虚拟手术中肝脏形变仿真系统

由于肝脏具有复杂的生物力学特性,因此在进行形变仿真时,计算量大而难以达到实时性要求;同时在对肝脏形变实时仿真时却很难达到真实感.为了解决这一矛盾,需要研究一种可以自动依据手术器械操控区域及施力大小而动态转换的混合模型,通过理论分析提出了一种肝脏网格与无网格混合模型的构建方法.实验结果表明,该算法具有较高的计算效率且形变具有较好效果,能够满足虚拟肝脏手术仿真的真实感与实时性要求,对虚拟肝脏软组织手术研究具有一定的指导意义.     

具有Levy变异和精英自适应竞争机制的蚁狮优化算法

针对蚁狮优化算法易陷入局部最优、收敛速度慢的缺点,本文提出一种具有Levy变异和精英自适应竞争机制的蚁狮优化算法。利用服从Levy分布的随机数对种群较差个体进行变异,可改善种群多样性提高算法的全局搜索能力;精英自适应竞争机制使得多个精英并行带领种群寻优,提高了算法的收敛速度,为避免较大计算量,并行竞争的精英个数会随着寻优代数增加而减少。同多个改进算法进行比较,结果表明本文所提算法具有更好的寻优精度和收敛速度。最后将本文改进算法应用于硅单晶热场温度模型的参数辨识,仿真结果说明该算法具有较好的参数辨识能力。     

Rapid Simulation of Laser Processing of Discrete Particulate Materials

The objective of this paper is to develop a computational model and corresponding solution algorithm to enable rapid simulation of laser processing and subsequent targeted zonal heating of materials composed of packed, discrete, particles. Because of the complex microstructure, containing gaps and interfaces, this type of system is extremely difficult to simulate using continuum-based methods, such as the Finite Difference Time Domain Method or the Finite Element Method. The computationally-amenable model that is developed captures the primary physical events, such as reflection and absorption of optical energy, conversion into heat, thermal conduction through the microstructure and possible phase transformations. Specifically, the features of the computational model are (1) a discretization of a concentrated laser beam into rays, (2) a discrete element representation of the particulate material microstructure and (3) a discrete element transient heat transfer model that accounts for optical (laser) energy propagation (reflection and absorption), its conversion into heat, the subsequent conduction of heat and phase transformations involving possible melting and vaporization. A discrete ray-tracking algorithm is developed, along with an embedded, staggered, iterative solution scheme, which is needed to calculate the optical-to-thermal conversion, particle-to-particle conduction and phase-transformations, implicitly. Numerical examples are given, focusing on concentrated laser beams and the effects of surrounding material conductivity, which draws heat away from the laser contact zone, thus affecting the targeted material state.     

环形激光陀螺的零偏热效应补偿模型

热效应对激光陀螺零偏的影响非常明显且难以精确建模。根据对环形激光陀螺的大量温度试验的数据,分析了温度变化、温度梯度与温变速率对陀螺零偏漂移影响的规律,进而提出了一种适用于工程应用的环形激光陀螺的零偏热效应补偿方法。经试验验证,此模型能在一定程度上改善热效应对环形激光陀螺零偏稳定性的影响,为建立最后的误差补偿模型,进一步提高环形激光陀螺的精度打下基础。     

三轴一体IMU组件中光纤环热分析

对某型号三轴一体光纤陀螺捷联惯导系统建立有限元模型,从结构角度分析了惯性测量单元（ IMU）中光源和加速度计等发热模块对光纤环温度场分布的影响。分析研究IMU组件在22℃常温稳态下的传热规律,表明光源与加速度计等热源所产生热量将不以传导方式在箱体与 IMU台体之间传递,对流与辐射传热对IMU温度分布影响较大;光源为主要热源,是造成Y,Z轴光纤环温度分布不均匀的主要原因;加速度计发热将影响X轴光纤环温度分布。通过＋60℃高温瞬态热分析,研究光纤环在极端环境下温度变化规律,表明系统在极端环境下随着温度上升而温度梯度递减,光纤环瞬态温差增大。稳态和瞬态热分析可指导惯导系统IMU部分结构热设计的改进。     

Simulation and fabrication for pin-to-plate microjoining by Nd:YAG laser

This paper reports the behavior of pin-to-plate microjoining using the pulsed Nd:YAG laser in the approaches of numerical simulation and metallurgical analysis. A three-dimensional pin-to-plate finite element model has been developed to analyze the transient temperature field and thermal stress distribution during the fabrication process of the laser joint to find a better joining condition. The thermal history of the plate was recorded during the joining process to verify the result of the finite element approach. Metallurgical analysis was done to understand microstructure transformation after joining. The result shows that the micro-pin can be successfully joined to the thin plate in the applied condition using CF50-2 commercial filler alloy. The numerical result is also in good agreement with experimental measurements after joining fabrication.     

