铁基激光熔覆合金设计及微观组织与性能研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">本文设计并制备了具有优良的强韧性能和高温性能的激光熔覆涂层。利用 SEM、TEMEY X－射线衍射仪等研究了涂层的化学成分、宏微观结构及其转变机制，同时研究了涂层强韧性及耐高温磨损性能及其影响机制。对激光溶覆涂层进行了合金化、微观组织、强韧化机制、加工工艺性能设计。合金系为 Fe-Cr-C-W-Ni，成分配比（质量分数），Fe:52-60%, Cr:24-30%, C:5-6%, W:4.5-7.5%, Ni:5-6%。强化机制为两相强化、亚结构强化及固溶强化，其中强化相为合金碳化物，基体相为合金元素过饱和度极高的韧性奥氏体。亚共晶及过共晶组织的领先凝固相分别为奥氏体及 M_7C_3 合金碳化物，两相共晶组织均为韧性相奥氏体和强化相 M_7C_3 合金碳化物。加工工艺控制领先凝固相的结构、组织演化及力学性能。熔覆组织在高温时效过程中形成大量新的碳化物。在过饱和奥氏体内部，可弥散析出细小的 Mc、M_2C 及 M_(23)C_6碳化物；在奥氏体与 M_7C_3 相界面，亚稳相M_7C_3发生原位转变，形成 M_(23)C_6 及 M_6C碳化物。激光熔覆合金具有较高的综合力学性能，熔覆涂层有高的显微硬度、优良的抗回火稳定性、显著的二次硬化特征、优异的抗磨粒磨损和冲击磨损性能。Fe-Cr-C-W-Ni 激光熔覆合金具有较低的裂纹形成倾向和良好的表面成形性能，这与奥氏体较高的高温韧塑性及合金的低熔点共晶特征密切相关；实验及理论分析表明，通过调整合金成分、激光工艺参数和后续热处理工艺，可获得具有不同强韧性能的熔覆涂层。span>

冲击压缩下玻璃等脆性材料中失效波的实验和理论研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">本文首先回顾了失效波的研究进展和存在的问题，然后在现有实验手段改进的基础上对目前失效波研究中尚存的问题进行了较系统的实验研究，并建立了理论模型，进行了失效波传播的数值模拟。本文实验工作给出了K9玻璃中失效波速度和冲击载荷之间的定量关系和产生失效波的最小载荷阈值；通过：（1）玻璃样品和飞片表面状况对萌生失效波的载荷阈值的影响；（2）高速摄影观测冲击压缩下玻璃样品表面和内置界面处失效波的萌生和冲击波后压缩区内损伤演化情况；（3）回收样品的显微分析和Ｘ光衍射分析等实验，证实失效波的萌生基本与冲击相变无关，主要与玻璃样品表面状况有关，失效波的本质是玻璃样品表面固有微裂纹和冲击瞬间在此处萌生的微裂纹系统向玻璃样品中扩展的宏观统计表现。高速摄影观测到冲击波后有破碎界面在移动，其速度明显高于同等加载条件下VISAR测试的失效波速度，据此推测失效波是由大量裂纹扩展的宏观表现一破碎界面和其后方声阻抗明显降低的移动界面组成。实验同时研究了微晶玻璃、高纯度石英玻璃以及碱石灰玻璃在冲击压缩下的动力学响应特性。本文关于失效波方面的研究工作结果大部分未见相关文献报道。实验工作对于深入了解玻璃中失效波的萌生机制和失效波的力学性质等是非常重要的，并有助于建立与失效波萌生、传播有关的理论模型。改进有关实验技术，是完成本文实验研究的必要条件，也是本文工作的重要组成部分。本文设计了一种高接收效率、景深可调的新型VISAR探头，其技术指标接近国外同类产品，成本不到国外同类产品的二十分之一，且结构是国内外同类产品中最简单的。本文设计高速摄影的阴影和纹影光路，用于观察冲击压缩下玻璃样品中冲击损伤和失效波的演化及发展规律，其技术优于国外同类实验。本文设计了两种冲击压力低于玻璃样品HEL值的爆轰加载装置，用于配合高速摄影诊断实验。爆轰驱动厚飞片装置的设计，在飞片的炸药透镜之间增加了一个空腔，既降低了飞片的速度，又有效避免了以往同类装置驱动厚飞片时经常遇到的层裂问题，并且成功地进行了与爆轰驱动有关的数值模拟及设计工作。本文在理想微裂系统演化理论的基础上，建立了描述失效波的理论模型，分别讨论了表面损伤、微裂纹扩展和微裂纹形核、长大对失效波萌生、传播的影响，给出了描述失效波扩展的损伤演化方程，结合冲击压缩下材料的Resende压缩损伤本构方程，进行了各种加载条件下玻璃样品后自由表面速度和玻璃体内应力分存的数值模拟，计算结果与实验结果相符，表明本文建立的理论模型是可靠的，能够反映冲击压缩下失效波的传播特性的基本力学特性。用飞片碰玻璃样品时，实验测量的表面速度时程曲线出现“过冲现象”。本文通过高速摄影和VISAR等测试手段从实验上对其进行了较系统的研究，发现这种现象与玻璃样品后自由表面的破碎有关，并且其萌生所需的载荷阈值与萌生失效波的载荷阈值接近。本文建立了描述这一破坏现象的损伤演化方程，对其进行了数值模拟。计算结果和实验结果吻合，表明本文对这一现象的解释是合理的。span>

