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Nonlinear vibrations of nano-beams accounting for nonlocal effect using a multiple scale method

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Science china(Technological Sciences) 2009年第3期52卷 617-621页

作者： c. w. LIM Department of Building and construction City University of Hong Kong

The nonlinear free transverse vibrations of a nano-beam on simple supports are investigated based on nonlocal elasticity theory. The governing equation is proposed by considering geometric nonlinearity due to finite stretching of the beam. The method of multiple scales is applied to the governing equa- tion to evaluate the nonlinear natural frequencies. Numerical examples are presented to demonstrate the analytical results and highlight the contributions of the nonlinear term and nonlocal effect.

关键词： nonlocal elasticity theory nanobeam natural frequency method of multiple scales

Dynamic behaviour of axially moving nanobeams based on nonlocal elasticity approach

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Acta Mechanica Sinica 2010年第5期26卷 755-765页

作者： c. w. Lim c. Li Ji-Lin Yu Department of Building and construction City Universityof Hong Kong Tat Chee Avenue KowloonHong Kong SAR China State Key Laboratory of Structural Analysis for IndustrialEquipment Dalian University of TechnologyDalian 116024 China Department of Modem Mechanics University of Scienceand Technology of China Hefei 230026 China

In this article, transverse free vibrations of axially moving nanobeams subjected to axial tension are studied based on nonlocal stress elasticity theory. A new higher-order differential equation of motion is derived from the variational principle with corresponding higher-order, non-classical boundary conditions. Two supporting conditions are investigated, i.e. simple supports and clamped supports. Effects of nonlocal nanoscale, dimensionless axial velocity, density and axial tension on natural frequencies are presented and discussed through numerical examples. It is found that these factors have great influence on the dynamic behaviour of an axially moving nanobeam. In particular, the nonlocal effect tends to induce higher vibration frequencies as compared to the results obtained from classical vibration theory. Analytical solutions for critical velocity of these nanobeams when the frequency vanishes are also derived and the influences of nonlocal nanoscale and axial tension on the critical velocity are discussed.

关键词： Axially moving nanobeams critical velocity Free vibration Natural frequency Nonlocal elasticity

cOMPARISON OF K-ε AND K-ω TURBULENcE MODELS FOR SIMULATION OF SHALLOw REcIRcULATING FLOwS IN AN OPEN cHANNEL

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Journal of Hydrodynamics 1998年第2期10卷 74-86页

作者： Zhang, Yifan Li, c.w. Nanjing Hydraulic Research Inst Nanjing China

The paper describes the comparison of commonly used k-ε and k-ω turbulence models for simulating the recirculating flows behind a sudden expansion in a straight open channel. The depth-average numerical model and the quasi three-dimensional multilayer model are presented respectively. The governing equations are split into three parts in the finite difference approach: advection, dispersion and propagation. comparison of the results computed with the k-ε and k-ω models shows that the two turbulence models have strong robustness in simulating the reattachment length of the recirculating flows. The computed results are found in good agreement with the measured data.

关键词： turbulence k-ω modek k-ε model finite difference channel expansion

NUMERIcAL SOLUTIONS OF INcOMPRESSIBLE EULER AND NAVIER-STOKES EQUATIONS BY EFFIcIENT DIScRETE SINGULAR cONVOLUTION METHOD

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Acta Mechanica Sinica 2000年第3期16卷 223-239页

作者： D.c.wan G.w.wei Shanghai Univ Shanghai Inst Appl Math&MechShanghai 200072Peoples R China Natl Univ Singapore Dept Computat SciSingapore 119260Singapore

An efficient discrete singular convolution (DSc) method is introduced to the numerical solutions of incompressible Euler and Navier-Stokes equations with periodic boundary conditions. Two numerical tests of two-dimensional Navier-Stokes equations with periodic boundary conditions and Euler equations for doubly periodic shear layer flows are carried out by using the DSc method for spatial derivatives and fourth-order Runge-Kutta method for time advancement, respectively. The computational results show that the DSc method is efficient and robust for solving tho problems of incompressible flows, and has the potential of being extended to numerically solve much broader problems in fluid dynamics.

关键词： incompressible flows periodic boundary DSc method fourth-order Runge-Kutta method

An Improved Evolvable Oscillator and Basis Function Set for control of an Insect-Scale Flapping-wing Micro Air Vehicle

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Journal of computer Science & Technology 2012年第5期27卷 966-978页

作者： John c.Gallagher Michael w.Oppenheimer Department of computer Science and Engineering Wright State University control Sciences Branch Air Force Research Laboratory

This paper introduces an improved evolvable and adaptive hardware oscillator design capable of supporting adaptation intended to restore control precision in damaged or imperfectly manufactured insect-scale flapping-wing micro air vehicles. It will also present preliminary experimental results demonstrating that previously used basis function sets may have been too large and that significantly improved learning times may be achieved by judiciously culling the oscillator search space. The paper will conclude with a discussion of the application of this adaptive, evolvable oscillator to full vehicle control as well as the consideration of longer term goals and requirements.

