Acta Mechanica Sinica最新文献

Direct numerical simulations of Mach 6 hypersonic flow over a 34° compression corner subject to steady jet are conducted. Distributions of skin friction coefficient, wall pressure, mean velocity and temperature, boundary layer thickness and Stanton number demonstrate that the flow changes dramatically in the shock wave/turbulent boundary layer interaction area. It is found that the steady jet has no effect on suppressing flow separation unexpectedly, but increases its spatial scale instead. Instantaneous flow structures show that the turbulence amplification can be observed after the application of flow control, and abundant Gortler-like vorticities appear, but the strength of the main shock decreases. Analyzing the wall fluctuating pressure signals using weighted power spectral density, we found an interesting thing. That is, although the low-frequency oscillation phenomenon induced by separation shock is suppressed by the steady jet, wall fluctuating pressure beneath the jet shock is oscillating at a frequency lower than 0.1u_∞/δ_ref. Results of coherent and intermittency factor reveal that it is related to the backand- forth movement of the jet shock itself.

The equiatomic refractory high-entropy alloys (RHEAs) exhibit the excellent performance at high temperatures, breaking through the upper limits of operating temperatures in the conventional high-temperature alloys. Here, the influences of chemistry and temperature on the deformation mechanisms of the equiatomic MoNbTaW RHEAs are investigated, using the large-scale atomic simulations. According to the microstructure evolution, a microstructure-based constitutive model is established to study the effects of the multiple strengthening mechanisms. The results show the jagged sharp fluctuations of the flow stress with the strain after the strain hardening. The increasing temperature reduces the strain-hardening rate and the amplitude of fluctuations in the flow stress, due to the reduction of the solute concentration for the annealed structure. The deformation twinning plays a certain role in the deformation mechanism in comparison with dislocation, and the local deformation is further accommodated via the dislocation-based plasticity, and amorphous nucleation in the grains. The existence of the ordered structure affects the stress and strain partition dependent upon the mechanical properties. The solid solution strengthening and grain boundary strengthening contribute considerably to the flow stress, and twinning strengthening contributes relatively little to the flow stress. Our atomic simulation and model give valuable insights into the deep understanding of chemistry and temperature related to the deformation behaviour of RHEAs.

Recently, flexible pressure and strain sensors have attracted the attention of researchers because of their high sensitivity, broad strain-sensing ability, and various forms. Flexible sensors have essential applications and broad market prospects in fields such as wearable electronics, intelligent machines, and structural health monitoring. At the same time, these emerging fields also require more significant performance requirements for flexible sensors. Conductive rubber composite materials have high tensile strength, high electromechanical sensitivity, and high stability, making them ideal for fabricating of high-performance flexible pressure sensors. Therefore, further improving the performance of conductive flexible rubber composite pressure sensors is developmental focus. In this review, the preparation and electromechanical response mechanisms of conductive polymer composites are summarized, and methods for improving the performance of flexible sensors through structural design are introduced, including conductive network structural design, substrate structural design, and conductive polymer composite structural design. In addition, the main applications of flexible pressure sensors are introduced. Finally, problems in developing flexible sensors are summarized, and future development directions are discussed.

Thin-walled structures are commonly used in the design of mechanical systems, and their flexible dynamic problems are the frontiers in engineering research. In this paper, a flexible multibody system modeling method based on the shell theory is firstly developed for the dynamics of ammunition manipulator with thin-walled structure. We obtained the kinematic equation of the Reissner-Mindlin shell structure based on the floating frame of reference formulation. The coupling of membrane deformation and bending deformation with rigid motion is integrated in the proposed model, which is the characteristic of Reissner-Mindlin shell different from solid structure. In order to overcome membrane-locking and shear-locking problems in shell element simulation, an edge-center based strain smoothing - discrete shear gap (ECSS-DSG) element is introduced. The ECSS-DSG method achieves better membrane and bending behavior, as well as effectively overcoming shear-locking. Accordingly, the ECSS-DSG shows better performance in the structural analysis. Based on these works, the parameters of the ammunition manipulator model are identified by combining with experimental results. Subsequently, the prediction of model dynamic response under various working conditions is verified, which shows its excellent robustness. Our research can not only provide theoretical support for the further study of the ammunition manipulator, but also provide reference for the study of the dynamics of multibody system with thin wall structure.

This paper presents an analytical solution for two-dimensional heterogeneous materials containing nano-fibers or pores, taking into account the Steigmann-Ogden elastic surface under far-field loading. The solution is validated against numerical results from the complex function method in recent literature, and the closed-form expressions for specific displacement and stress fields are provided. The effects of surface elasticity parameters, surface residual stress, fiber/pore size, and far-field load on local stress distribution are numerically investigated. Results show that surface elasticity parameters can disturb internal stresses within the fiber domain, while surface bending stiffness parameters significantly impact stress concentrations, which is different from the uniform stress distribution in the classical Eshelby problem. The analytical expressions reveal interesting phenomena, e.g., the stress/displacement fields of fiber composites under hydrostatic load are only related to the surface Lamé parameters, and the non-constant coefficient in the analytical expression under shear load is kept twice of that under uniaxial tensile load, which are first reported in this paper. The developed solution is crucial for accurately capturing the mechanical responses of nanocomposites with significant surface effects.

