Analytical solutions for equivalent elastic compliance of cubic lattice structures subjected to hypergravity conditions

In order to comprehensively understand the mechanical behavior of biological entities and aerospace applications subjected to hypergravity environments,we delve into the impact of hypergravity on the equivalent compliance of cubic lattice structures.Capitalizing on the periodic spatial distribution,we employ a unit cell methodology to deduce the homogenized stress-strain relationship for the lattice structures,subsequently obtaining the associated equivalent compliance.The equivalent compliance can be conveniently reduced to instances without hypergravity influence.Furthermore,numerical simulations are executed to validate the derivations and to illustrate that hypergravity indeed affects the mechanical properties of lattice structures.We introduce a non-dimensional hypergravity factor,which quantifies the impact of hypergravity magnitude relative to the Young’s modulus of the base material.Our findings reveal that the hypergravity factor influences perpendicular compliance quadratically and parallel compliance linearly.Simultaneously,the perpendicular shear compliance remains unaffected,whereas the parallel shear compliance experiences an inverse effect.Additionally,the lattice structure transforms into a gradient material oriented in the hypergravity direction,consequently generating a scale effect.

Study on the influence of pneumatic probe on the wake flow field of transonic turbine cascade

The pneumatic probe is widely used for contact measurements in turbomachinery flow field research.However,it inevitably interferes with the original flow field,leading to additional errors,particularly in wake flow fields or transonic regions with significant pressure gradients.This study employed Reynolds-Averaged Navier-Stokes delete and high-fidelity numerical simulation to investigate the impact of an inserted pneumatic probe on the wake flow field of a transonic turbine blade and compared it to the baseline flow field.Results indicate that the probe causes the shock waves premature occurrence in the high subsonic wake region near the turbine blade trailing edge.These shock waves affect vortex shedding by thickening the boundary layer near the trailing edge and changing the shedding pattern from high-frequency-low-energy to low-frequencyhigh-energy.In addition,the extra flow loss is incurred,and the blade's heat transfer characteristic is changed.This research provides a reference for testing experiments in complex transonic flow fields,guiding experimental researchers to minimize instrument interference with the original flow field.

CORC电缆中超导带材在缠绕、弯曲和扭曲变形下的应变分析

CORC(conductor on round core)超导电缆是一种基于REBCO(rare-earth barium copper oxide)导体的高温超导电缆,已经成为迄今为止最灵活的高温超导(HTS)导体之一.然而,由于超导REBCO陶瓷材料的脆性,临界电流表现出敏感的应变依赖性,即临界电流与应变密切相关.该材料的应变一旦超过临界值,就会引发明显的衰减.除此之外,由于带材缠绕和电缆绞合后的复杂变形,切实存在一个实际性的挑战,即如何准确预测电缆超导带材在各种变形下的轴向应变.在本文中,我们建立了分析模型,可以用来准确计算CORC电缆的超导带材在电缆内芯上缠绕、电缆弯曲和扭曲变形等实际条件下的轴向应变.并且通过比较分析结果与有限元数值结果和实验数据,验证了分析模型,即超导带的临界电流随CORC电缆的核心直径而变化.在轴向应变结果的基础上,进一步确定了临界参数,即超导层的轴向应变恰好达到临界应变时的参数,如临界核心直径、临界弯曲直径和临界绞距等.对CORC电缆中REBCO超导带材的应变分析研究将有助于CORC电缆的加工和进一步设计高水平的超导线圈和磁体.

基于物理信息的神经网络方法在梁非线性屈曲分析中的应用

本文提出了一种基于物理信息的神经网络(PINN)框架来分析在Winkler-Pasternak基础上的三维功能梯度FG多孔细长梁的非线性屈曲行为.使用简单的网络,PINNs只需要很少的训练数据来获得较高的精度.在这项工作中使用的强大工具可以处理任何类型的偏微分方程.我们使用深度学习平台TensorFlow和DeepXDE库来设计我们的网络.在本研究中,PINNs框架从梁系统的控制微分方程和边界条件中获取信息,并输出临界非线性屈曲载荷.利用哈密顿原理,考虑几何的非线性,建立了数学模型.通过将建模框架应用于各种边界条件情况以及三维功能梯度FG指标等物理参数对非线性力学行为的影响,仔细检验了建模框架的准确性.最后,利用广义微分求积法(GDQM)对PINNs结果进行了验证.研究结果表明,所提出的PINN框架能够较好地表征三维功能梯度FG多孔细长梁的非线性屈曲行为.此外,PINN预测梁的非线性屈曲行为的速度比数值方法快71倍.

