Acta Mechanica Sinica
     Acta Mechanica Sinica
  Acta Mechanica Sinica--2015, 31 (2)   Published: 20 May 2015
On hairpin vortex generation from near-wall streamwise vortices Hot!
Yinshan Wang, Weixi Huang, Chunxiao Xu

The generation of a hairpin vortex from near-wall streamwise vortices is studied via the direct numerical simulation (DNS) of the streak transient growth in the minimal channel flow at Reτ = 400. The streak profile is obtained by conditionally averaging the DNS data of the fully developed turbulent channel flow at the same Reynolds number. The near-wall streamwise vortices are produced by the transient growth of the streak which is initially subjected to the sinuous pe...

An inverse problem to estimate an unknown order of a Riemann-Liouville fractional derivative for a fractional Stokes' first problem for a heated generalized second grade fluid Hot!
Bo Yu, Xiaoyun Jiang, Haitao Qi

In this paper, we propose a numerical method to estimate the unknown order of a Riemann-Liouville fractional derivative for a fractional Stokes' first problem for a heated generalized second grade fluid. The implicit numerical method is employed to solve the direct problem. For the inverse problem, we first obtain the fractional sensitivity equation by means of the digamma function, and then we propose an efficient numerical method, that is, the Levenberg-Marquardt algorithm based on a fracti...

Thermodynamic and rate variational formulation of models for inhomogeneous gradient materials with microstructure and application to phase field modeling Hot!
Svyatoslav Gladkov, Bob Svendsen

In this work, thermodynamic models for the energetics and kinetics of inhomogeneous gradient materials with microstructure are formulated in the context of continuum thermodynamics and material theory. For simplicity, attention is restricted to isothermal conditions. The materials of interest here are characterized by (1) first- and secondorder gradients of the deformation field and (2) a kinematic microstructure field and its gradient (e.g., in the sense of director, micromorphic or Cosserat...

Identification of the elastic stiffness of composites using the virtual fields method and digital image correlation Hot!
Lebin Jiang, Baoqiao Guo, Huimin Xie

This paper presents an effective methodology for characterizing the mechanical parameters of composites using digital image correlation combined with the virtual fields method. By using a three-point bending test configuration, this method can identify all mechanical parameters of thematerialwith merely a single test. Successful results verified that this method is especially effective for characterizing composite materials. In this study, the method is applied to measure the orthotropic elas...

How does a woodpecker work? An impact dynamics approach Hot!
Yuzhe Liu, Xinming Qiu, Tongxi Yu, Jiawei Tao, Ze Cheng

To understand how a woodpecker is able accelerate its head to such a high velocity in a short amount of time, a multi-rigid-segment model of a woodpecker's body is established in this study. Based on the skeletal specimen of the woodpecker and several videos of woodpeckers pecking, the parameters of a three-degree-of-freedom system are determined. The high velocity of the head is found to be the result of a whipping effect, which could be affected by muscle torque and tendon stiffness. The me...

Mechanical properties of irradiated multi-phase polycrystalline BCC materials Hot!
Dingkun Song, Xiazi Xiao, Jianming Xue, Haijian Chu, Huiling Duan

Structure materials under severe irradiations in nuclear environments are known to degrade because of irradiation hardening and loss of ductility, resulting from irradiation-induced defects such as vacancies, interstitials and dislocation loops, etc. In this paper, we develop an elastic-viscoplastic model for irradiated multi-phase polycrystalline BCC materials in which the mechanical behaviors of individual grains and polycrystalline aggregates are both explored. At the microscopic grain sca...

An elastic model for bioinspired design of carbon nanotube bundles Hot!
Xiaoyu Sun, Zuoqi Zhang, Yuanjie Xu, Yongwei Zhang

Collagen fibers provide a good example of making strong micro-or mesoscale fibers from nanoscale tropocollagen molecules through a staggered and crosslinked organization in a bottom-up manner. Mimicking the architectural features of collagen fibers has been shown to be a promising approach to develop carbon nanotube (CNT) fibers of high performance. In the present work, an elastic model is developed to describe the load transfer and failure propagation within the bioinspired CNT bundles, and ...

Noise-induced bistable switching dynamics through a potential energy landscape Hot!
Yuanhong Bi, Zhuoqin Yang, Xiangying Meng, Qishao Lu

Interlinked positive feedback loops, an important building block of biochemical systems, can induce bistable switching, leading to long-lasting state changes by brief stimuli. In this work, prevalent mutual activation between two species as another positive feedback is added to a generic interlinked positive-feedback-loop model originating from many realistic biological circuits. A stochastic fluctuation of the positive feedback strength is introduced in a bistable interval of the feedback st...

A piezoelectric energy harvester based on internal resonance Hot!
Liqun Chen, Wenan Jiang

A vibration-based energy harvester is essentially a resonatorworking in a limited frequency range. To increase the working frequency range is a challenging problem. This paper reveals a novel possibility for enhancing energy harvesting via internal resonance. An internal resonance energy harvester is proposed. The excitation is successively assumed as the Gaussian white noise, the colored noise defined by a second-order filter, the narrow-band noise, and exponentially correlated noise. The co...

Experimental studies on active control of a dynamic system via a time-delayed absorber Hot!
Jian Xu, Yixia Sun

The traditional passive absorber is fully effective within a narrow and certain frequency band. To solve this problem, a time-delayed acceleration feedback is introduced to convert a passive absorber into an active one. Both the inherent and the intentional time delays are included. The former mainly comes from signal acquiring and processing, computing, and applying the actuation force, and its value is fixed. The latter is introduced in the controller, and its value is actively adjustable. ...

Mechanokinetics of receptor-ligand interactions in cell adhesion Hot!
Ning Li, Shouqin Lü, Yan Zhang, Mian Long

Receptor-ligand interactions in blood flow are crucial to initiate such biological processes as inflammatory cascade, platelet thrombosis, as well as tumor metastasis. To mediate cell adhesion, the interacting receptors and ligands must be anchored onto two apposing surfaces of two cells or a cell and a substratum, i.e., two-dimensional (2D) binding, which is different from the binding of a soluble ligand in fluid phase to a receptor, i.e., three-dimensional (3D) binding. While numerous works...

Tissue level microstructure and mechanical properties of the femoral head in the proximal femur of fracture patients Hot!
Linwei Lü, Guangwei Meng, He Gong, Dong Zhu, Jiazi Gao, Yubo Fan

This study aims to investigate the regional variations of trabecular morphological parameters and mechanical parameters of the femoral head, as well as to determine the relationship between trabecular morphological and mechanical parameters. Seven femoral heads from patients with fractured proximal femur were scanned using a micro-CT system. Each femoral head was divided into 12 sub-regions according to the trabecular orientation. One 125mm3 trabecular cubic model was reconstructed...

Cell biomechanics and its applications in human disease diagnosis Hot!
Yasaman Nematbakhsh, Chwee Teck Lim

Certain diseases are known to cause changes in the physical and biomechanical properties of cells. These include cancer, malaria, and sickle cell anemia among others. Typically, such physical property changes can result in several fold increases or decreases in cell stiffness, which are significant and can result in severe pathology and eventual catastrophic breakdown of the bodily functions.While there are developed biochemical and biological assays to detect the onset or presence of disease...

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