Concurrent topology optimization of structures and material orientations is a hot topic over the past decades. However, how to avoid the local optima of such problems is quite challenging. To handle this issue, a method combining the discrete material optimization method and continuous fiber orientation optimization method is proposed in our previous work, referred to as discrete-continuous parameterization (DCP), which takes advantage of the global search capability of discrete methods and the full design space of continuous methods. However, the DCP method requires too many design variables, resulting in a huge computational burden. Hence, we provide an improved DCP method to reduce the number of design variables and at the same time without sacrificing the convexity of the optimization problem in this work. In the proposed method, an extended multimaterial interpolation is firstly developed, which is capable of reducing the number of design variables greatly. Then, we integrate the proposed interpolation into the DCP method, generating an improved DCP method for the concurrent optimization of structural topology and fiber orientation. Several benchmark optimization examples show that the proposed method can greatly reduce the risk of falling into local optima with much fewer design variables.

Endovascular embolization of arteriovenous malformations (AVMs) in the brain usually requires injecting liquid embolic agents (LEAs) to reduce blood flow through the malformation. In clinical procedures, the feeding artery into which the LEAs are injected, and the amount of LEAs needs to be carefully planned preoperatively. Computational fluid dynamics can simulate the injecting process of LEAs in nidus and evaluate the therapeutic effects of different procedures preoperatively. Applying a porous media model avoided the difficulties of geometric modeling of AVMs, and the complex vascular network structure within the nidus was reproduced. The multi-phase flow was applied to simulate the interaction between LEAs and blood. The viscosity of LEAs is determined by the concentration of its solute ethylene-vinyl alcohol copolymer (EVOH). The diffusion process of the solvent dimethyl sulfoxide (DMSO) was calculated by solving the species transport equation. The coagulation of LEAs was simulated by constructing the relationship between the concentration of EVOH and viscosity. The numerical simulation method of LEAs for injection and coagulation was tested on two patient-specific AVMs. The calculations predicted the flow direction of the LEAs in the nidus. The morphology of the injected LEAs could be well visualized by 3D rendering. Quantitative analysis was conducted, including flow rate changes at the feeding arteries and draining veins. The embolization process of AVMs with LEAs can be simulated by computational fluid dynamics (CFD) methods to show the therapeutic effects of different embolization procedure planning, the optimal treatment plan can be determined.

In the present study, the influences of wall disturbances on coherent structures and the corresponding turbulent transport in supersonic turbulent boundary layers are investigated. The free-stream Mach number is set as 2.0. The database is scrutinized to present the instantaneous distributions and spectral properties of momentum and heat transfer related flow quantities. In their important roles in enhancing the momentum and heat transfer, the wall disturbances lead to the energetic turbulent fluctuations with the length scales of the wall disturbance wavelength and its harmonics in the near-wall region. The large-scale motions are also excited with the spanwise length scales of the turbulent boundary layer thicknesses, which, by reasoning, is caused by nonlinear interactions between wall disturbances and turbulent motions at some certain scales. They also produce intense density and pressure fluctuations that penetrate the boundary layer by deforming the sonic surfaces and radiate towards the free stream, where the fluctuations remain isentropic processes in nature. During this process, the steady wall disturbances are distorted by the turbulence, therefore endued with the features of multi-scale and multi-frequency instead of remaining energetic at a single wavelength or frequency.

Hypergravity can be realized by creating a field imposed by centripetal acceleration in a centrifuge apparatus. Such an apparatus is often used to test soil response in geotechnical engineering problems. Here we present the potential usage of a centrifuge apparatus to study various topics in hydrodynamics. The scaling law associated with hydrodynamics is first reviewed, and the advantage of controlling the body force is described. One of the perceived disadvantages in such experiments is the unwanted presence of the Coriolis effect in the centrifuge. However, we propose exploiting this effect to our advantage to study geophysical fluid-dynamic problems that occur particularly in the equatorial region.

To improve the effect of the traditional two-dimensional trapezoidal cavity in promoting mixing and combustion, a three-dimensional cavity with slots equipped on the cavity rear edge was proposed and investigated in a model supersonic combustor in this work. Experiments are conducted at a direct-connect test facility, where the traditional two-dimensional trapezoidal cavity (baseline) case and the slots cavity case are compared. The results show that the overall combustion along the combustor is intensified by the slots cavity. Specifically, the wall static pressure around the cavity increases by 6.1% at the lower injection pressure and 12.7% at the higher injection pressure. Based on the quasi-one-dimensional analysis, the estimated combustion efficiency increases by more than 20% due to the slots cavity. For the slots case, the combustion area and flamebase location are less affected by the injection pressure than that for the baseline case. As a result, the combustion that the slots cavity stabilizes is more stable than the combustion that the baseline cavity stabilizes. The increased exchange area, the bigger length-to-depth ratio, and the transverse flow and streamwise vortices induced by the slots may be responsible for the enhanced mixing and combustion.