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TRPIV investigation of space-time correlation in turbulent flows over flat and wavy walls

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Acta Mechanica Sinica 2014年第4期30卷 468-479页

作者： Wei Wang Xin-Lei Guan Nan Jiang Department of mechanics Tianjin University Tianjin Key laboratory of Modern Engineering mechanics The State Key laboratory of Nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

The present experimental work is devoted to in- vestigate a new space-time correlation model for the turbulent boundary layer over a flat and a wavy walls. A turbulent boundary layer flow at Reo = 2460 is measured by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The space-time correlations of instantaneous streamwise fluctuation velocity are calculated at 3 different wall-normal locations in logarithmic layer. It is found that the scales of coherent structure increase with moving far away from the wall. The growth of scales is a manifestation of the growth of prevalent coherent structures in the turbulent boundary layer like hairpin vortex or hairpin packets when they lift up. The resulting contours of the space-time correlation exhibit elliptic-like shapes rather than straight lines. It is suggested that, instead of Taylor hypothesis, the elliptic model of the space-time correlation is valid for the wallbounded turbulent flow over either a flat wall or a wavy wall. The elliptic iso-correlation curves have a uniform preferred orientation whose slope is determined by the convection velocity. The convection velocity derived from the space-time correlation represents the velocity at which the large-scale eddies carry small-scale eddies. The sweep velocity rep- resents the distortions of the small-scale eddies and is intimately associated with the fluctuation velocity in the logarithmic layer of turbulent boundary layers. The nondimensionalized correlation curves confirm that the elliptic model is more proper for approximating the space-time correlation than Taylor hypothesis, because the latter can not embody the small-scale motions which have non-negligible distortions. A second flow over a wavy wall is also recorded using TRPIV. Due to the combined effect of shear layers and the adverse pressure gradient, the space-time correlation does not show an elliptic-like shape at some specific heights over the wavy wall, but in the outer region of the wav

关键词： Tomographic particle image velocimetry Tur-bulent boundary layer Wavy wall Space-time correlation Taylor hypothesis Elliptic model

Optimal transient growth in turbulent pipe flow

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Applied Mathematics and mechanics(English Edition) 2015年第8期36卷 1057-1072页

作者： Yang SONG Chunxiao XU Weixi HUANG Guixiang CUI Key laboratory of Applied mechanics Department of Engineering MechanicsTsinghua University

The optimal transient growth process of perturbations driven by the pressure gradient is studied in a turbulent pipe flow. A new computational method is proposed, based on the projection operators which project the governing equations onto the sub- space spanned by the radial vorticity and radial velocity. The method is validated by comparing with the previous studies. Two peaks of the maximum transient growth am- plification curve are found at different Reynolds numbers ranging from 20 000 to 250 000. The optimal flow structures are obtained and compared with the experiments and DNS results. The location of the outer peak is at the azimuthal wave number n = 1, while the location of the inner peak is varying with the Reynolds number. It is observed that the velocity streaks in the buffer layer with a spacing of 100δv are the most amplified flow structures. Finally, we consider the optimal transient growth time and its dependence on the azimuthal wave length. It shows a self-similar behavior for perturbations of different scales in the optimal transient growth process.

关键词： turbulent pipe flow optimal transient growth coherent structure

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 2.25 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

A new thermo-elasto-plasticity constitutive theory for polycrystalline metals

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Acta Mechanica Sinica 2015年第3期31卷 338-348页

作者： Cen Chen Qiheng Tang Tzuchiang Wang State Key laboratory of Nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

In this study, the behavior of polycrystalline metals at different temperatures is investigated by a new thermo-elasto-plasticity constitutive theory. Based on solid mechanical and interatomic potential, the constitutive equa- tion is established using a new decomposition of the deformation gradient. For polycrystalline copper and magnesium, the stress-strain curves from 77 to 764 K （copper）, and 77 to 870 K （magnesium） under quasi-static uniaxial loading are calculated, and then the calculated results are compared with the experiment results. Also, it is determined that the present model has the capacity to describe the decrease of the elastic modulus and yield stress with the increasing temperature, as well as the change of hardening behaviors of the polycrystalline metals. The calculation process is simple and explicit, which makes it easy to implement into the applications.