均匀来流绕旋转及旋转振荡圆柱绕流研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">该文通过数值方法求解二维不可压Navier-Stokes方程，对均匀来流中静止、旋转和旋转振荡圆柱绕流进行了系统的数值模拟.该文采用有限体积法对控制方程进行离散，选用元结构化四边形网格剖分计算区域，关于速度-压力耦合的处理使用了SIMPLEC方法.经过了大量的数值模拟，分盺166L鸬玫搅苏饧钢秩屏鞯氖的Ｄ饨峁?该文重点是用快速傅里叶变换（FFT）方法对旋转振荡圆柱绕流中的频率耦合现象进行研究，并分析在不同频率耦合作用下涡形成、发展和脱落的规律.span>

基于多矩的有限体积浸入边界法研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">浸入边界法（Immersed Boundary Method）是计算流体力学中求解具有复杂、移动边界流动问题的一类有效途径，该方法在笛卡尔坐标系上离散求解流体控制方程，并通过在控制方程中添加相应源相来代表浸入边界。尽管浸入边界法借助其简单、高效的显著特点在计算流体力学应用中显示出极强的生命力，特别是针对复杂的实际流动及动边界流动问题有着无可比拟的优势，但仍有许多问题需要进一步的研究。 本论文基于浸入边界方法及多矩VSIAM3（Volume/Surface Integrated Average Multi-Moment Method）格式提出了一种不可压缩流体求解数值格式。不可压N-S方程使用VSIAM3格式进行法进行离散，引入浸入边界法处理复杂、移动流动边界条件，使用虚拟网格方法计算动量方程修正项，同时还考虑了对连续方程的修正。VSIAM3格式是一种基于多矩的有限体积法，在方程的离散中总是使用两种或两种以上的矩，如：VIA（Volume Integrated Average）和SIA（Surface Integrated Average）。而不同的矩在求解过程中依据不同形式的控制方程使用不同的离散方法进行更新。VSIAM3格式更多的局地自由度及同时使用交错网格和同位网格的特点使浸入边界法的实施更加便利、高效。研究中，浸入边界法不仅应用于处理动力边界条件，同样可以处理热动力边界条件。 研究中对大量经典算例进行了数值实验，包括一维线性初始问题、方腔流问题、二维绕静止及振荡圆柱流动、三维绕球流动及热对流问题等。数值结果同实验值及其它计算结果保持一致，该算法可准确、高效处理具有复杂、移动边界及存在热对流的不可压流动问题，为实际应用打下了基础。span>

Flow-induced vibrations of long circular cylinders modeled by coupled nonlinear oscillators

The dynamics of long slender cylinders undergoing vortex-induced vibrations (VIV) is studied in this work. Long slender cylinders such as risers or tension legs are widely used in the field of ocean engineering. When the sea current flows past a cylinder, it will be excited due to vortex shedding. A three-dimensional time domain model is formulated to describe the response of the cylinder, in which the in-line (IL) and cross-flow (CF) deflections are coupled. The wake dynamics, including in-line and cross-flow vibrations, is represented using a pair of non-linear oscillators distributed along the cylinder. The wake oscillators are coupled to the dynamics of the long cylinder with the acceleration coupling term. A non-linear fluid force model is accounted for to reflect the relative motion of cylinder to current. The model is validated against the published data from a tank experiment with the free span riser. The comparisons show that some aspects due to VIV of long flexible cylinders can be reproduced by the proposed model, such as vibrating frequency, dominant mode number, occurrence and transition of the standing or traveling waves. In the case study, the simulations show that the IL curvature is not smaller than CF curvature, which indicates that both IL and CF vibrations are important for the structural fatigue damage.