曲线网格下带有自由流和涡流保存特性的低耗散数值方案

本文提出了一种改进的THINC数值方案,用于在曲线网格下保持自由来流和等熵涡特性,其中采用对称保守度量方法来消除坐标转换系数在数值离散中诱导的几何误差.由于非等间距的网格会导致原本带有固定陡度参数的THINC方案的数值精度和计算稳定性降低.因此,本文引入了一种带有自适应陡度参数的THINC方案用于减少数值耗散,其中陡度参数根据网格间隔的大小建立方程实现自适应缩放.数值测试表明,与原始THINC方案相比,新的THINC方案能够有效地在一般结构化曲线网格下保持自由流和涡旋保存,并且能够更加精准地拟合流场中的各类型间断和小尺度结构.此外,通过边界变差最小(BVD)原理,将改进的THINC方案与加权本质非振荡方案(WENO)相结合,兼顾了光滑解和间断解的数值精度.本文在波浪形网格和随机网格下设置了一系列一维和二维数值算例,分别测试了在有限差分和有限体积框架下改进的THINC方法与低耗散的WENO-THINC综合方案的性能.数值验证结果表明改进后的数值方案可以有效地减少非物理振荡和数值耗散,相较于原始方案和同类型数值方案有较明显的优势.

A SHORT BIOGRAPHY

Dr.Yang Lu is currently a"HKU-100 Scholar"Professor in the Department of Mechanical Engineering at The University of Hong Kong(HKU).He received his BS degree in Physics from Nanjing University and PhD degree in Mechanical Engineering from Rice University,and did his postdoctoral research in the Nanomechanics Lab at MIT.During 2012-2022,he worked at City University of Hong Kong from Assistant Professor to Full Professor,and served as Director of Nano-Manufacturing Laboratory(NML)at CityU Shenzhen Research Institute.

Effect of rib height on turbulent flow and heat transfer of kerosene in rectangular duct

The coupling effects of rib heights and fluid properties on turbulent convective heat transfer of kerosene flow through the rectangular duct on the ribbed bottom wall are studied numerically in this paper.The numerical simulation is based on the ten components surrogate model of kerosene and the Reynolds average method combined with the re-normalized group(RNG)k-εturbulence model.The turbulent vortex structures and heat transfer characteristics of kerosene flowing over rectangular ribs of different heights are obtained.The results show that three dimensional vortices are generated by the ribs,and the vortices alter local flow significantly,leading to both enhanced and reduced convective heat transfer at different locations near the ribs.In addition,it is found that with the increase of rib height,the average Nusselt number and the wall friction factor on the ribbed wall also increase.For the present study,the maximum heat transfer enhancement rate of kerosene flow is 72.16%,and the ratio of rib-to-duct height is 0.75.

Time-varying stiffness analysis of double-row tapered roller bearing based on the mapping structure of bearing stiffness matrix

Time-varying stiffness is one of the most important dynamic characteristics of rolling element bearings.The method of analyzing the elements in the bearing stiffness matrix is usually adopted to investigate the characteristics of bearing stiffness.Linear mapping structure of the bearing stiffness matrix is helpful to understand the varying compliance excitation and its influence on vibration transmission.In this study,a method to analyze the mapping structure of bearing stiffness matrix is proposed based on the singular value decomposition of block matrices in the stiffness matrix.Not only does this method have the advantages of coordinate transformation independence and unit independence,but also the analysis procedure involved is geometrically intuitive.The time-varying stiffness matrix of double-row tapered bearing is calculated and analyzed using the proposed method under two representative load cases.The principal stiffnesses and principal axes defined in the method together indicate the dominant and insignificant stiffness properties with the corresponding directions,and the vibration transmission properties are also revealed.Besides,the coupling behaviors between different shaft motions are found during the analysis of mapping structure.The mechanism of the generation of varying compliance excitation is also revealed.

Analysis of antisym metric cross-ply laminates using high-order shear deformation theories:a meshless approach

For many years finite element method(FEM)was the chosen numerical method for the analysis of composite structures.However,in the last 20 years,the scientific community has witnessed the birth and development of several meshless methods,which are more flexible and equally accurate numerical methods.The meshless method used in this work is the natural neighbour radial point interpolation method(NNRPIM).In order to discretize the problem domain,the NNRPIM only requires an unstructured nodal distribution.Then,using the Voronoi mathematical concept,it enforces the nodal connectivity and constructs the background integration mesh.The NNRPIM shape functions are constructed using the radial point interpolation technique.In this work,the displacement field of composite laminated plates is defined by high-order shear deformation theories.In the end,several antisymmetric cross-ply laminates were analysed and the NNRPIM solutions were compared with the literature.The obtained results show the efficiency and accuracy of the NNRPIM formulation.

