Discrete time transfer matrix method for dynamics of multibody system with flexible beams moving in space

In this paper, by defining new state vectors and developing new transfer matrices of various elements mov- ing in space, the discrete time transfer matrix method of multi-rigid-flexible-body system is expanded to study the dynamics of multibody system with flexible beams moving in space. Formulations and numerical example of a rigid- flexible-body three pendulums system moving in space are given to validate the method. Using the new method to study the dynamics of multi-rigid-flexible-body system mov- ing in space, the global dynamics equations of system are not needed, the orders of involved matrices of the system are very low and the computational speed is high, irrespec- tive of the size of the system. The new method is simple, straightforward, practical, and provides a powerful tool for multi-rigid-flexible-body system dynamics.

Study of heat transfer by using DEM–CFD method in a randomly packed pebble-bed reactor

The pebble-bed reactor is one of the most promising designs for the nuclear energy industry. In this paper,a discrete element method-computational fluid dynamics(DEM-CFD) approach that includes thermal conduction, radiation, and natural convection mechanisms was proposed to simulate the thermal-fluid phenomena after the failure of forced circulation cooling system in a pebble-bed core. The whole large-scale packed bed was created using the DEM technique, and the calculated radial porosity of the bed was validated with empirical correlations reported by researchers. To reduce computational costs, a segment of the bed was extracted, which served as a good representative of the large-scale packed bed for CFD calculation. The temperature distributions simulated with two different fluids in this DEM-CFD approach were in good agreement with SANA experimental data. The influence of the natural convection mechanism on heat transfer must be taken into account for coolants with strong convective capacity. The proposed DEM-CFD methodology offers a computationally efficient and widely applied method for understanding the heat transfer process in a pebble-bed core. The method can also be easily extended to assess the passive safety features of newly designed fluoride-salt-cooled pebble-bed reactors.

Numerical modelling of resonance mechanisms in jointed rocks using transfer functions

Resonance effects in parallel jointed rocks subject to stress waves are investigated using transfer functions,derived from signals generated through numerical modelling.Resonance is important for a range of engineering situations as it identifies the frequency of waves which will be favourably transmitted.Two different numerical methods are used for this study,adopting the finite difference method and the combined discrete element-finite difference method.The numerical models are validated by replicating results from previous studies.The two methods are found to behave similarly and show the same resonance effects;one operating at low frequency and the other operating at relatively high frequency.These resonance effects are interpreted in terms of simple physical systems and analytical equations are derived to predict the resonant frequencies of complex rock masses.Low frequency resonance is shown to be generated by a system synonymous with masses between springs,described as spring resonance,with an equal number of resonant frequencies as the number of blocks.High frequency resonance is generated through superposition of multiple reflected waves developing standing waves within intact blocks,described as superposition resonance.While resonance through superposition has previously been identified,resonance based on masses between springs has not been previously identified in jointed rocks.The findings of this study have implications for future analysis of multiple jointed rock masses,showing that a wave travelling through such materials can induce other modes of propagation of waves,i.e.spring resonance.

基于CFD-DEM耦合的梯级溜槽的设计与分析

针对传统物料转运过程中溜槽和输送带磨损严重、出口处粉尘浓度过高的问题,建立含臂架的梯级溜槽几何模型,采用基于计算流体力学与离散单元法(CFD-DEM)耦合的数值模拟方法,分析了臂架对转运溜槽的磨损以及对其出口处粉尘排放浓度的影响。仿真结果表明:含臂架的梯级溜槽可以有效控制物料流的速度和方向,降低对溜槽内表面的冲击磨损,降低出口处的粉尘量。

低载波比牵引系统的感应电机特征根离散化模型研究

在大功率和高速电机驱动领域,电机控制系统将运行于低载波比工况。传统的一阶欧拉、二阶双线性等降阶离散化模型在低载波比下由于离散化误差过大,对应的状态观测将出现幅值和相位的稳态误差,严重时甚至出现发散不收敛现象。针对上述问题,该文提出了感应电机特征根离散化模型。通过构建感应电机的复矢量模型状态空间方程,将满秩的状态转移矩阵进行对角化,得到状态转移矩阵的精确离散化结果,该模型在低载波比时仍具有较高的离散化精度。同时,提出了一种基于伯德图的离散化误差定量分析方法,通过定量对比不同离散化模型和连续域模型之间观测变量的幅值和相位误差,从理论上证明了提出方法的优越性。最后,通过仿真和实验验证了上述感应电机特征根离散化模型在低载波比下均具有良好的稳态精度与暂态跟随性能。