关键词： evolvable and adaptive hardware micro air vehicle evolutionary algorithm

Dynamic Response of composite Materials with 2D Reduced Micromorphic Model

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Acta Mechanica Solida Sinica 2022年第4期35卷 603-615页

作者： A.R.El Dhaba c.w.Lim Department of Mathematics Faculty of ScienceDamanhour UniversityDamanhourEgypt Department of Architecture and civil Engineering City University of Hong KongKowloonHong Kong SARChina

In this article,we introduce a complete set of constitutive relations and field equations for the linear reduced micromorphic *** further investigate the internal variables and their relationship in the case of two-dimensional(2D)wave *** dynamic response is investigated for composite materials,which is due to an external wave in two dimensions applied at the boundary of the considered *** solutions for the model are unavailable at this stage due to dependency of the field equations on spatial and time variables in a complicated manner.A finite element approach is adopted to derive approximate solutions for the field equations,and numerical finite element solutions for the internal fields are presented in detail and discussed.

关键词： composite materials Finite element method(FEM) Reduced micromorphic model wave propagation

Laser additive manufacturing of biodegradable Mg-based alloys for biomedical applications: A review

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Journal of Magnesium and Alloys 2022年第4期10卷 915-937页

作者： c.L.wu w.J.Xie H.c.Man Department of Industrial and Systems Engineering The Hong Kong Polytechnic UniversityHung HomKowloonHong KongChina

Metallic implants are widely used in internal fixation of bone fracture in surgical *** are mainly used for providing mechanical support and stability during bone reunion,which usually takes a few months to *** implants made of stainless steels,Ti-based alloys and cocrMo alloys have been widely used for orthopedic reconstruction due to their high strength and high corrosion *** metallic implants will remain permanently inside the body after implantation,and a second surgery after bone healing is needed because the long-term presence of implant will lead to various *** implant removal surgery not only incurs expenditure,but also risk and psychological *** a consequence,studies on the development of biodegradable implants,which would degrade and disappear in vivo after bone reunion is completed,have drawn researchers’*** this connection,Mg-based alloys have shown great potentials as promising implant materials mainly due to their low density,inherent biocompatibility,biodegradability and mechanical properties close to those of ***,the high degradation rate of Mg-based implants in vivo is the biggest hurdle to *** from materials selection,a fixation implant is ideally tailor-made in size and shape for an individual case,for best surgical ***,laser additive manufacturing(LAM),with the advent of sophisticated laser systems and software,is an ideal process to solve these *** this paper,we reviewed the progress in LAM of biodegradable Mg-based alloys for biomedical *** effect of powder properties and laser processing parameter on the formability and quality was thoroughly *** microstructure,phase constituents and metallurgical defects formed in the LAMed samples were *** mechanical properties,corrosion resistance,biocompatibility and antibacterial properties of the LAMed samples were summarized and compared with samples fabricated by

关键词： Laser additive manufacturing Magnesium alloys Implant Biomedical applications Mechanical properties Biodegradability

A Novel Application of Multi-Resonant Dissipative Elastic Metahousing for Bearings

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Acta Mechanica Solida Sinica 2021年第4期34卷 449-465页

作者： Muhammad c.w.Lim N.S.Vyas Department of Architecture and civil Engineering City University of Hong KongKowloonHong Kong SARChina Department of Mechanical Engineering Indian Institute of Technology KanpurKanpur208016India

Bearing as an important machine element is widely used for industrial and automotive *** certain operational speed,bearings induce disturbing vibrations and noises that affect machine service life,productivity and passenger comfort in case of vehicle *** elastic metamaterials have caught considerable attention of scientific community due to their effective medium properties and peculiar dynamic characteristics including frequency bandgaps that can be effectively applied to attenuate and control undesirable vibration and *** a substantial amount of theoretical work for effective medium characteristics and dynamic properties of acoustic/elastic metamaterials has been reported,the practical design and application of these composite structures for real-life engineering problems still remain *** present study intends to investigate a potential application of dissipative elastic metamaterials in controlling the bearing-generated vibration and noises over an ultrawide frequency *** study is based on a simple analytical model together with rigorous finite element numerical *** has been established that the dissipative characteristic of resonant system caused by larger material mismatch broadens the local resonance bandgaps beyond the bounding resonance frequency at the cost of wave *** order to achieve broadband vibration and noise control,multi-resonant composite structures are embedded inside the bearing housing in five different *** reported results revealed the presence of broadband wave attenuation zone distributed from 3 to 52 kHz with consideration of material *** bearing-generated vibration and noises lying inside the wave attenuation zone will be *** feasibility study provides a new concept for the design and application of acoustic/elastic metamaterials in the bearing industry to improve machine service life and to enhance productivity and passenger comfo

关键词： Bearing metahousing Dissipative elastic metamaterials Frequency Local resonance bandgap Vibration attenuation zone

Amplitude modulation and extreme events in turbulent channel flow

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Acta Mechanica Sinica 2018年第1期34卷 1-9页

作者： Y.c.Yao w.X.Huang c.X.Xu AML Department of Engineering Mechanics Tsinghua University

Amplitude modulation of near-wall turbulence by large-scale structures in the outer layer is investigated by direct numerical simulation of turbulent channel flows at Reynolds number Re= 540, 1000, 2000. The effect of modulation is obvious in the two-point cross-section correlation map, and the correlation coefficients increase significantly with the Reynolds number. The influence of modulation is reflected in the tail of the probability density function of the near-wall flow signals, which expands as the Reynolds number increases. The flatness factor provides a quantitative description of the high fluctuation events due to modulation. Vortical structures associated with modulation are revealed by conditionally averaging the flow field of the near-wall extreme events, providing a depiction of how the influence of the large-scale structures penetrate towards the near-wall region.

关键词： wall turbulence Amplitude modulation Flatness Extreme events