A new fully nonisothermal coupled model for the simulation of ice sheet flow

A shallow ice thermocoupled model for the complex nonlinear polythermal ice sheet dynamics is proposed and solved by means of efficient numerical methods. A novelty is the obstacle problem formulation associated to a nonlinear integro-differential equation (with nonlocal temperature dependent coefficients) for the ice sheet profile. This formulation is motivated by the free boundary feature and the influence of the temperature on the profile (fully nonisothermal model). Concerning the temperature equation, a dynamically prescribed surface temperature, obtained from an Energy Balance model corrected by the altitude effect, is posed. As the profile and temperature equations are fully coupled, a nonlinear PDE system governing the upper ice sheet profile, the velocity field, the temperature and the basal stress is stated. In addition to the numerical difficulties associated to the new profile equation, several techniques have been considered for the numerical solution of the temperature, velocity and basal magnitudes. Discussions concerning the nonlinear dynamics of the different involved magnitudes and the improvement in their computed values with respect to previous works are also presented.     

液态金属自由表面膜流MHD效应的数值模拟

对液态金属自由表面膜流在强磁场下的磁流体力学效应进行了数值模拟研究,获得了液态金属自由表面的形状、截面流速分布及截面上的电动势分布,从而能对膜流的一些磁流体动力学行为作出解释。数值计算结果与理论分析和实验结果符合较好。由实验和数值模拟结果可以得出,液态金属膜流通过强磁场时,磁场会阻碍膜流的运动。     

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分析了磁流体力学效应对液态金属自由表面射流稳定性的影响。从射流的感应电势、电流、速度等方面,解释射流在磁场中稳定的原因。数值计算结果验证了理论分析。     

Numerical simulation of bubble dynamics in a Phan-Thien–Tanner liquid: Non-linear shape and size oscillatory response under periodic pressure

We study non-linear bubble oscillations driven by an acoustic pressure with the bubble being immersed in a viscoelastic, Phan-Thien–Tanner liquid. Solution is provided numerically through a method which is based on a finite element discretization of the Navier–Stokes flow equations. The proposed computational approach does not rely on the solution of the simplified Rayleigh–Plesset equation, is not limited in studying only spherically symmetric bubbles and provides coupled solutions for the velocity, stress fields and bubble interface. We present solutions for non-spherical bubbles, with asphericity being addressed by means of Legendre polynomials or associated Legendre functions. A parametric investigation of the bubble dynamical oscillatory response as a function of the fluid rheological properties shows that the amplitude of bubble oscillations drastically increases as liquid elasticity (quantified by the Deborah number) increases or as liquid viscosity decreases (quantified by the Reynolds number). Extensive numerical calculations demonstrate that increasing elasticity and/or viscosity of the surrounding liquid tend to stabilize the shape anisotropy of an initially non-spherical bubble. Results are shown for pressure amplitudes 0.2–2 MPa and Deborah, Reynolds numbers in the intervals of 1–8 and 0.094–1.256, respectively.     

A volume of fluid approach for crystal growth simulation

A new approach to simulating the dendritic growth of pure metals, based on a recent volume of fluid (VOF) method with PLIC (piecewise linear interface calculation) reconstruction of the interface, is presented. The energy equation is solved using a diffuse-interface method, which avoids the need to apply the thermal boundary conditions directly at the solid front. The thermal gradients at both sides of the interface, which are needed to obtain the front velocity, are calculated with the aid of a distance function to the reconstructed interface. The advection equation of a discretized solid fraction function is solved using the unsplit VOF advection method proposed by López et al. [J. Comput. Phys. 195 (2004) 718–742] (extended to three dimensions by Hernández et al. [Int. J. Numer. Methods Fluids 58 (2008) 897–921]), and the interface curvature is computed using an improved height function technique, which provides second-order accuracy. The proposed methodology is assessed by comparing the numerical results with analytical solutions and with results obtained by different authors for the formation of complex dendritic structures in two and three dimensions.     