Simulating the laser micromachining of a 3D flexible structure

My work is to improve the generation of 3-D flexible structures by laser micromachining. The purpose is to extract the material by an ablation of matter in order to achieve “V” shape for multi-bend structure. This multi-bend part will be the key for 3-D structures. For that, a frequency multiplied Nd:YAG laser provided with a marking trigger and equipped with a galvanometric head was used. For optimizing the laser irradiation time and power a thermal modeling of the laser matter interaction for a continuous laser impact case is presented. The paper models numerically the localized three-dimensional temperature distribution in a polyimide caused by a moving Gaussian laser beam by Finite element method (FEM) analysis. The beam’s penetration depth, which can be described with an absorption coefficient, depends on the ambient temperature. The model examines the penetration depth and the influences of the laser motion on the transient temperature distribution. As the maximum temperature is mesh dependent we decided to choose 10 μm initial mesh with 1.01 mesh grow, so the laser beam diameter occurs 20 μm. The overall heat flux and temperature distribution on a macroscopic level are both accurate.     

Thermal conductivity measurement of liquids in a microfluidic device

A new microfluidic-based approach to measuring liquid thermal conductivity is developed to address the requirement in many practical applications for measurements using small (microlitre) sample size and integration into a compact device. The approach also gives the possibility of high-throughput testing. A resistance heater and temperature sensor are incorporated into a glass microfluidic chip to allow transmission and detection of a planar thermal wave crossing a thin layer of the sample. The device is designed so that heat transfer is locally one-dimensional during a short initial time period. This allows the detected temperature transient to be separated into two distinct components: a short-time, purely one-dimensional part from which sample thermal conductivity can be determined and a remaining long-time part containing the effects of three-dimensionality and of the finite size of surrounding thermal reservoirs. Identification of the one-dimensional component yields a steady temperature difference from which sample thermal conductivity can be determined. Calibration is required to give correct representation of changing heater resistance, system layer thicknesses and solid material thermal conductivities with temperature. In this preliminary study, methanol/water mixtures are measured at atmospheric pressure over the temperature range 30–50°C. The results show that the device has produced a measurement accuracy of within 2.5% over the range of thermal conductivity and temperature of the tests. A relation between measurement uncertainty and the geometric and thermal properties of the system is derived and this is used to identify ways that error could be further reduced.     

Nonlinear thermal system identification using fractional Volterra series

Linear fractional differentiation models have already proven their efficacy in modeling thermal diffusive phenomena for small temperature variations involving constant thermal parameters such as thermal diffusivity and thermal conductivity. However, for large temperature variations, encountered in plasma torch or in machining in severe conditions, the thermal parameters are no longer constant, but vary along with the temperature. In such a context, thermal diffusive phenomena can no longer be modeled by linear fractional models. In this paper, a new class of nonlinear fractional models based on the Volterra series is proposed for modeling such nonlinear diffusive phenomena. More specifically, Volterra series are extended to fractional derivatives, and fractional orthogonal generating functions are used as Volterra kernels. The linear coefficients are estimated along with nonlinear fractional parameters of the Volterra kernels by nonlinear programming techniques. The fractional Volterra series are first used to identify thermal diffusion in an iron sample with data generated using the finite element method and temperature variations up to 700 K. For that purpose, the thermal properties of the iron sample have been characterized. Then, the fractional Volterra series are used to identify the thermal diffusion with experimental data obtained by injecting a heat flux generated by a 200 W laser beam in the iron sample with temperature variations of 150 K. It is shown that the identified model is always more accurate than the finite element model because it allows, in a single experiment, to take into account system uncertainties.     

Estimating soil thermal properties from sequences of land surface temperature using hybrid Genetic Algorithm–Finite Difference method

Most models used in land surface hydrology, vadose zone hydrology, and hydro-climatology require an accurate representation of soil thermal properties (soil thermal conductivity and volumetric heat capacity). Various empirical relations have been suggested to estimate soil thermal properties. However, they require many input parameters such as soil texture, mineralogical composition, porosity and water content, which are not always available from laboratory experiments and field measurements. In this paper, to overcome the above challenge, a hybrid numerical method, Genetic Algorithm–Finite Difference (GA–FD), is proposed to estimate soil thermal properties using land surface temperature (LST) as the only input. The genetic algorithm (GA) optimization method coupled with the finite difference (FD) modeling technique is a viable hybrid approach for estimating soil thermal properties. The finite difference method is employed to solve the heat diffusion equation and simulate LST, while a robust optimization technique (GA) is used to retrieve soil thermal properties by minimizing the difference between observed and simulated LST. Furthermore, a generalization of the hybrid model is developed for inhomogeneous soil, in which soil thermal properties are not constant throughout the soil slab. The proposed model is applied to the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). The results show that the proposed hybrid numerical method is able to estimate soil thermal properties accurately, and therefore effectively eliminate the need for the unavailable soil parameters which are required by empirical methods for determining the soil thermal conductivity and volumetric heat capacity. Remarkably, the temporal variation of the retrieved soil thermal conductivity is consistent with the volumetric water content, even though no water content information is used in the model.