A particle-resolved heat-particle-fluid coupling model by DEM-IMB-LBM

Multifield coupling is frequently encountered and also an active area of research in geotechnical engineering.In this work,a particle-resolved direct numerical simulation(PR-DNS)technique is extended to simulate particle-fluid interaction problems involving heat transfer at the grain level.In this extended technique,an immersed moving boundary(IMB)scheme is used to couple the discrete element method(DEM)and lattice Boltzmann method(LBM),while a recently proposed Dirichlet-type thermal boundary condition is also adapted to account for heat transfer between fluid phase and solid particles.The resulting DEM-IBM-LBM model is robust to simulate moving curved boundaries with constant temperature in thermal flows.To facilitate the understanding and implementation of this coupled model for non-isothermal problems,a complete list is given for the conversion of relevant physical variables to lattice units.Then,benchmark tests,including a single-particle sedimentation and a two-particle drafting-kissing-tumbling(DKT)simulation with heat transfer,are carried out to validate the accuracy of our coupled technique.To further investigate the role of heat transfer in particle-laden flows,two multiple-particle problems with heat transfer are performed.Numerical examples demonstrate that the proposed coupling model is a promising high-resolution approach for simulating the heat-particle-fluid coupling at the grain level.

散煤转运溜槽结构优化设计

针对散煤转运溜槽工作中存在的积料、堵料、偏载、粉尘过大问题展开优化设计。首先建立理论计算模型,进行溜槽概念设计;其次对实际转运散煤特性进行测试,获得固体密度、松散密度、颗粒弹性恢复系数、壁面摩擦角及内摩擦角等相关基本参数。再次借助L箱流动性实验进行离散元参数标定,进而进行散煤流动离散元仿真分析。这种理论计算和离散元分析相互结合的优势,能够快速有效实现散煤转运溜槽的结构优化设计,从而提高散煤输送能力,同时从根源上减少冲击粉尘的产生。最后通过工程应用验证了该方法的合理性和有效性。

Three-dimensional heat transfer in a particulate bed in a rotary drum studied via the discrete element method

We simulated three-dimensional heat transfer inside a horizontal rotating drum using the discrete element method and a thermal conduction model.The aim was to determine the effect of end-wall heating on thermal behavior of a granular bed.The simulation showed that the end-wall heating significantly affects the axial temperature profile of the bed,particularly when the length-to-diameter ratio is low.Particles near the wall heated faster and became more thermally uniform than those in the center of the drum.The region affected by the end heating gradually increased over time.Increasing the rotation speed enhanced the heat conduction rate,and increasing the fill level reduced the mean temperature and thermal uniformity of the granular bed.Heat transfer was also simulated for drums with different length-to-diameter ratios.

Numerical Solution of MHD Flow of Micropolar Fluid with Heat and Mass Transfer towards a Stagnation Point on a Vertical Plate

The paper investigates the numerical solution of problem of magnetohydrodynamic (MHD) micropolar fluid flow with heat and mass transfer towards a stagnation point on a vertical plate. In this study, we consider both strong concentrations (n = 0) and weak concentrations (n = 1/2). The governing equations have been transformed into nonlinear ordinary differential equations by applying the similarity transformation and have been solved numerically by using the finite difference method (FDM) and analytically by using (DTM). The effects of various governing parameters, namely, material parameter, radiation parameter, magnetic parameter, Prandtl number, Schmidt number, chemical reaction parameter and Soret number on the velocity, microrotation, temperature and concentration have been computed and discussed in detail through some figures and tables. In order to verify the accuracy of the present results, we have compared these results with the analytical solutions by using the differential transform method (DTM) and the multi-step differential transform method (MDTM). It is observed that this approximate numerical solution is in good agreement with the analytical solution.