CICC导体轧制过程有限元模拟

利用显式动力学有限元分析软件ANSYS/LS-DYNA对CICC导体的轧制过程进行了模拟,得到了导体在不同轧制道次中所需的轧制力以及轧制道次对导体形状和残余应力的影响。轧制力的计算结果与参考的实验数据吻合,能为导体轧制成型设备的研制提供依据。     

自由上浮气泡运动特性的光滑粒子流体动力学模拟

基于虚功原理, 在Hu X Y等和Grenier N等的研究结果基础上推导了多相流光滑粒子流体动力学(smoothed particle hydrodynamics, SPH)控制方程, 采用精度较高的黏性力和表面张力模型, 发展了一套适用于具有大密度比和大黏性比界面的多相流SPH方法. 首先, 通过施加人工位移修正, 适当背景压力和异相界面力, 使得计算全程粒子分布相对均匀, 改善了界面处的失稳现象, 防止了异相界面处粒子的非物理性穿透; 在此基础上, 利用方形流体团振荡模型对表面张力模型进行了验证, 数值结果与解析解甚为吻合; 然后采用上浮气泡经典数值算例对比研究了不同黏性力计算方法、不同核函数的适用性以及人工位移修正的效果; 最后, 对单个气泡的上浮、变形、撕裂以及垂向两个气泡的追赶、融合等现象进行了模拟, 初步揭示了气泡上浮过程中各种有趣物理现象的细节过程和动力学机理.     

低温气液两相流数值计算

低温工质两相流动在很多场合都存在,研究深冷剂在两相流动时参数的分布有着很重要的作用.文中采用一维均相流模型对液氮循环流动过程进行稳态数值模拟,得到了循环流动过程中液氮含汽率、压力和温度沿管程方向的分布情况,并对由于不同回路条件引起的不同结果进行了分析.     

ITER聚变装置重力支撑静应力数值模拟

应用ANSYS软件对ITER聚变装置重力支撑结构进行了静应力有限元分析,模拟了重力支撑在工作过程中多种工况的应力变化情况。静应力计算结果表明,支撑结构各部件的应力低于材料的许用应力极限,因此支撑结构,尤其是21块韧性板部件能在各种工况下安全运行。     

大气压射频氧气放电特性的数值模拟研究

基于漂移扩散近似,对大气压下氧气射频放电产生的等离子体建立了一维流体模型,包括了电子、正离子和负离子的连续性方程、电子能量方程及泊松方程。采用有限差分法对所建立的模型进行了自洽的数值模拟,得到了相应的数值结果。通过对所得数值结果的分析,研究了大气压下氧气放电的基本特性。研究表明,大气压氧气放电中等离子体密度可达到10¹²cm^-3 ,电子温度约在1.23eV,放电中主要的负离子是O^-离子;随着电压峰值的增加,电子密度、电子温度都增加,但n_O-/n_e 的比率先增加到7.5%又后减小到5%,在电压峰值为700V 时达到最大。     

随机填充增殖剂球床内载气流动特性数值模拟

固态氚增殖包层是聚变堆及聚变-裂变混合堆产氚包层的重要候选结构之一,其球床通道内载气流动特性将影响氚提取效率。利用离散元方法（DEM）生成随机填充增殖剂球床,通过径向孔隙率分布验证其合理性,计算流体力学（CFD）模拟计算其通道内气体流场特征。模拟得到：球床内吹扫氦气流速随孔隙率波动并随入口流速增大而均匀增大,通道内氦气流向及流速变化显著,Blake-Kozeny方程可良好预测该随机填充球床通道压降。     

近海平面大气湍流对光波传输影响的数值模拟

针对近海平面大气湍流的特点,采用多相位屏法,对平面波在不同湍流强度下的近海平面的传播进行了数值模拟。对最终的能量分布与在真空中传播相同距离的传播结果进行了比较。结果表明,湍流对光波强度的影响显著,且湍流强度越大,光波强度起伏越大。     

Numerical simulation on partial coalescence of a droplet with different impact velocities

Partial coalescence is a complicated flow phenomenon. In the present study, the coalescence process is simulated with the volume of fluid(VOF) method. The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence. The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet turning into a prolate shape and the final pinch-off. The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off. With the droplet impact velocity increasing, the droplet shape will get close to a sphere before the pinch-off. When the shape gets close enough to a sphere, the partial coalescence shifts to full coalescence. The effect of film thickness on the coalescence process is also investigated. With large film thickness,partial coalescence happens, while with small film thickness, full coalescence happens. In addition, the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient but decreases with the increase of viscosity and initial droplet diameter. And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.     

Numerical simulation of Bessel beams by FDTD employing the superposition principle

Finite difference time domain (FDTD) method is adopted to build a Bessel beams simulation model according to homogeneousness and linearity of the Maxwell equations in source-free region. Validation for this model is confirmed by comparing the simulation results with the theoretical results solved with a vector Helmholtz equation in free space and good agreement with maximum error 2% has been demonstrated. It is indicated that FDTD could be an effective approach to analyze other complicated models of Bessel beams in source-free region by means of superposition principle.     

