This work studies the spontaneous droplet flow in tubes with different wettability gradient and number of segments by using a particle-based numerical method, many-body dissipative particle dynamics (MDPD). The study aims to understand how the different contact angle sets (gradient contact angle distribution along with different segments of a tube) and the number of segments could affect the droplet flow velocity and time to reaching a stable state. Simulation results show that even with the same wettability gap, a contact angle set with higher values of contact angle along the tube segments can drive the droplet to flow faster in the tubes; moreover, with the same contact angle set and tube length, the tubes with fewer segments can allow a faster flow of droplet, thus a shorter time to reaching the stable state. The numerical findings in this work can provide a new idea and direction for the design of droplet-based microfluidic systems.

Droplets sliding on surfaces always exhibit an advancing and a receding contact angle. When exerting different driving forces on the droplet to force it to slide at different velocities, the droplet would alter its shape to adapt to the new motion. Hence, different advancing/receding contact angles are likely to be observed, leading to the multiple contact angle hysteresis on a given surface. To verify this hypothesis, many-body dissipative particle dynamics is employed to perform the sliding simulation on both chemically homogeneous and heterogeneous surfaces. By ensuring the droplet sliding in uniform motions under different driving forces, the advancing/receding contact angles are recorded for analysis. Simulation results show that, for homogeneous surfaces, a larger driving force can result in both larger advancing contact angle and smaller receding contact angle, while for heterogeneous surfaces, increasing the driving force only results in smaller receding contact angles. For both cases, multiple contact angle hysteresis can be observed. These observations are contrary to the currently prevailing opinion, which believes that the contact angle hysteresis should be unique on given surfaces. Our findings would advance the understanding of wetting phenomena and possibly inspire new guidance for the design of functional interfaces.

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage.

The present work simulates a concept about how to drive droplet flowing through non-wetting (hydrophobic) capillaries without any external force by using many-body dissipative particle dynamics. By decorating the capillary segments with wettability gradients, a droplet with proper radius can be absorbed by the non-wetting capillaries and then constantly flow through the capillary. The simulation results show the droplet can keep flowing through the whole capillaries under certain wettability gradients and the flow velocity also depends on the degree of the wettability gradients. The average wettability of the whole capillary is also essential for the continuous flowing, higher non-wetting capillaries can still keep the flowing with low wettability gradients due to less surface adhesion. A strategy on how to achieve longer flow pathway is also presented. It is also find that unbalanced uptake of droplet via lateral heterogeneous surfaces cannot stir the inside flow of the droplet. The simulation results could inspire the new design of microfluidics in which the transportation of droplet is an important aspect.

•Propose a novel boundary condition in dissipative particle dynamics system.•The boundary condition takes the interfacial factors into consideration.•The boundary condition is quite easy to implement.•The boundary condition keeps wettability well.•The boundary condition can be applied in any complex solid surfaces.•The boundary condition mimics the fluid/solid interaction quite realistically.

《液压与气压传动》课程是机械专业本科生一门重要的基础课程.但是,中关大学在该课程的教学过程中教学内容、教学方式以及实践环节等方面都存在一定的差异,这些差异为我国高校工科专业《液压与气压传动》课程的改革和创新指出了一个方向.

To obtain the accurate analysis model of bird impact for aircraft structures,the numerical model of bird impact on LY1 2-CZ aluminum alloy flat plates was established based on the explicit finite element code of PAM-CRASH.Johnson-Cook equation was adopted to describe the constitutive model of LY1 2-CZ aluminum alloy.The stress-strain curves at 4 different strain rates of LY1 2-CZ were measured by dynamic tensile tests using electronic universal testing machine and split Hopkinson tensile bar (SHTB),and four constants in Johnson-Cook equation were fitted.Bird was modeled by smooth particle hydrodynamics (SPH ) method,and Monaghan EOS equation was introduced to simulate the bird material.Two corresponding bird impact experiments were implemented,and the strains of testing points were obtained.The calculation results were compared with the bird impact experimental results.The results show that the good agreement between numerical calculation and experimental results is obtained, which illuminates the bird constitutive model,the aluminum alloy constitutive model and the calculation method for high speed are reasonable and reliable.

Site preference of alloying elements in DO22-Ni3V phase was investigated using phase-field and first-principles method. The concentrations of alloying elements on sublattices of DO22-Ni3V phase were quantitatively studied using phase-field model based on microscopic diffusion equations. The phase-field computation results demonstrate that the concentration differences of alloying elements on the NiI and NiII site are attributed to the coordination environment difference. Host atoms Ni and substitutional ternary additions Al prefer to occupy NiI site. Antisite atoms V show site preference on the NiII site. Further reason of site preference of alloying elements on the two different Ni sites were studied using first-principles method to calculate the electronic structure of DO22-Ni3V phase. Calculation of density of states, orbitals population and charge population of the optimized Ni3V structure found that the electronic structures of NiI and NiII sites are different. Electronic structure difference, which is caused by coordination environment difference, is the essential reason for site selectivity behaviors of alloying elements on NiI and NiII sites.