Two-dimensional Vortex-induced Vibrations of Submerged Floating Tunnel Tethers

A vortex-induced vibration (VIV) model is presented for predicting the nonlinear dynamic response of submerged floating tunnel (SFT) tethers which are subjected to wave, current and tunnel oscillatory displacements at their upper end in horizontal and vertical directions. A nonlinear fluid force formula is introduced in this model, and the effect of the nonlinearity of tether is investigated. First, the tunnel is stationary and the tether vibrates due to the vortices shedding. The calculated results show that the cross-flow amplitude of VIV decreases compared with the linear model. However the in-line amplitude of VIV increases. Next, the periodical oscillation of tunnel is considered. The oscillation caused by wave forces plays the roles of parametric exciter and forcing exciter to the VIV of tether. The time history of displacement of the tether mid-span is obtained by the proposed model. It is shown that the in-line amplitude increases obviously and the corresponding frequency is changed. The cross-flow amplitude exhibits a periodic behavior.

聚二甲基硅氧烷（PDMS）表面液滴动力学的实验研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">液滴是自然界中普遍存在的一种物质形态。非连续微流体（液滴）是近年来微流体技术重要发展方向之一。对液滴的产生、启动、移动、合并、分离和碰撞过程的研究对于航天、微纳系统、电子显示、计算机冷却、喷墨、生物医学等学科领域有着重要的应用价值。液滴属于软物质，其力学性质介于流体和固体之间，其类固体（solid-like）行为来自于曲率产生的Laplace压力和表面张力的约束。对液滴动力学行为的研究有着重要的学术价值。 本文的主要工作是针对生物微电子机械系统（Bio-MEMS）以及柔性微纳电子加工中常用的材料聚二甲基硅氧烷（Polydimethylsiloxane，PDMS）为基底的液滴动力学实验研究。 液滴是一个理想的微反应器，许多实验可以集成在一个液滴或多个液滴内完成。液滴本身的动力学特性对于实验的完成效率和质量有着重要的影响。液滴的微操控技术包括多相流法、电润湿法、热毛细管法、介电泳法等。液滴的动力学特性受到基底的影响非常大，包括基频、振动模态、运动过程等均随基底的润湿性、弹性模量的变化而有所变化。 在Bio-MEMS以及柔性微纳电子加工当中，PDMS扮演着越来越重要的角色，尤其是PDMS的润湿性和电润湿特性。目前的PDMS在Bio-MEMS当中主要是用于制备各种微流道。常见的问题主要是一方面PDMS是疏水材料，影响流体的输运。另一方面是液体在这种低Reynolds数情况下不易混合，反应效率低。本文提出了在PDMS表面溅射纳米厚度的金来减小PDMS表观接触角的方法。这种方式在特定喷金量的情况下可以在PDMS表面产生多层次的压应力波纹。这种压应力波纹对于柔性微纳电子加工，以及微流道中加速流体混合有着非常重要的作用。 电润湿是另一种可以使PDMS亲水化的方法。实验证明，PDMS具有较好的电润湿性质。此外电润湿也是目前操纵液滴的主要方式。目前一个常见的问题是电击穿现象阻碍了驱动电压的低压化，且低Reynolds数情况下液滴的混合效率偏低。此外电极还会由于少量电解的发生导致腐蚀及对液体样品的污染。本文提出了接触式的电润湿，在电极逐渐触碰液滴的过程中，液滴发生百Hz的失稳振动，稳定后接触角减小。这种电润湿模式可以有效的提高临界击穿电压，避免液滴被腐蚀后的电极污染，同时可以加快液滴的混合效率。其失稳特征时间在10 ms量级，这恰是所用液滴特征尺度在1 mm左右的电润湿器件的最快响应时间。并采用液滴振动的理论估算了液滴的失稳时间，同时还考虑了基底润湿性对液滴振动过程的影响。 液滴的启动是电润湿操控液滴过程中的重要环节。通常的液滴启动都是在非连续基底上依靠逻辑电路产生的电势变化来驱动液滴。无论是逻辑电路的设计还是驱动装置的加工都非常复杂。本文首次实现了在超疏水生物样品荷叶上的液滴启动，启动速度为数十毫米/秒，启动时间为10 ms量级。并利用PDMS成功的仿制了荷叶结构实现了超疏水的PDMS表面，荷叶同仿荷叶的PDMS超疏水表面具有相近的润湿性。 在数字微流体操控液滴的过程中，液滴的合并涉及液滴的碰撞，而且MEMS系统当中利用液滴撞击进行冷却的实验已经有所开展。同时理解液滴碰撞还对许多领域包括生物、化学、喷墨、大气物理等有着非常重要的作用。本文实验研究了Weber数和毛细数对液滴碰撞过程的影响，通过改变Weber数和毛细数得到了四种不同的响应模式。span>