关键词： Thermo-elasto-plasticity constitutive theory Yield stress Hardening behaviors Finite temperature

Morphological, Structural, Thermal and Tensile Properties of High Density Polyethylene Composites Reinforced with Treated Argan Nut Shell Particles

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Journal of Bionic Engineering 2015年第1期12卷 129-141页

作者： Hamid Essabir Mounir EI Achaby EI Moukhtar Hilali Rachid Bouhfid AbouEIkacem Qaiss Moroccan Foundation for Advanced Science Innovation and Research (MAScIR) Institute of Nanomaterials and Nanotechnology (NANOTECH) Laboratory of Polymer Processing Rabat Morocco National School of Applied Sciences Ibn Zohr University Laboratory Mechanics Processes Energy and Environment Agadir Morocco

High Density Polyethylene （HDPE） composites reinforced with treated bio-filler from Argan-Nut Shell （ANS） at various filler contents are prepared by extrusion and injection molding processes. The microstructures of the composites are charac- terized by Fourier Transform Infrared Spectroscopy （FTIS） and Scanning Electron Microscopy （SEM）; the thermal stability is analyzed by Thermogravimetric Analysis （TGA）, and their mechanical properties are investigated by dynamical mechanical analysis and rheological testing. The morphological and structural results indicate an improvement in adhesion between the ANS fillers and HDPE matrix upon alkali treatment. The mechanical properties of the composites show a significant increase in young＇s modulus with the addition of filler, a gain of 58% is marked compared to neat polymer, Thermal analysis reveals that the incorporation of bio-filler in polymer results in a decrease in decomposition temperatures. This research offers an ecological alternative to upgrade the valorization of abundant and unexploited Moroccan resources. In addition, the possibility of finding uses for ANS in composite manufacturing will help open new markets for what is normally considered waste or for use in low value products.

关键词： argan tree argan nut shell particles polymer composite melt extrusion mechanical properties

Improving the thermal shock resistance of ceramics by crack arrest blocks

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Science China(Technological Sciences) 2016年第6期59卷 913-919页

作者： WANG Yan Wei XIA Biao SU Hong Hong CHEN Hang FENG Xue Applied mechanics laboratory Department of Engineering Mechanics Tsinghua University Center for mechanics and Materials Tsinghua University Department of Engineering mechanics Chongqing University

Ceramics used in the high temperature environment are inevitably subjected to sudden temperature change, which may lead to catastrophic thermal shock failure due to the intrinsic brittleness of ceramics. In this paper, an experimental platform is designed to realize the in-situ observation during the thermal shock experiments. Experimental results show that all the cracks initiate from one of the edge midpoints and propagate to another one for square specimens. Such experimental observation is consistent with the maximum tensile stress zone with the maximum temperature gradient given by the finite element method(FEM). The different crack modes resulting from different heating rates after thermal shock experiments are observed and analyzed. Comparison between different clamping methods is conducted to study the effects of boundary conditions on the thermal shock experiments. Furthermore, in order to improve the thermal shock performance of alumina ceramics, crack arrest blocks are added near the edge midpoint. The thickness, shape and arrangement of the blocks are systematically investigated to understand the mechanism of improvement of thermal shock resistance.