阴极金属微凸起电场增强因子数值模拟

 利用有限元方法对无穷大平板二极管中阴极表面金属微凸起的微观电场增强因子进行了计算研究,给出了微凸起微观电场增强因子随凸起参数的变化规律,拟合得到了微观电场增强因子的简单实用的经验表达式,并与文献中的近似公式进行了比较。模拟的结果表明,对锥状球头微凸起的电场增强因子,文献给出的近似公式误差较大,应避免使用。     

表面垂直裂痕诱发瑞利波散射的数值分析

激光激发的声表面波为材料表面缺陷的检测提供了有力的工具.针对含缺陷材料在模型边界上的复杂性,建立了基于平面应变的有限元模型并选取了相同厚度但含有不同深度的表面裂痕的单层铝板进行了对比计算,得到了声表面波经过不同深度的表面裂痕时产生的反射及透射信号波形的时域特征.进而引入了基于Wigner-Ville分布理论的时-频分析方法计算裂痕前、后散射的瞬态表面波的能量在时间-频率平面内分布的情形.结果显示：声表面波接近中心频率的某一频率成分在经过深度小于其中心波长的表面缺陷时,随着裂痕深度的增加,对应于该频率的反射系数呈现单调递增的趋势;而透射系数呈现递减的特征,这一结果可以为激光超声检测表面缺陷提供一种定量的表征手段.     

Numerical simulation of the ultrasonic waves generated by ring-shaped laser illumination patterns

The generation of ultrasound in aluminum plate subjected to ring-shaped laser beam illumination has been studied quantitatively by using the finite element method. The superposition effects of surface acoustic waves on the top surface and the bulk ultrasonic waves on the rear surface of specimen have been obtained in a single simulation. The typical displacement profiles of the bulk ultrasonic wave at various depths along the central axis of the ring are obtained for three different radii, and the effect of the ring radius on the focal depths of the compression and shear mode are determined. The numerical results confirm that the focal depth of a bulk acoustic mode is determined by the directivity patterns of the acoustic mode generated by point-like laser sources via a thermoelastic mechanism, which depends on the physical constants of elastic medium.     

低温冷冻靶温度动态特性的数值模拟研究

惯性约束聚变点火成功的关键之一在于靶丸内形成均匀的氘氚冰层,靶丸周围的温度场对冰层质量有很大影响.首先通过实验靶系统实验验证了数值计算模型的可靠性,在此模型的基础上,对低温冷冻靶装置的热物理问题特别是温度动态特性问题展开了数值模拟,重点考察冷环温度波动时,温度传递衰减过程的规律以及各影响因素对于温度传递衰减过程的影响.结果显示:冷环温度一定时,填充气体压力降低、填充气体中氦气比例增大,靶丸表面温度均匀性提高;当冷环温度波动时,温度波动的周期减小、振幅减小、填充气体压力升高、填充气体中氦气比例降低有利于控制靶丸表面温度波动;冷环温度波动的周期适中、振幅减小、填充气体压力降低、填充气体中氦气比例提高有利于改善靶丸表面温度均匀性.研究结果对实验中冷冻靶合理配置各参数实现温度控制具有重要参考价值.     

超声速流场条件下激光辐照耦合效应数值模拟

激光辐照结构物包含复杂的多物理场耦合问题,其存在流、热、固多种机制的耦合效应。结合计算流体力学(CFD)和有限元方法,对超声速条件下的激光辐照平板问题进行了热流固耦合分析。采用CFD方法得到平板附近流场分布,利用有限元方法计算平板的温度分布,并将二者结合起来实现流体和固体间的数据交互。理论分析确定了流场效应的最主要影响参数为来流马赫数与攻角。对于不同马赫数,激光区域在6 Ma条件下存在温度的谷值,小于等于6 Ma条件下主要体现为冷却效应,而6 Ma以上主要体现为气动加热效应。攻角增大会导致激光区流体质量流量的增加,使冷却效应更加明显。最后综合分析了流场气动加热和冷却两种效应的产生机制。     

外加磁场下真空沿面闪络特性数值模拟

以单粒子模型和带电粒子运动方程为基础,采用蒙特卡罗方法,编写了真空沿面闪络过程计算程序,研究了外加磁场对真空沿面闪络过程的影响,主要是对二次电子发射及电子束的雪崩运动的影响。研究表明：外加磁场的存在,改变了绝缘体表面电子的运动,进而影响到绝缘体表面电荷的分布,从而在宏观上对绝缘体的耐受电压产生影响;外磁场抑制真空沿面闪络的效果与磁场的空间分布有关,磁场加在阴极附近时产生的效果优于加在阳极附近。