Configuration optimization of hypersonic vehicles under transitional flow conditions

The rarefied gas effects on several configurations are investigated under hypersonic flow conditions using the direct simulation Mont Carlo method. It is found that the Knudsen number, the Mach number, and the angle of attack all play a mixed role in the aerodynamics of a flat plate. The ratio of lift to drag decreases as the Knudsen number increases. Studies on 3D delta wings show that the ratio of lift to drag could be increased by decreasing the wing thickness and/or by increasing the wing span. It is also found that the waveriders could produce larger ratio of lift to drag as compared with the delta wing having the same length, wing span, and cross section area.

IP方法模拟硬盘读写头气膜润滑问题

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">本论文中采用IP方法模拟硬盘读写头的气膜润滑问题，并选择耗时但精准的DSMC方法作为该问题中IP方法的检验标准，IP方法计算得到的读写头表面压力分布及读写头所受净升力为读写头的设计提供可靠的参考数据。 首先，将读写头复杂表面简化为平面并假定尺寸为微米（远小于真实的毫米），微米尺寸时可以得到DSMC的模拟结果用于比较。IP法对该问题的模拟结果与DSMC的模拟结果完全吻合。 其次，选定一个真实的读写头表面几何，但仍然假定尺寸为微米。IP法在该问题的应用过程中遇到四点困难，我们提出或采取了相应的解决办法。在复杂表面几何的读写头气膜润滑问题中，IP法的模拟结果与DSMC的模拟结果也相符一致。 最后完成真实几何、真实尺寸（毫米）的读写头气膜润滑问题的模拟，这也是本论文的最终研究目标。IP法模拟该问题时需要采用大网格，为此我们构造二维检验模型验证了大网格的合理性。 真实几何读写头气膜润滑问题的模拟结果表明，在流场的局部区域压力沿着高度方向变化显著，这意味着读写头问题的传统计算方法（即概括性雷诺方程）在该局部区域不成立，故用它计算得到的结果值得认真检验，但文献中尚未给出过这类检验。这正是该论文的研究背景和采用新方法（IP方法）的原因。 本论文研究的问题属于过渡领域、低速流动问题，故本论文还介绍了该领域另外一种粒子模拟方法（LVDSMC方法）以及我们对该方法提出的一些改进。 关键词：硬盘读写头，气膜润滑，概括性雷诺方程，IP方法，DSMC方法，过渡领域，LVDSMC方法span>

