através dos parâmetros de compressibilidade previamente conhecidos daquele material. Foi aplicada a correção devido à submersão do aterro e, para isso, foi desenvolvida neste trabalho uma nova metodologia de cálculo para este efeito, considerando a submersão do aterro de forma gradual em todo o seu desenvolvimento matemático. Através da diferença entre o recalque total verificado nos ensaios e o recalque primário calculado, determinou-se a magnitude do recalque secundário. Com isso, estimou-se o OCR secundário do local. Este parâmetro surge da teoria que considera que os recalques secundários também ocorrem devido à variação das tensões efetivas, mas, nesse caso, horizontais. As principais conclusões desta pesquisa apontam para um valor de OCR secundário maior do que aquele já conhecido da argila de Sarapuí, que serviu de base para a referida teoria. Além disso, a proposta de cálculo do efeito da submersão se mostrou fácil e rápida de ser aplicada e forneceu resultados próximos daqueles obtidos pelas metodologias usuais.

Surface roughness noise prediction for silent aircraft experimental design SAX-40

Surface roughness noise is a potentially important contributor to airframe noise. In this paper, noise assessment due to surface roughness is performed for a conceptual Silent Aircraft design SAX-40 by means of a prediction model developed in previous theoretical work and validated experimentally. Estimates of three idealized test cases show that surface roughness could produce a significant noise level above that due to the trailing edge at high frequencies. Roughness height and roughness density are the two most significant parameters influencing surface roughness noise, with roughness height having the dominant effect. The ratio of roughness height to boundary-layer thickness is the relevant non-dimensional parameter and this decreases in the streamwise direction. The candidate surface roughness is selected for SAX-40 to meet an aggressive noise target and keep surface roughness noise at a negligible level. Copyright © 2008 by Yu Liu and Ann P. Dowling.

Controlled growth of aligned ZnO nanowires

The growth of vertically aligned zinc oxide nanowires (ZnO NW) using a simple vapor deposition method system is reported. The growth properties are studied as a function of the Au catalyst layer thickness, pressure, deposition temperature, and oxygen ratio. It was found that the diameter and density of the nanowires is controlled mostly by the growth temperature and pressure. The alignment of the nanowires depends on a combination of three factors including the pressure, temperature and the oxygen ratio. Our results implicates the growth occurs by a vapor liquid solid (VLS) process [1].

Micro-vortex generator flow control for supersonic engine inlets

To investigate the flow control potential of micro-vortex generators for supersonic mixed-compression inlets, a basic model experiment has been designed which combines a normal shock wave with a subsonic diffuser. The diffuser is formed by a simple expansion corner, with a divergence angle of 6 degrees. The diffuser entry Mach numbers were M=1.3 and M=1.5 and a number of shock locations relative to the corner position were tested. Flow control was applied in the form of counter-rotating micro-vanes with heights of approximately 20% of boundary layer thickness. Furthermore, corner fences where employed to reduce sidewall effects. It was found that micro-vortex generators were able to significantly reduce the extent of flow separation under all conditions, but could not eliminate it altogether. Corner fences also demonstrated potential for improving the flow in rectangular cross section channels and the combination of corner fences with micro-vortex generators was found to give the greatest benefits. At M=1.3 the combination of corner fences and micro-vanes placed close to the diffuser entry could prevent separation for a wide range of conditions. At the higher diffuser entry Mach number the benefits of flow control were less significant although a reduction of separation size and an improved pressure recovery was observed. It is thought that micro-vortex generators can have significant flow control potential if they are placed close to the expected separation onset and when the adverse pressure gradient is not too far above the incipient separation level. The significant beneficial effects of corner fences warrant a more comprehensive further investigation. It is thought that the control methods suggested here are capable of reducing the bleed requirement on an inlet, which could provide significant performance advantages.

VGs for a normal SBLI with a downstream diffuser

New types of vortex generators for boundary layer control were investigated experimentally in a flow field which contains a Mach 1.4 normal Shockwave followed by a subsonic diffuser. A parametric study of device height and distance upstream of the normal shock was undertaken with two novel devices: ramped-vanes and split-ramps. Flowfield diagnostics included high-speed Schlieren, oil flow visualization, and pitot-static pressure measurements. A number of flowfield parameters including flow separation, pressure recovery, centerline incompressible boundary layer shape factor, and shock stability were analyzed and compared to the baseline. All configurations tested yielded an elimination of centerline flow separation with the presence of the vortex generators. However, the devices also tended to increase the three-dimensionality of the flow with increased side-wall interaction. When located 25δo upstream of the normal shock, the largest ramped-vane device (whose height was about 0.75 the incoming uncontrolled boundary layer thickness, δo) yielded the smallest centerline incompressible shape factor and the least streamwise oscillations of the normal shock. However, additional studies are needed to better understand the corner interaction effects, which are substantial. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.