关键词： thermal shock ultra-high temperature ceramics crack arrest blocks in-situ dynamic observation

Micro- and nano-mechanics in China： A brief review of recent progress and perspectives

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Science China(Physics,mechanics & Astronomy) 2018年第7期61卷 18-31页

作者： ZhiPing Xu QuanShui Zheng Applied mechanics laboratory and Center for Nano and Micro mechanics Department of Engineering Mechanics Tsinghua UniversityBeijing 100084 China

The past three decades have witnessed the explosion of nanoscience and technology, where notable research eftbrts have been made in synthesizing nanomaterials and controlling nanostructures of bulk materials. The uncovered mechanical behaviors of structures and materials with reduced sizes and dimensions pose open questions to the community of mechanicians, which expand the framework of continuum mechanics by advancing the theory, as well as modeling and experimental tools. Researchers in China have been actively involved into this exciting area, making remarkable contributions to the understanding of nanoscale mechanical processes, the development of multi-scale, multi-field modeling and experimental techniques to resolve the processing-microstructures-properties relationship of materials, and the interdisciplinary studies that broaden the subjects of mechanics. This article reviews selected progress made by this community, with the aim to clarify the key concepts, methods and applications of micro- and nano-mechanics, and to outline the perspectives in this fast-evolving field.

关键词： micro- and nano-mechanics multi-scale and multi-field methods nanotechnology nanomaterials nanostructurcs

Analytical solutions for plane problem of functionally graded magnetoelectric cantilever beam

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Applied Mathematics and mechanics(English Edition) 2015年第7期36卷 955-970页

作者： Yanmei YUE Xiaofen YE Kaiyu XU Shanghai Institute of Applied Mathematics and mechanics Shanghai Key Laboratory of Mechanics in Energy EngineeringDepartment of MechanicsShanghai University

In this paper, an exact analytical solution is presented for a transversely isotropic functionally graded magnetomelectro-elastic （FGMEE） cantilever beam, which is subjected to a uniform load on its upper surface, as well as the concentrated force and moment at the free end. This solution can be applied for any form of gradient distribution. For the basic equations of plane problem, all the partial differential equations governing the stress field, electric, and magnetic potentials are derived. Then, the expressions of Airy stress, electric, and magnetic potential functions are assumed as quadratic polynomials of the longitudinal coordinate. Based on all the boundary conditions, the exact expressions of the three functions can be determined. As numerical examples, the material parameters are set as exponential and linear distributions in the thickness direction. The effects of the material parameters on the mechanical, electric, and magnetic fields of the cantilever beam are analyzed in detail.

关键词： functionally graded material (FGM) analytical solution magnetoelectrie(ME) material cantilever beam plane stress problem

Bioinspired hierarchical helical nanocomposite macrofibers based on bacterial cellulose nanofibers

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National Science Review 2020年第1期7卷 73-83页

作者： Huai-Ling Gao Ran Zhao Chen Cui Yin-Bo Zhu Si-Ming Chen Zhao Pan Yu-Feng Meng Shao-Meng Wen Chuang Liu Heng-An Wu Shu-Hong Yu Division of nanomaterials & Chemistry Hefei National Laboratory for Physical Sciences at the Microscale CAS Center for Excellence in Nanoscience Hefei Science Center of CAS Department of Chemistry University of Science and Technology of China CAS Key laboratory of Mechanical Behavior and Design of Materials Department of Modern Mechanics CAS Center for Excellence in Complex System Mechanics University of Science and Technology of China

Bio-sourced nanocellulosic materials are promising candidates for spinning high-performance sustainable macrofibers for advanced applications. Various strategies have been pursued to gain nanocellulose-based macrofibers with improved strength. However, nearly all of them have been achieved at the expense of their elongation and toughness. Inspired by the widely existed hierarchical helical and nanocomposite structural features in biosynthesized fibers exhibiting exceptional combinations of strength and toughness, we report a design strategy to make nanocellulose-based macrofibers with similar characteristics. By combining a facile wet-spinning process with a subsequent multiple wet-twisting procedure, we successfully obtain biomimetic hierarchical helical nanocomposite macrofibers based on bacterial cellulose nanofibers, realizing impressive improvement in their tensile strength, elongation and toughness simultaneously. The achievement certifies the validity of the bioinspired hierarchical helical and nanocomposite structural design proposed here. This bioinspired design strategy provides a potential platform for further optimizing or creating many more strong and tough nanocomposite fiber materials for diverse applications.

关键词： bioinspired nanocomposite hierarchical helical macrofibers strength and toughness bacterial cellulose