冲压发动机喷气催化复合效应研究

<span style="font-family: 'Trebuchet MS', 'Lucida Sans Unicode', Arial, sans-serif; line-height: 22px">在高超声速飞行条件下，流入冲压发动机燃烧室并降至低速的空气的温度随飞行马赫数增加而愈来愈高。燃料与高温空气混合燃烧释放的化学能中的一部分将转化为解离能。这些解离能在长度受限的尾喷管中难以充分复合形成推力，使冲压发动机推力在高超声速范围内随飞行马赫数增大而下降，难以满足高超声速飞行器的推进要求。 与亚燃冲压发动机相比，流入超燃冲压发动机燃烧室的空气的温度在同样飞行马赫数条件下将明显降低，上述困难可大大缓解。然而目前超燃冲压发动机还存在关键性难点有待克服。若保持现有亚燃冲压发动机的吸气与燃烧方式，通过催化促进燃气解离组分在尾喷管膨胀过程中复合，可以增大冲压发动机的推力，满足高超声速飞行器的推进要求，为高超声速飞行器推进提供新的选择。 本论文主要研究内容如下： （1） 研究了亚燃冲压发动机燃烧室内燃气解离能与飞行马赫数的关系。通过对冻结流、平衡流和有限化学反应速率的流动的数值计算，确定了回收解离能增大推力的潜力。 （2） 以双爆轰技术为基础，建立起一套地面燃气产生装置。所产生的燃气的组分、温度和压力均与冲压发动机在高空飞行时燃气完全相同。调试出总温3200K、总压20Bar（对应来流马赫数6）和试验时间17.5ms以及总温4000K、总压5Bar（对应来流马赫数8）和试验时间12.5ms两种状态参数的试验用燃气。 （3） 建立了基于动量守恒原理的通过皮托管测压力换算推力的测量方法。对催化复合增大推力的实验而言，一般要进行特定流动条件下喷水与未喷水两种情况下推力大小的比较，其精度可以达到2%甚至更高。 （4） 完成了尾喷管喉道下游管壁喷水试验，成功释放出高温燃气中的解离能，有效增大了推力，证实了催化增推的想法是可行的。在来流马赫数6的条件下获得了11.0%的推力增量；在来流马赫数8.0的条件下也获得了11.7%的推力增加。span>

Incidence of mayfly (Povilla sp.) infestation of wooden infrastructures of artisanal fishers in the lagoon and marine fisheries locations of Ogun State, Nigeria

A diagnostic survey of the incidence of mayfly (Povilla sp.) infestation of wooden infrastructures of the artisanal fishers in ten (10) lagoons and marine fishing villages of Ogun States (Nigeria) was carried out through the application of structured questionnaire and participatory Rural Appraisal interviews. The demographic, infrastructural and socioeconomic characteristics of the ten fishing villages sampled were derived and analyzed. The infestation which occurs all year round is found to be most prevalent (70%) in the wet season, increasing proportionally with salinity from 56% (brackish water); to 63% (marine water). The life-span of Povilla sp. is reduced from 55% to 62% (freshwater); 41% (brackish water) and 38% (marine water). Annual financial loss of N10,000.00 per fisher or N80,000,000.00 to the 8000 artisanal fishers affected in Ogun State is discussed. It is recommended that fishers should preferably use non-wood crafts and infrastructures while adopting appropriate management strategies for containing the existing infestation

The Experimental and Numerical Studies on Gas Production from Hydrate Reservoir by Depressurization

A set of experimental system to study hydrate dissociation in porous media is built and some experiments on hydrate dissociation by depressurization are carried out. A mathematical model is developed to simulate the hydrate dissociation by depressurization in hydrate-bearing porous media. The model can be used to analyze the effects of the flow of multiphase fluids, the kinetic process and endothermic process of hydrate dissociation, ice-water phase equilibrium, the variation of permeability, convection and conduction on the hydrate dissociation, and gas and water productions. The numerical results agree well with the experimental results, which validate our mathematical model. For a 3-D hydrate reservoir of Class 3, the evolutions of pressure, temperature, and saturations are elucidated and the effects of some main parameters on gas and water rates are analyzed. Numerical results show that gas can be produced effectively from hydrate reservoir in the first stage of depressurization. Then, methods such as thermal stimulation or inhibitor injection should be considered due to the energy deficiency of formation energy. The numerical results for 3-D hydrate reservoir of Class 1 show that the overlying gas hydrate zone can apparently enhance gas rate and prolong life span of gas reservoir.

Grain growth in protoplanetary disks

The majority of young, low-mass stars are surrounded by optically thick accretion disks. These circumstellar disks provide large reservoirs of gas and dust that will eventually be transformed into planetary systems. Theory and observations suggest that the earliest stage toward planet formation in a protoplanetary disk is the growth of particles, from sub-micron-sized grains to centimeter- sized pebbles. Theory indicates that small interstellar grains are well coupled into the gas and are incorporated to the disk during the proto-stellar collapse. These dust particles settle toward the disk mid-plane and simultaneously grow through collisional coagulation in a very short timescale. Observationally, grain growth can be inferred by measuring the spectral energy distribution at long wavelengths, which traces the continuum dust emission spectrum and hence the dust opacity. Several observational studies have indicated that the dust component in protoplanetary disks has evolved as compared to interstellar medium dust particles, suggesting at least 4 orders of magnitude in particle- size growth. However, the limited angular resolution and poor sensitivity of previous observations has not allowed for further exploration of this astrophysical process.