鼓泡流态化气泡间相互作用下相间传质过程的模拟

气泡-乳化相相间传质行为是流化床反应器设计与调控的关键问题,气泡间的相互作用增加了相间传质过程的复杂性。本文利用CFD-DEM方法对三维射流鼓泡床内垂直分布的双气泡传质过程展开模拟,基于建立的对流-扩散传质系数独立确定方法,分析了气泡存在相互作用时不同传质机制的演变过程,对比了双气泡与单气泡传质过程的差异性。研究结果表明,与单气泡相比,上方气泡的存在会使得下方气泡轴向拉伸,减小了下方气泡体积并提高了气泡上升速度。对相间传质过程而言,上方气泡浓度的释放增加了下方气泡的扩散传质阻力,且使其出现逆向对流传质现象。不同入射时间间隔也会进一步影响传质过程,气泡复杂的传质过程将是气泡自身气流特性、形状演变与气泡间浓度干扰等多因素作用的结果。

A numerical and experimental comparison of heat transfer in a quasi two-dimensional packed bed

Heat transfer plays a major role in many industrial processes taking place in packed beds.An accurate and reliable simulation of the heat exchange between particles is therefore crucial for a reliable operation and to optimize the processes in the bed.The discrete ordinates method(DOM)provides an established numerical technique to model radiative heat transfer in granular media that offers the possibility to consider the directional dependence of the radiation propagation.In this work,DOM is compared with Monte Carlo ray tracing,which provides an alternative method for heat transfer simulations.Geomet-rically simple configurations are used to investigate the influence of the angular discretization on the accuracy of the results and the computation time in both methods.The obtained insights are then transferred to a more complex configuration of a quasi two-dimensional test rig consisting of metal rods for which also experimental results are available.Our results show that both DOM and Monte Carlo ray tracing allow for an accurate simulation of heat transfer in packed beds.Monte Carlo ray tracing requires thereby computation times that are surprisingly competitive(although still somewhat slower)compared to DOM and allows for an easier computation of highly accurate reference solutions.In our preliminary comparison to the experimental test rig,Monte Carlo ray tracing also provides the advantage that it can more easily model highly specular materials such as stainless steel.Both methods are comparable for diffuse materials such as magnesium oxide.

INTENSITY CALCULATION IN STRONGLY ANISOTROPICALLY SCATTERING ATMOSPHERE WITH DISCRETE ORDINATE METHOD

The problems in some previous discrete ordinate method computations of radiative transfer are discussed, and a new manipulation is presented, which overcomes the difficulties in intensity calculations with the discrete ordinate method, and can be used to obtain accurate intensities with less streams. Even in strongly anisotropically scattering atmospheres the intensities in all directions including forward and backward directions of the direct radiation can be obtained satisfactorily.

Discrete Duality Finite Volume Discretization of the Thermal-P_(N) Radiative Transfer Equations on General Meshes

The discrete duality finite volume method has proven to be a practical tool for discretizing partial differential equations coming from a wide variety of areas of physics on nearly arbitrary meshes.The main ingredients of the method are:(1)use of three meshes,(2)use of the Gauss-Green theorem for the approximation of derivatives,(3)discrete integration by parts.In this article we propose to extend this method to the coupled grey thermal-P_(N) radiative transfer equations in Cartesian and cylindrical coordinates in order to be able to deal with two-dimensional Lagrangian approximations of the interaction of matter with radiation.The stability under a Courant-Friedrichs-Lewy condition and the preservation of the diffusion asymptotic limit are proved while the experimental second-order accuracy is observed with manufactured solutions.Several numerical experiments are reported which show the good behavior of the method.

炉膛辐射换热计算的一种改进的离散传递法

在Lockwood和Shah于1981年提出的离散传递法中,求解沿特征射线的辐射传递方程时没有将入射散射项计入。基于作者最近提出的带微小加权离散坐标格式,文中提出了一种改进的离散传递法:首先采用一个常规离散坐标系列来求解辐射强度场,然后再由此强度场向特征射线的辐射传递方程提供源函数(包括入射散射项)。这样,改进的离散传递法就可以处理有散射的多维辐射问题。最后采用2个二维测试问题对这一方法进行了验证,证明该法能适用于任何场合。

辐射离散传播法在三维圆柱腔体辐射传热计算中的应用

运用空间解析几何理论与数值计算相结合的方法,实现了辐射离散传播法(DTM)在三维圆柱腔体内辐射传热计算的应用。采用坐标转换建立了辐射射线方程,通过直接求解所有发射点上各立体角内的辐射射线与各辐射单元体的交点,确定射线经过的路径及各交点与发射点的距离,然后按距离远近对交点进行排序,得到适合DTM法求解辐射能量传递方程的交点顺序。运用该方法对圆柱腔体内辐射换热进行三维计算,得到与精确解基本相符的结果;将DTM法运用于煤粉燃烧火焰辐射换热的计算,得到的温度场与实验结果基本一致,表面辐射热流密度分布合理,由此表明本文设计的方法是可行的。

