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Gas kinetic algorithm for flows in Poiseuille-like microchannels using Boltzmann model equation

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Science China(Physics,mechanics & Astronomy) 2005年第4期48卷 496-512页

作者： LI Zhihui1,2. ZHANG Hanxin1 & FU Song2.⒈ National Laboratory for CFD, Hypervelocity Aerodynamics Institute,China Aerodynamics Research and Development Center,Mianyang 62.000,China 2.department of engineering {2.,Tsinghua University,Beijing 100084,China 1. National Laboratory for CFD Hypervelocity Aerodynamics Institute China Aerodynamics Research and Development Center 621000 Mianyang China 2. department of engineering mechanics Tsinghua University 100084 Beijing China

The gas-kinetic unified algorithm using Boltzmann model equation have been extended and developed to solve the micro-scale gas flows in Poiseuille-like micro-channels from Micro-Electro-Mechanical Systems (MEMS). The numerical modeling of the gas kinetic boundary conditions suitable for micro-scale gas flows is presented. To test the present method, the classical Couette flows with various Knudsen numbers, the gas flows from short microchannels like plane Poiseuille and the pressure-driven gas flows in two-dimensional short microchannels have been simulated and compared with the approximate solutions of the Boltzmann equation, the related DSMC results, the modified N-S solutions with slip-flow boundary theory, the gas-kinetic BGK-Burnett solutions and the experimental data. The comparisons show that the present gas-kinetic numerical algorithm using the mesoscopic Boltzmann simplified velocity distribution function equation can effectively simulate and reveal the gas flows in microchannels. The numerical experience indicates that this method may be a powerful tool in the numerical simulation of micro-scale gas flows from MEMS.

关键词： Boltzmann model equation, discrete velocity ordinate method, finite-difference scheme, Couette flow, Poiseuille flow, gas flow in short-microchannel.

mechanics of flexible and stretchable piezoelectrics for energy harvesting

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Science China(Physics,mechanics & Astronomy) 2015年第9期58卷 39-51页

作者： CHEN Ying LU BingWei OU DaPeng FENG Xue department of engineering {2. Tsinghua University Center for mechanics and Materials Tsinghua University

As rapid development in wearable/implantable electronic devices benefit human life in daily health monitoring and disease treatment medically, all kinds of flexible and/or stretchable electronic devices are booming, together with which is the demanding of energy supply with similar mechanical property. Due to its ability in converting mechanical energy lying in human body into electric energy, energy harvesters based on piezoelectric materials are promising for applications in wearable/implantable device's energy supply in a renewable, clean and life-long way. Here the mechanics of traditional piezoelectrics in energy harvesting is reviewed, including why piezoelectricity is the choice for minor energy harvesting to power the implantable/wearable electronics and how. Different kinds of up to date flexible piezoelectric devices for energy harvesting are introduced, such as nanogenerators based on Zn O and thin and conformal energy harvester based on PZT. A detailed theoretical model of the flexible thin film energy harvester based on PZT nanoribbons is summarized, together with the in vivo demonstration of energy harvesting by integrating it with swine heart. Then the initial researches on stretchable energy harvesters based on piezoelectric material in wavy or serpentine configuration are introduced as well.

关键词： piezoelectric effect energy harvesting flexible electronics stretchable electronics

Size and strain rate effects in tensile strength of penta-twinned Ag nanowires

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Acta Mechanica Sinica 2017年第4期33卷 792-800页

作者： Xuan Zhang Xiaoyan Li Huajian Gao Centre of Advanced mechanics and Materials Department of Engineering Mechanics Tsinghua University School of engineering Brown University

Penta-twinned Ag nanowires（pt-AgNWs） have recently attracted much attention due to their interesting mechanical and physical properties. Here we perform largescale atomistic simulations to investigate the influence of sample size and strain rate on the tensile strength of pt-AgNWs. The simulation results show an apparent size effect in that the nanowire strength（defined as the critical stress for dislocation nucleation） increases with decreasing wire diameter. To account for such size effect, a theoretical model involving the interaction between an emerging dislocation and the twin boundary has been developed for the surface nucleation of dislocations. It is shown that the model predictions are in quantitative agreement with the results from atomistic simulations and previous experimental studies in the literatures. The simulations also reveal that nanowire strength is strain-rate dependent, which predicts an activation volume for dislocation nucleation in the range of 1–10b^3,where b is the magnitude of the Burgers vector for a full dislocation.

关键词： nucleation tensile dislocation modulus apparent diameters decreasing magnitude crystalline penta

Kutta-Joukowski force expression for viscous flow

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Science China(Physics,mechanics & Astronomy) 2015年第2期58卷 90-94页

作者： LI Juan XU YiZhe WU ZiNiu department of engineering {2. Tsinghua University

The Kutta Joukowski(KJ) theorem, relating the lift of an airfoil to circulation, was widely accepted for predicting the lift of viscous high Reynolds number flow without separation. However, this theorem was only proved for inviscid flow and it is thus of academic importance to see whether there is a viscous equivalent of this theorem. For lower Reynolds number flow around objects of small size, it is difficult to measure the lift force directly and it is thus convenient to measure the velocity flow field solely and then, if possible, relate the lift to the circulation in a similar way as for the inviscid KJ theorem. The purpose of this paper is to discuss the relevant conditions under which a viscous equivalent of the KJ theorem exists that reduces to the inviscid KJ theorem for high Reynolds number viscous flow and remains correct for low Reynolds number steady flow. It has been shown that if the lift is expressed as a linear function of the circulation as in the classical KJ theorem, then the freestream velocity must be corrected by a component called mean deficit velocity resulting from the wake. This correction is small only when the Reynolds number is relatively large. Moreover, the circulation, defined along a loop containing the boundary layer and a part of the wake, is generally smaller than that based on inviscid flow assumption. For unsteady viscous flow, there is an inevitable additional correction due to unsteadiness.

