Design and application of tunable phononic crystals (PnCs) are attracting increasing interest due to their promising capabilities to manipulate acoustic and elastic waves effectively. This paper investigates topology optimization of the magnetorheological (MR) materials including PnCs for opening the tunable and wide bandgaps. Therein, the bandgap tunability of the PnCs is achieved by shear modulus variation of MR materials under a continuously changing applied magnetic field. The pseudo elemental densities representing the bi-material distribution inside the PnC unit cell are taken as design variables and interpolated with an artificial MR penalization model. An aggregated bandgap index for enveloping the extreme values of bandgap width and tunable range of the MR included smart PnCs is proposed as the objective function. In this context, the sensitivity analysis scheme is derived, and the optimization problem is solved with the gradient-based mathematical programming method. The effectiveness of the proposed optimization method is demonstrated by numerical examples, where the optimized solutions present tunable and stably wide bandgap characteristics under different magnetic fields. The tunable optimized PnCs based device that can provide a wider tunable bandgap range is also explored.

Mechanosensors are the most important organelles for osteocytes to perceive the changes of surrounding mechanical environment. To evaluate the biomechanical effectiveness of collagen hillock, cell process and primary cilium in lacunar-canalicular system (LCS), we developed pressure-electricity-structure interaction models by using the COMSOL Multiphysics software to characterize the deformation of collagen hillocks- and primary cilium-based mechanosensors in osteocyte under fluid flow and electric field stimulation. And mechanical signals (pore pressure, fluid velocity, stress, deformation) were analyzed in LCS. The effects of changes in the elastic modulus of collagen hillocks, the number and location of cell processes, the length and location of primary cilia on the mechanosensitivity and the overall poroelastic responses of osteocytes were studied. These models predict that the presence of primary cilium and collagen hillocks resulted in significant stress amplifications (one and two orders of magnitude larger than osteocyte body) on the osteocyte. The growth of cell process along the long axis could stimulate osteocyte to a higher level than along the short axis. The Mises stress of the basal body of primary cilia near the top of osteocyte is 8 Pa greater than that near the bottom. However, the presence of collagen hillocks and primary cilium does not affect the mechanical signal of the whole osteocyte body. The established model can be used for studying the mechanism of bone mechanotransduction at the multiscale level.

Particle motion in confined shear flow of viscoelastic fluids is very common in nature and has a wide range of applications. Understanding and mastering the motion characteristics of particles in viscoelastic fluids has important academic value and practical significance. In this paper, we first introduce the related equations and characteristic parameter, and then emphasize the following issues: the lateral equilibrium position of particle; interaction and aggregation of multiple particles; the chain structure formed by multiple particles; and the motion of non-spherical particle. Finally, some unresolved issues, challenges, and future research directions are highlighted.

The internal length scale (ILS) is a dominant parameter in strain gradient plasticity (SGP) theories, which helps to successfully explain the size effect of metals at the microscale. However, the ILS is usually introduced into strain gradient frameworks for dimensional consistency and is model-dependent. Even now, its physical meaning, connection with the microstructure of the material, and dependence on the strain level have not been thoroughly elucidated. In the current work, Aifantis’ SGP model is reformulated by incorporating a recently proposed power-law relation for strain-dependent ILS. A further extension of Aifantis’ SGP model by including the grain size effect is conducted according to the Hall-Petch formulation, and then the predictions are compared with torsion experiments of thin wires. It is revealed that the ILS depends on the sample size and grain size simultaneously; these dependencies are dominated by the dislocation spacing and can be well described through the strain hardening exponent. Furthermore, both the original and generalized Aifantis models provide larger estimated values for the ILS than Fleck-Hutchinson’s theory.

In this paper, elastic metasurfaces composed of zigzag units are proposed to manipulate flexural waves at a deep subwavelength scale. Through the parameter optimization of the genetic algorithm, units with full transmission and full phase control can be found, while the width is only one-fifth of the wavelength. The outstanding capability of the units is explained by analyzing their wave fields. The flat and the curved metasurfaces for focusing are designed and simulated, showing excellent performance. Experimental results of the flat metasurface show that the incident wave energy at the focal point is enhanced over 6 times, verifying the simulation results. The proposed metasurfaces could be useful in the design of compact and efficient elastic devices.