求解辐射传递方程的离散坐标法

阐述了含吸收散射性介质三维空腔内辐射传递方程的离散坐标解法。讨论了入射散射项积分格式的选取，以及假散射和射线效应对解精度的影响。对三维矩形炉膛内辐射传递过程进行了数值模拟，并与区域法和离散传递法进行比较。比较结果表明离散坐标法具有较好的精度，是目前燃烧室内辐射传热过程数值模拟的一种较好的方法。

基于离散单元法的丝状颗粒传热数学模型

丝状颗粒作为一类长径比较大的非球形颗粒,其传热特性及相关技术广泛应用于工农业生产的诸多领域。但目前颗粒在运动过程中传热问题的研究还很不充分,特别是对于丝状颗粒,更是缺乏有效的数学模型进行描述。从颗粒传热机理出发,提出了一种基于离散单元法的丝状颗粒传热模型,模型中综合考虑了颗粒碰撞(接触)传热、颗粒的内部导热以及颗粒与气体间的对流换热。利用该模型,对固定床中堆积丝状颗粒的热量迁移过程进行了数值模拟,着重比较了各种传热方式对传热过程的影响。研究表明,对流换热对整体传热量的贡献较大。此外,还获得了不同工况下颗粒温度随时间的变化规律。

基于颗粒尺度的离散颗粒传热模型

颗粒间传热在诸多工业过程中有着十分重要的作用。详细考虑颗粒间传热机理,对颗粒间各传热途径建模,包括颗粒内部导热、颗粒粗糙表面传热、颗粒表面气膜及接触颗粒间隙气膜传热,并与离散颗粒模型(DEM)耦合,建立颗粒尺度下离散颗粒传热模型。以固定床为对象,考察颗粒粒径、颗粒比热容、颗粒热导率及压缩负载对固定床有效传热系数的影响,并将本文计算值和文献的实验值及模型预测值对比,结果表明,该模型可定量预测固定床有效传热系数。本文建立的离散颗粒传热模型为合理预测颗粒体系内的传热提供了一种有效方法。

铝合金厚板淬火表面换热系数的离散解析求法

为了快速准确求取铝合金厚板淬火过程的换热系数,对淬火热传导过程进行分析。首先,将换热系数解析过程假设为淬火温度离散化的,并且是相邻离散点可进行迭代优化的计算过程。然后,分步解析求解了各离散温度区间的换热系数,最后完成了数据修正和仿真计算还原。结果表明,该方法获得的换热系数,可以使实验冷却曲线与计算冷却曲线较好的吻合,从而证明这种计算方法的可行性,并在文末对该方法的误差来源和特点进行了分析。

飞行器后机身蒙皮红外辐射特性(英文)

随着红外探测和红外制导技术的迅猛发展,飞行器的生存力受到越来越大的威胁,如何降低飞行器的红外辐射特征成为提高隐身能力的关键技术之一,排气系统是飞行器后半球的主要红外辐射源。为了研究飞行器后机身蒙皮的红外辐射特性,针对飞行器的后机身蒙皮,结合涡扇发动机引射喷管的CFD计算,采用离散传递法计算了典型作战飞行器在8～14μm波段的红外辐射强度分布。在红外计算过程中,考虑了发动机固体壁面的发射和反射,飞机蒙皮的发射,燃气内CO2、H2O和CO的吸收与发射作用;并通过冷却、隔热等措施降低后机身发动机热影响区蒙皮的温度对红外辐射特征的影响进行了研究。计算结果表明:后机身发动机热影响区温度降低70 K,在红外探测的主要威胁方位头向5°范围内可以降低整机红外辐射强度9.1%,30°可以降低15.4%,60°可以降低17.5%,在正下方90°,可以降低18.9%;在此基础上采用低反射率可以在各个方向上进一步降低飞机的红外辐射强度,除了正后向及附近排气系统喷管腔体影响强烈扇区间以外,都能降低到20%以内。计算结果和方法为飞行器红外隐身设计提供了有益参考。