As part of my thesis, I embarked in an observational program to search for evidence of radial variations in the dust properties across a protoplanetary disk, which may be indicative of grain growth. By making use of high angular resolution observations obtained with CARMA, VLA, and SMA, I searched for radial variations in the dust opacity inside protoplanetary disks. These observations span more than an order of magnitude in wavelength (from sub-millimeter to centimeter wavelengths) and attain spatial resolutions down to 20 AU. I characterized the radial distribution of the circumstellar material and constrained radial variations of the dust opacity spectral index, which may originate from particle growth in these circumstellar disks. Furthermore, I compared these observational constraints with simple physical models of grain evolution that include collisional coagulation, fragmentation, and the interaction of these grains with the gaseous disk (the radial drift problem). For the parameters explored, these observational constraints are in agreement with a population of grains limited in size by radial drift. Finally, I also discuss future endeavors with forthcoming ALMA observations.

Nonlinear optics and wavelength translation via cavity-optomechanics

The field of cavity-optomechanics explores the interaction of light with sound in an ever increasing array of devices. This interaction allows the mechanical system to be both sensed and controlled by the optical system, opening up a wide variety of experiments including the cooling of the mechanical resonator to its quantum mechanical ground state and the squeezing of the optical field upon interaction with the mechanical resonator, to name two.

In this work we explore two very different systems with different types of optomechanical coupling. The first system consists of two microdisk optical resonators stacked on top of each other and separated by a very small slot. The interaction of the disks causes their optical resonance frequencies to be extremely sensitive to the gap between the disks. By careful control of the gap between the disks, the optomechanical coupling can be made to be quadratic to first order which is uncommon in optomechanical systems. With this quadratic coupling the light field is now sensitive to the energy of the mechanical resonator and can directly control the potential energy trapping the mechanical motion. This ability to directly control the spring constant without modifying the energy of the mechanical system, unlike in linear optomechanical coupling, is explored.

Next, the bulk of this thesis deals with a high mechanical frequency optomechanical crystal which is used to coherently convert photons between different frequencies. This is accomplished via the engineered linear optomechanical coupling in these devices. Both classical and quantum systems utilize the interaction of light and matter across a wide range of energies. These systems are often not naturally compatible with one another and require a means of converting photons of dissimilar wavelengths to combine and exploit their different strengths. Here we theoretically propose and experimentally demonstrate coherent wavelength conversion of optical photons using photon-phonon translation in a cavity-optomechanical system. For an engineered silicon optomechanical crystal nanocavity supporting a 4 GHz localized phonon mode, optical signals in a 1.5 MHz bandwidth are coherently converted over a 11.2 THz frequency span between one cavity mode at wavelength 1460 nm and a second cavity mode at 1545 nm with a 93% internal (2% external) peak efficiency. The thermal and quantum limiting noise involved in the conversion process is also analyzed and, in terms of an equivalent photon number signal level, are found to correspond to an internal noise level of only 6 and 4 times 10x^-3 quanta, respectively.

We begin by developing the requisite theoretical background to describe the system. A significant amount of time is then spent describing the fabrication of these silicon nanobeams, with an emphasis on understanding the specifics and motivation. The experimental demonstration of wavelength conversion is then described and analyzed. It is determined that the method of getting photons into the cavity and collected from the cavity is a fundamental limiting factor in the overall efficiency. Finally, a new coupling scheme is designed, fabricated, and tested that provides a means of coupling greater than 90% of photons into and out of the cavity, addressing one of the largest obstacles with the initial wavelength conversion experiment.

Effect of nonuniform seismic input on arch dams

Standard earthquake analyses of civil engineering structures use uniform ground motions even though considerable variations in both amplitude and phase can occur along the foundation interface for long-span bridges and large dams. The objective of this thesis is to quantify the effect that these nonuniformities have on the structural response.

The nonuniform, free-field motions of the foundation interface are assumed to be caused by incident plane body waves. The medium in which these waves travel is a linear, elastic half-space containing a canyon of uniform cross section in which the structure is placed. The solutions for the free-field motions that are due to incident SH, P and SV waves are calculated using the boundary element method.

An analysis of Pacoima (arch) dam located near Los Angeles, California, is performed for both uniform and nonuniform excitations. The important effect of nonuniformities in the free-field motions, sometimes leading to a decrease in the dam response and sometimes to an increase, is quantified.