The constitutive behavior of microcrystals remains mysterious since very little, or no information regarding plastic deformation in the measured stress-strain curve is available due to plastic instability. Furthermore, the measured stress-strain curves vary greatly under different control modes, while constitutive behavior should remain unaffected by test methods. Beyond these reasons, probing the real constitutive behavior of microcrystals has long been a challenge because the nonlinear dynamical behaviors of micromechanical testing systems are unclear. Here, we perform and carefully analyze the experiments on single-crystal aluminum micropillars under displacement control and load control. To interpret these experimental results, a lumped-parameter physical model based on the principle of micromechanical testing is developed, which can directly relate nonlinear dynamics of the micromechanical testing system to the constitutive behavior of microcrystals. This reveals that some stages of the measured stress-strain curve attributed to the control algorithm are not related to constitutive behavior. By solving the nonlinear dynamics of the micromechanical testing system, intense plastic instability (large strain burst) starting from the equilibrium state is attributed to the strain-softening stage of microcrystals. Parametric studies are also performed to reduce the influence of plastic instability on the measured responses. This study provides critical insights for developing various constitutive models and designing a reliable micromechanical testing system.

This study is about an analytical attempt that explores the two-dimensional concentration distribution of a solute in an open channel flow. The solute undergoes reversible sorption at the channel bed. The method of multiple scales is used to find the two-dimensional concentration distribution, which is important for modern day application in industry, environmental risk assessment, etc. Study deduces an analytic expression of two-dimensional concentration distribution for an open channel flow with sorptive channel bed. Effects of retention parameter, Damkohler number on the solute dispersion are also discussed in this paper. Results reveal that slow or strong kinetics (small value of Damkohler number) increases solute dispersion. It is also observed that for slow phase exchange kinetics between bulk flow and small retentive channel bed, solute concentration distribution will uniform faster than their inert counterpart.

We develop and assess a model of the turbulent burning velocity $s_{\text{T}}$ over a wide range of conditions. The aim is to obtain an explicit $s_{\text{T}}$ model for turbulent combustion modeling and flame analysis. The model consists of sub models of the stretch factor and the turbulent flame area. The stretch factor characterizes the flame response of turbulence stretch and incorporates detailed chemistry and transport effects with a lookup table of laminar counterflow flames. The flame area model captures the area growth based on Lagrangian statistics of propagating surfaces and considers the effects of turbulence length scales and fuel characteristics. The present model predicts $s_{\text{T}}$ via an algebraic expression without free parameters. We assess the model using 490 cases of the direct numerical simulation or experiment reported from various research groups on planar and Bunsen flames over a wide range of conditions, covering fuels from hydrogen to n-dodecane, pressures from 1 to 30 atm, lean and rich mixtures, turbulence intensity ratios from 0.1 to 177.6, and turbulence length ratios from 0.5 to 66.7. Despite the scattering $s_{\text{T}}$ data in the literature, the comprehensive comparison shows that the proposed $s_{\text{T}}$ model has an overall good agreement over the wide range of conditions, with the averaged modeling error of 28.1%.

Wheel/rail rolling contact is a highly nonlinear issue affected by the complicated operating environment (including adhesion conditions and motion attitude of train and track system), which is a fundamental topic for further insight into wheel/rail tread wear and rolling contact fatigue (RCF). The rail gauge corner lubrication (RGCL) devices have been installed on the metro outer rail to mitigate its wear on the curved tracks. This paper presents an investigation into the influence of RGCL on wheel/rail non-Hertzian contact and rail surface RCF on the curves through numerical analysis. To this end, a metro vehicle-slab track interaction dynamics model is extended, in which an accurate wheel/rail non-Hertzian contact algorithm is implemented. The influence of RGCL on wheel/rail creep, contact stress and adhesion-slip distributions and fatigue damage of rail surface are evaluated. The simulation results show that RGCL can markedly affect wheel/rail contact on the tight curves. It is further suggested that RGCL can reduce rail surface RCF on tight curves through the wheel/rail low-friction interactions.

Oscillation of fluid flow may cause the dynamic instability of nanotubes, which should be valued in the design of nanoelectromechanical systems. Nonlinear dynamic instability of the fluid-conveying nanotube transporting the pulsating harmonic flow is studied. The nanotube is composed of two surface layers made of functionally graded materials and a viscoelastic interlayer. The nonlocal strain gradient model coupled with surface effect is established based on Gurtin-Murdoch’s surface elasticity theory and nonlocal strain gradient theory. Also, the size-dependence of the nanofluid is established by the slip flow model. The stability boundary is obtained by the two-step perturbation-Galerkin truncation-Incremental harmonic balance (IHB) method and compared with the linear solutions by using Bolotin’s method. Further, the Runge-Kutta method is utilized to plot the amplitude-frequency bifurcation curves inside/outside the region. Results reveal the influence of nonlocal stress, strain gradient, surface elasticity and slip flow on the response. Results also suggest that the stability boundary obtained by the IHB method represents two bifurcation points when sweeping from high frequency to low frequency. Differently, when sweeping to high frequency, there exists a hysteresis boundary where amplitude jump will occur.