Survey of the mechanisms of power take-off(PTO)devices of wave energy converters

Ocean wave energy conversion as one of the renewable clean energy sources is attracting the research interests of many people.This review introduces different types of power take-off(PTO)technology of wave energy converters.The novelty of this paper is to present advantages and disadvantages of the linear direct and indirect drive PTO devices for ocean wave energy conversion.The designs and optimizations of PTO systems of ocean wave energy converters have been studied from reviewing the recently published literature.The novel mechanical designs of the PTO systems have been compared and investigated in order to increase the energy harvesting efficiency.

Two-point statistics of coherent structures in turbulent flow over riblet-mounted surfaces

Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.

Bulging intervertebral disc:an asymptotic elasticity solution

A bulging intervertebral disc (IVD) occurs when pressure on a spinal disc damages the once healthy disc,causing it to compress or change its normal shape.In medicine,most attention has been paid clinically to diagnosis of and treatment for such problems,which little effect has been made to understand such issues from a mechanics perspective,i.e.,the bulging deformation of the soft IVD induced by excessive compressive load.We report herein a simple elasticity solution to understand the bulging disc issue.For simplicity,the soft IVD is modeled as an incompressible circular composite layer consisting of an inner nucleus and outer annulus,sandwiched between two vertebral segments which are much stiffer than the IVD and can be treated as rigid bodies.Without adopting any assumptions regarding prescribed displacements or stresses,we obtained the stress and displacement fields within the composite layer when a certain compressive stain is applied via an asymptotic approach.This asymptotic approach is very simple and accurate enough for prediction of the bugling profile of the IVD.We also performed finite-element modeling (FEM) to validate our solutions;the predicted stress and displacement fields inside the composite are in good agreement with the FEM results.

Influences of affiliated components and train length on the train wind

The induced airfl w from passing trains,which is recognized as train wind,usually has adverse impacts on people in the surroundings,i.e.,the aerodynamic forces generated by a high-speed train＇s wind may act on the human body and endanger the safety of pedestrians or roadside workers.In this paper,an improved delayed detached eddy simulation（IDDES） method is used to study train wind.The effects of the affiliate components and train length on train wind are analyzed.The results indicate that the aff liated components and train length have no effect on train wind in the area in front of the leading nose.In the downstream and wake regions,the longitudinal train wind becomes stronger as the length of the train increases,while the transverse train wind is not affected.The presence of affiliate components strengthens the train wind in the near fiel of the train because of strong fl w solid interactions but has limited effects on train wind in the far field.

Folding of multi-layer graphene sheets induced by van der Waals interaction

Graphene sheets are extremely flexible, and thus small forces, such as van der Waals interaction, can induce significant out-of-plane deformation, such as folding. Folded graphene sheets show racket shaped edges, which can significantly affect the electrical properties of graphene. In this paper, we present combined theoretical and computational studies to reveal the folding behavior of multi-layer graphene sheets. A nonlinear theoretical model is established to determine the critical length of multilayer graphene sheets for metastable and stable folding, and to accurately predict the shapes of folded edges. These results all show good agree- ment with those obtained by molecular dynamics simulations.

On aerodynamic noises radiated by the pantograph system of high-speed trains

Pantograph system of high-speed trains become significant source of aerodynamic noise when travelling speed exceeds 300 km/h. In this paper, a hybrid method of non-linear acoustic solver （NLAS） and Ffowcs Williams-Hawkings （FW-H） acoustic analogy is used to predict the aerodynamic noise of pantograph system in this speed range. When the simulation method is validated by a benchmark problem of flows around a cylinder of finite span, we calculate the near flow field and far acoustic field surrounding the pantograph system. And then, the frequency spectra and acoustic attenuation with distance are analyzed, showing that the pantograph system noise is a typical broadband one with most acoustic power restricted in the medium-high frequency range from 200 Hz to 5 kHz. The aerodynamic noise of pantograph systems radiates outwards in the form of spherical waves in the far field. Analysis of the overall sound pressure level （OASPL） at different speeds exhibits that the acoustic power grows approximately as the 4th power of train speed. The comparison of noise reduction effects for four types of pantograph covers demonstrates that only case 1 can lessen the total noise by about 3 dB as baffles on both sides can shield sound wave in the spanwise direction. The covers produce additional aerodynamic noise themselves in the other three cases and lead to the rise of OASPLs.