The dynamic mechanical behaviors of 7075-T6 aluminum alloy at various strain rates were measured by dynamic tensile tests using the electronic universal testing machine, high velocity testing system and split Hopkinson tensile bar (SHTB). Stress-strain curves at different rates were obtained. The results show that the strain rate hardening effect of 7075-T6 aluminum alloy is significant. By modifying the strain rate hardening term in the Johnson-Cook constitutive model, a new Johnson-Cook (JC) constitutive model of 7075-T6 aluminum alloy was obtained. The improved Johnson-Cook model matched the experiment results very well. With the Johnson-Cook constitutive model, numerical simulations of tensile tests at different rates for 7075-T6 aluminum alloy were conducted. According to tensile loading and stress-strain relation of 7075-T6 aluminum alloy, calculation results were compared with experimental results. Accuracy of the modified Johnson-Cook constitutive equation was further proved. (C) 2014 Elsevier B.V. All rights reserved.

Local dent is a typical damage of aircraft, however, very few studies have investigated the effect of dents on the residual ultimate strength of the aluminum plate under different loading conditions, especially under tensions. In this study, drop-weight impact tests were conducted on the 2024-T3 aluminum alloy plate to generate 5 types of dents, and then tension tests were performed on the dented specimens to investigate the effects of the dents on the residual ultimate strength. Results show that the residual ultimate strength and the fracture displacement of the plate are significantly affected by the local dents. With increase in the depths of spherical, V-shape, and elliptical dents, the ultimate strengths decrease in quadratic trend. Whereas, for the conical and U-shape dent, linear relationships were observed between dent depth and the ultimate strength. The mathematical formulas between the ultimate strength reduction rate and the dent depth reduction factors were derived through regression analysis on the experimental results. (C) 2015 Published by Elsevier Ltd.

An optimization method combining improved genetic algorithm with sequential quadratic programming was proposed for the design of reusable launch vehicle reentry trajectory.The advantages of being insensitive to initial values and global convergence of genetic algorithm(GA),and rapid convergence and high precision of sequential quadratic programming(SQP)were developed.The weakness including solution vibration of GA and small convergence radius,being sensitive to initial values and easy to fall into a local extremum of SQP was overcome.The improved genetic algorithm with simulated annealing penalty function was employed to globally search design space and sequential quadratic programming for local optimization,while the direct collocation method was used to discretize optimal control problem into nonlinear programming problem.A global high-precision solution can be obtained without initial guess.Results show the correctness,effectiveness,insensitive to initial values and good robustness of the algorithm.

To solve trajectory optimization problem of reusable launch vehicle(RLV), a hybrid HPSO algorithm was proposed. Combining improved particle swarm optimization algorithm with sequential quadratic programming, a new particle swarm arrangement scheme was adopted to introduce time-varying inertia factors for adjusting searching velocity, and to improve the update scheme for velocity and position. Flight dynamic equations, engine model, aerodynamic model and constraint conditions of ascend trajectory optimization problems were obtained. The optimization design steps including collocation discretization, improved multi-neighborhood PSO and HPSO hybrid algorithm were determined, and the minimum fuel consumption problem was optimized and analysed. The calculation results show that a global high-precision solution of RLV trajectory optimization can be obtained even without a proper initial guess. The proposed algorithm can solve the trajectory optimization problem with good correctness, effectiveness and robustness.

*† A hybrid optimization method combining an improved genetic algorithm with sequential quadratic programming is proposed for the optimum design of the reentry trajectory of a reusable launch vehicle. The advantages of the genetic algorithm of insensitivity to the initial values and global convergence and the advantages of sequential quadratic programming of rapid convergence and high precision were obtained. The weaknesses of the genetic algorithm, including oscillation of the solution, and the weaknesses of sequential quadratic programming, including a small convergence radius, sensitivity to the initial values, and ease of falling into a local extremum, were overcome. An improved genetic algorithm with a simulated-annealing penalty function was employed to search the design space globally, sequential quadratic programming was used for local optimization, and direct collocation was used to discretize the optimal-control problem into a nonlinear programming problem. A global high-precision solution could be obtained without an initial guess because of the reduced sensitivity to the initial values. The results show the correctness, effectiveness, and robustness of the algorithm.

A direct numerical approach for space vehicle trajectory optimization was proposed. The direct transcription with Simpson quadrature was used for discretization of constrained optimal control problem. The genetic algorithm with simulated annealing penalty function was used for global search. The application to space vehicle orbit transfer optimization indicated that this method could improve the sensitive problem of initial guess effectively, attain to the global optimal result and avoid the appearance of ill-conditioned gradient. It is a good reference for the preliminary schematic trajectory design of space vehicle as well.

飞行器设计中计算的工作量很大,需要使用一种计算能力强的计算机语言对其数据进行处理。由于Fortran语言在这方面的优势,很多算法都是用Fortran编写的。而VC~(++)擅长图形界面处理,适用于人机对话和交互处理。本文探讨了VC~(++)和Fortran混合编程,重点阐述了其中的一些关键性的技术,并将其运用于飞行器可靠性的数据计算中。