In this work, we propose incorporating the finite cell method (FCM) into the absolute nodal coordinate formulation (ANCF) to improve the efficiency and robustness of ANCF elements in simulating structures with complex local features. In addition, an adaptive subdomain integration method based on a triangulation technique is devised to avoid excessive subdivisions, largely reducing the computational cost. Numerical examples demonstrate the effectiveness of the proposed method in large deformation, large rotation and dynamics simulation.

Electrostatic torsional micromirrors are widely applied in the fields of micro-optical switches, optical attenuators, optical scanners, and optical displays. In previous lectures, most of the micromirrors were twisted along the uniaxial or biaxial direction, which limited the range of light reflection. In this paper, a quasicrystal torsional micromirror that can be deflected in any direction is designed and the dynamic model of the electrostatically driven micromirror is established. The static and dynamic phenomena and pull-in characteristics are analyzed through the numerical solution of the strain gradient theory. The results of three kinds of mirror deflection directions are compared and analyzed. The results show the significant differences in the torsion models with different deflection axis directions. When the deflection angle along the oblique axis reaches 45°, the instability voltage is the smallest. The pull-in instability voltage increases with the increment of phonon-phason coupling elastic modulus and phason elastic modulus. The permittivity of quasicrystal, the strain gradient parameter, and the air damping influence the torsion of the micromirror dynamic system. A larger pull-in instability voltage generates with the decrease of surface distributed forces.

To understand the macroscopic mechanical behaviors of responsive DNA hydrogels integrated with DNA motors, we constructed a state map for the translocation process of a single FtsK_C on a single DNA chain at the molecular level and then investigated the movement of single or multiple FtsK_C motors on DNA chains with varied branch topologies. Our studies indicate that multiple FtsK_C motors can have coordinated motion, which is mainly due to the force-responsive behavior of individual FtsK_C motors. We further suggest the potential application of motors of FtsK_C, together with DNA chains of specific branch topology, to serve as strain sensors in hydrogels.

Sediment-water interfaces are important interfaces for lakes, which are related to most environmental and ecological problems. Wind-induced waves cause secondary pollution via sediment resuspension. Since the coupling mechanism of water, resuspended sediments, and phosphorus affects the release of phosphorus (P) near the interface, a coupled model was explored for two sediment types with different adsorption-desorption capabilities to examine sediment resuspension and P release. The relationships among wind speed, wave characteristics, sediment distribution and P concentration were obtained. For different sediments, the unit sediment desorption release is negatively correlated with wind speed. When sediments are resuspended under low or moderate wind speed, the P concentration in the overlying water increases abruptly, hampering diffusion. P release exhibits the characteristics of concentrated release in a small region and changes the water environment rapidly.

In this study, we propose valley phononic crystals that consist of a hexagonal aluminum plate with six chiral arrangements of ligaments. Valley phononic crystals were introduced into a topological insulator (TI) system to produce topologically protected edge waves (TPEWs) along the topological interfaces. The implementation of chiral topological edge states is different from the implementation of topological edge states of systems with symmetry. Unlike the conventional breaking of mirror symmetry, a new complete band with topological edge modes gap was opened up at the Dirac point by tuning the difference in lengths of the ligaments in the chiral unit cells. We investigated the dispersion properties in chiral systems and applied the dispersion properties to waveguides on the interfaces to achieve designable route systems. Furthermore, we simulated the robust propagation of TPEWs in different routes and demonstrated their immunity to backscattering at defects. Finally, the existence of the valley Hall effect in chiral systems was demonstrated. The study findings may lead to the further study of the topological states of chiral materials.

With the rapid development of artificial intelligence techniques such as neural networks, data-driven machine learning methods are popular in improving and constructing turbulence models. For high Reynolds number turbulence in aerodynamics, our previous work built a data-driven model applicable to subsonic airfoil flows with different free stream conditions. The results calculated by the proposed model are encouraging. In this work, we aim to model the turbulence of transonic wing flows with fully connected deep neural networks, where there is less research at present. The proposed model is driven by two flow cases of the ONERA (Office National d’Etudes et de Recherches Aérospatiales) wing and coupled with the Navier-Stokes equation solver. Four subcritical and transonic benchmark cases of different wings are used to evaluate the model performance. The iteration process is stable, and final convergence is achieved. The proposed model can be used to surrogate the traditional Reynolds averaged Navier-Stokes turbulence model. Compared with the data calculated by the Spallart-Allmaras model, the results show that the proposed model can be well generalized to the test cases. The mean relative error of the drag coefficient at different sections is below 4% for each case. This work demonstrates that modeling turbulence by data-driven methods is feasible and that our modeling pattern is effective.