Dynamics of cantilever plates and its hybrid vibration control

Applying Lagrange-Germain＇s theory of elas- tic thin plates and Hamiltonian formulation, the dynamics of cantilever plates and the problem of its vibration control are studied, and a general solution is finally given. Based on Hamiltonian and Lagrangian density function, we can obtain the flexural wave equation of the plate and the relationship between the transverse and the longitudinal eigenvalues. Based on eigenfunction expansion, dispersion equations of propagation mode of cantilever plates are deduced. By satisfying the boundary conditions of cantilever plates, the natural frequencies of the cantilever plate structure can be given. Then, analytic solution of the problem in plate structure is obtained. An hybrid wave/mode control approach, which is based on both independent modal space control and wave control methods, is described and adopted to analyze the active vibration control of cantilever plates. The low-order （controlled by modal control） and the high-order （controlled by wave control） frequency response of plates are both improved. The control spillover is avoided and the robustness of the system is also improved. Finally, simulation results are analyzed and discussed.

Ultrasonic beam steering using Neumann boundary condition in multiplysics

The traditional one-dimensional ultrasonic beam steering has time delay and is thus a complicated problem. A numerical model of ultrasonic beam steering using Neumann boundary condition in multiplysics is presented in the present paper. This model is based on the discrete wave number method that has been proved theoretically to satisfy the continuous conditions. The propagating angle of novel model is a function of the distance instead of the time domain. The propagating wave fronts at desired angles are simulated with the single line sources for plane wave. The result indicates that any beam angle can be steered by discrete line elements resources without any time delay.

Unsteady flow of viscoelastic fluid between two cylinders using fractional Maxwell model

The unsteady flow of an incompressible fractional Maxwell fluid between two infinite coaxial cylinders is studied by means of integral transforms. The motion of the fluid is due to the inner cylinder that applies a time dependent tor- sional shear to the fluid. The exact solutions for velocity and shear stress are presented in series form in terms of some generalized functions. They can easily be particularized to give similar solutions for Maxwell and Newtonian fluids. Fi- nally, the influence of pertinent parameters on the fluid motion, as well as a comparison between models, is highlighted by graphical illustrations.

Nonlinear magneto-electric response of a giant magnetostrictive/piezoelectric composite cylinder

In this study, we investigate the nonlinear cou- pling magneto-electric （ME） effect of a giant magnetostric- tive/piezoelectric composite cylinder. The nonlinear consti- tutive relations of the ME material are taken into account, and the influences of the nonlinear material properties on the ME effect are investigated for the static and dynamic cases, respectively. The influences of different constraint conditions on the ME effect are discussed. In the dynamic case considering nonlinear material properties, the double frequency ME response （The response frequency is twice the applied magnetic frequency） is obtained and discussed, which can be used to explain the experiment phenomenon in which the input signal with frequency f is converted to the output signal with 2f in ME laminated structures. Some calculations on nonlinear ME effect are conducted. The obtained results indicate that the nonlinear material properties affect not only the magnitude of the ME effect in the static case but also the ME response frequency in the dynamic case

Modeling rock failure using the numerical manifold method followed bythe discontinuous deformation analysis

A complete rock failure process usually involves opening/sliding of preexisting discontinuities as well as frac- turing in intact rock bridges to form persistent failure sur- faces and subsequent motions of the generated rock blocks. The recently developed numerical manifold method （NMM） has potential for modelling such a complete failure process. However, the NMM suffers one limitation, i.e., unexpected material domain area change occurs in rotation modelling. This problem can not be easily solved because the rigid body rotation is not represented explicitly in the NMM. The discontinuous deformation analysis （DDA） is specially de- veloped for modelling discrete block systems. The rotation- induced material area change in the DDA modelling can be avoided conveniently because the rigid body rotation is represented in an explicit form. In this paper, a transition technique is proposed and implemented to convert a NMMmodelling to a DDA modelling so as to simulate a complete rock failure process entirely by means of the two methods, in which the NMM is adopted to model the early fracturing as well as the transition from continua to discontinua, while the DDA is adopted to model the subsequent motion of the generated rock blocks. Such a numerical approach also im- proves the simulation efficiency greatly as compared with a complete NMM modelling approach. The fracturing of a rock slab with pre-existing non-persistent joints located on a slope crest and the induced rockfall process are simulated. The validity of the modelling transition from the NMM to the DDA is verified and the applicability of the proposed nu- merical approach is investigated.