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.

Use of automatic sand pourers for loose sand models

The consistency of laboratory sand model preparation for physical testing is a fundamental criterion in representing identical geotechnical issues at prototype scale. This objective led to the development of robotic apparatus to eliminate the non-uniformity in manual pouring. Previous studies have shown consistent sand models with high relative density between 50 to 90% produced by the automatic moving-hopper sand pourer at the University of Cambridge, based primarily on a linear correlation to flow rate. However, in the case of loose samples, the influence of other parameters, particularly the drop height, becomes more apparent. In this paper, findings on the effect of flow rate and drop height are discussed in relation to the layer thickness and relative density of loose sand samples. Design charts are presented to illustrate their relationships. The effect of these factors on different sand types is also covered to extend the use of the equipment. © 2010 Taylor & Francis Group, London.

Tensile response of passivated films with climb-assisted dislocation glide

The tensile response of single crystal films passivated on two sides is analysed using climb enabled discrete dislocation plasticity. Plastic deformation is modelled through the motion of edge dislocations in an elastic solid with a lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The dislocation motion in the films is by glide-only or by climb-assisted glide whereas in the surface passivation layers dislocation motion occurs by glide-only and penalized by a friction stress. For realistic values of the friction stress, the size dependence of the flow strength of the oxidised films was mainly a geometrical effect resulting from the fact that the ratio of the oxide layer thickness to film thickness increases with decreasing film thickness. However, if the passivation layer was modelled as impenetrable, i.e. an infinite friction stress, the plastic hardening rate of the films increases with decreasing film thickness even for geometrically self-similar specimens. This size dependence is an intrinsic material size effect that occurs because the dislocation pile-up lengths become on the order of the film thickness. Counter-intuitively, the films have a higher flow strength when dislocation motion is driven by climb-assisted glide compared to the case when dislocation motion is glide-only. This occurs because dislocation climb breaks up the dislocation pile-ups that aid dislocations to penetrate the passivation layers. The results also show that the Bauschinger effect in passivated thin films is stronger when dislocation motion is climb-assisted compared to films wherein dislocation motion is by glide-only. © 2012 Elsevier Ltd.

Turbine blade tip heat transfer in low speed and high speed flows

In this paper, high and low speed tip flows are investigated for a high-pressure turbine blade. Previous experimental data are used to validate a CFD code, which is then used to study the tip heat transfer in high and low speed cascades. The results show that at engine representative Mach numbers the tip flow is predominantly transonic. Thus, compared to the low speed tip flow, the heat transfer is affected by reductions in both the heat transfer coefficient and the recovery temperature. The high Mach numbers in the tip region (M>1.5) lead to large local variations in recovery temperature. Significant changes in the heat transfer coefficient are also observed. These are due to changes in the structure of the tip flow at high speed. At high speeds, the pressure side corner separation bubble reattachment occurs through supersonic acceleration which halves the length of the bubble when the tip gap exit Mach number is increased from 0.1 to 1.0. In addition, shock/boundary-layer interactions within the tip gap lead to large changes in the tip boundary-layer thickness. These effects give rise to significant differences in the heat-transfer coefficient within the tip region compared to the low-speed tip flow. Compared to the low speed tip flow, the high speed tip flow is much less dominated by turbulent dissipation and is thus less sensitive to the choice of turbulence model. These results clearly demonstrate that blade tip heat transfer is a strong function of Mach number, an important implication when considering the use of low speed experimental testing and associated CFD validation in engine blade tip design. Copyright © 2009 by ASME.

Factors affecting seismic response of submarine slopes

The response of submerged slopes on the continental shelf to seismic or storm loading has become an important element in the risk assessment for offshore structures and "local" tsunami hazards worldwide. The geological profile of these slopes typically includes normally consolidated to lightly overconsolidated soft cohesive soils with layer thickness ranging from a few meters to hundreds of meters. The factor of safety obtained from pseudo-static analyses is not always a useful measure for evaluating the slope response, since values less than one do not necessarily imply slope failure with large movements of the soil mass. This paper addresses the relative importance of different factors affecting the response of submerged slopes during seismic loading. The analyses use a dynamic finite element code which includes a constitutive law describing the anisotropic stress-strain-strength behavior of normally consolidated to lightly overconsolidated clays. The model also incorporates anisotropic hardening to describe the effect of different shear strain and stress histories as well as bounding surface principles to provide realistic descriptions of the accumulation of the plastic strains and excess pore pressure during successive loading cycles. The paper presents results from parametric site response analyses on slope geometry and layering, soil material parameters, and input ground motion characteristics. The predicted maximum shear strains, permanent deformations, displacement time histories and maximum excess pore pressure development provide insight of slope performance during a seismic event. © 2006 Author(s). This work is licensed under a Creative Commons License.

Thin film coil design considerations for wireless power transfer in flat panel display

Wireless power transfer is experimentally demonstrated by transmission between an AC power transmitter and receiver, both realised using thin film technology. The transmitter and receiver thin film coils are chosen to be identical in order to promote resonant coupling. Planar spiral coils are used because of the ease of fabrication and to reduce the metal layer thickness. The energy transfer efficiency as a function of transfer distance is analysed along with a comparison between the theoretical and the experimental results. © 2012 Materials Research Society.