关键词： lift force Kutta Joukowski expression viscous flow

Application of Arnoldi method to boundary layer instability

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Chinese Physics B 2015年第12期24卷 379-391页

作者：张永明罗纪生 department of mechanics Tianjin University Tianjin Key Laboratory of Modern engineering mechanics

The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory（LST） for two-dimensional（2.） disturbance on one-dimensional（1D） basic flow, comparison with LST for three-dimensional（3D） disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2. basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence（FST）. Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien–Schlichting（TS） waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous＇ in secondary instability.

关键词： Arnildi method linear stability theory Floquet theory secondary instability

Near-wall behaviors of oblique-shock-wave/turbulent-boundary-layer interactions

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Applied Mathematics and mechanics(English Edition) 2017年第10期38卷 1357-1376页

作者： Mingsheng YE Ming DONG department of mechanics Tianjin University Tianjin Key Laboratory of Modern engineering mechanics

A direct numerical simulation （DNS） on an oblique shock wave with an incident angle of 33.2. impinging on a Mach 2.2. supersonic turbulent boundary layer is performed. The numerical results are confirmed to be of high accuracy by comparison with the reference data. Particular efforts have been made on the investigation of the near-wall behaviors in the interaction region, where the pressure gradient is so significant that a certain separation zone emerges. It is found that, the traditional linear and loga- rithmic laws, which describe the mean-velocity profiles in the viscous and meso sublayers, respectively, cease to be valid in the neighborhood of the interaction region, and two new laws of the wall are proposed by elevating the pressure gradient to the leading order. The new laws are inspired by the analysis on the incompressible separation flows, while the compressibility is additionally taken into account. It is verified by the DNS results that the new laws are adequate to reproduce the mean-velocity profiles both inside and outside the interaction region. Moreover, the normalization adopted in the new laws is able to regularize the Reynolds stress into an almost universal distribution even with a salient adverse pressure gradient （APG）.

关键词： shock wave turbulent boundary layer direct numerical simulation (DNS),adverse pressure gradient (APG) separation

Effects of shear-thinning/thickening nature on the stability of Couette-like flow with uniform crossflow

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Acta Mechanica Sinica 2013年第6期29卷 806-810页

作者： Yu-Quan Liu Jie Peng Ke-Qin Zhu department of engineering {2. Tsinghua University

The linear stability of wall-injected pressure- driven Couette-like flow in power-law fluids is studied. Previous study on this kind of flow for Newtonian fluids by Nicoud and Angilella [Phys. Rev. E 56, 3000 （1997）] was extended to power-law fluids to understand the effects of shear-thinning/thickening nature on the flow stability. A related expression between the critical crossflow Reynolds number for Newtonian fluids and that for power-law fluids is obtained as the streamwise Reynolds number is large enough based on numerical computations, and verified theoretically in the case of a limiting condition with the power-law index.

关键词： Power-law fluid Couette-like flow - Crossflow Linear stability

Self-shaping of bioinspired chiral composites

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Acta Mechanica Sinica 2014年第4期30卷 533-539页

作者： Qing-Qing Rong Yu-Hong Cui Takahiro Shimada Jian-Shan Wang Takayuki Kitamura Tianjin Key Laboratory of Modern engineering mechanics Department of MechanicsTianjin University department of Mechanical engineering and Science Kyoto University

Self-shaping materials such as shape memory polymers have recently drawn considerable attention owing to their high shape-changing ability in response to changes in ambient conditions, and thereby have promising applications in the biomedical, biosensing, soft robotics and aerospace fields. Their design is a crucial issue of both theoretical and technological interest. Motivated by the shape-changing ability of Towel Gourd tendril helices during swelling/deswelling, we present a strategy for realizing self-shaping function through the deformation of micro/nanohelices. To guide the design and fabrication of selfshaping materials, the shape equations of bent configurations, twisted belts, and helices of slender chiral composite are developed using the variation method. Furthermore, it is numerically shown that the shape changes of a chiral composite can be tuned by the deformation of micro/nanohelices and the fabricated fiber directions. This work paves a new way to create self-shaping composites.

关键词： Self-shaping · Bioinspired chiral composites ·Micro/nanohelices · Tendrils

A simulation based on the cosserat continuum model of the vortex structure in granular materials

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Acta Mechanica Solida Sinica 2015年第2期28卷 182-186页

作者： Cun Yu Xihua Chu Yuanjie Xu department of engineering {2. Wuhan University

Displacement fluctuation is the difference between the real displacement and the affine displacement in deforming granular materials. The discrete element method （DEM） is widely used along with experimental approaches to investigate whether the displacement fluctuation represents the vortex structure. Current research suggests that the vortex structure is caused by the cooperative motion of particle groups on meso-scales, which results in strain localization in granular materials. In this brief article, we investigate the vortex structure using the finite element method （FEM） based on the Cosserat cor[tinuum model. The numerical example focuses on the relationship between the vortex structure and the shear bands under two conditions：（a） uniform granular materials; （b） granular materials with inclusions. When compared with distributions of the effective strain and the vortex structure, we find that the vortex structure coexists with the strain localization and originates from the stiffness cooperation of different locations in granular materials at the macro level.

关键词： granular materials vortex structure strain localization Cosserat continuum model