国内外微型飞行器研究现状及技术特点

微型飞行器(MAV)是集航空设计、航空制造、微机械、电子、高性能数字计算机于一体的多学科交叉的综合系统。本文简述了目前国内外微型飞行器的研究现状以及微型飞行器的技术特点,并对微型飞行器的发展趋势进行了展望。     

精确的互相关算法在超声波流速测量中的应用

互相关算法能够比较准确地计算出声波在流速正反2个方向上的传播时间,进而得出时间差,并根据时间差求出流体的流速。但离散的互相关函数峰值的精度取决于采样间隔的大小,间隔减小精度会相应提高,但在实际的应用中计算量也会大幅度提高。在以单片机为核心的测量仪表中,这种规模的计算通常是不会被采用的,找到一种更简便的计算方法是解决问题的关键。在离散的互相关函数峰值附近,利用抛物线算法可以得出更精确的互相关函数峰值。这种方法允许采样间隔较大,计算时间可大幅度缩短,使同一测量精度的计算时间从3.7 s缩短到0.9 s左右。     

电磁振动式微扑翼飞行器动态特性研究

电磁驱动具有驱动电压低、作用力大等优点, 是微机械领域一种重要驱动方式。本文设计了一种电磁振动式微扑翼飞行器，在此基础上，建立了电磁振动式微扑翼飞行器非线性动力学模型，研究了不同电磁力激励下系统动态特性，获得了系统固有频率，并得出方波和正弦半波电磁力驱动能够产生较大扑动幅度的结论。最后，研究了电磁振动式微扑翼飞行器机-磁耦合非线性系统动态特性，研究结果表明电磁振动式微扑翼飞行器适合采用正弦半波电压激励，而且通过结构改进措施，提高了扑动的对称性和稳定性。     

Analytical and numerical solutions of the unsteady 2D flow of MHD fractional Maxwell fluid induced by variable pressure gradient

The nonlocal property of the fractional derivative can supply more precise mathematical models for depicting flow dynamics of complex fluid which cannot be modelled appropriately by normal integer order differential equations. This paper studies the analytical and numerical methods of unsteady 2D flow of Magnetohydrodynamic (MHD) fractional Maxwell fluid in a rectangular pipe driven by variable pressure gradient. The governing equation is formulated with Caputo time dependent fractional derivatives whose orders are distributed in interval (0, 2). A challenge is to firstly obtain the exact solution by combining modified separation of variables method with Mikusiński-type operational calculus. Meanwhile, the numerical solution is also obtained by the implicit finite difference method whose validity has been confirmed by the comparison with the exact solution constructed. Different to the most classical works, both the stability and convergence analysis of two-dimensional multi-term time fractional momentum equation are derived. Based on numerical analysis, the results show that the velocity increases with the rise of the fractional parameter and relaxation time. While an increase in the values of Hartmann number leads to a slower velocity in the rectangular pipe.     

On the Stability and Accuracy of the Spectral Difference Method

In this article, it is shown that under certain conditions, the spectral difference (SD) method is independent of the position of the solution points. This greatly simplifies the design of such schemes, and it also offers the possibility of a significant increase in the efficiency of the method. Furthermore, an accuracy and stability study, based on wave propagation analysis, is presented for several 1D and 2D SD schemes. It was found that higher than second-order 1D SD schemes using the Chebyshev–Gauss–Lobatto nodes as the flux points have a weak instability. New flux points were identified which produce accurate and stable SD schemes. In addition, a weak instability was also found in 2D third- and fourth-order SD schemes on triangular grids. Several numerical tests were performed to verify the analysis.     

Analysis and measurements of mixing in pressure-driven microchannel flow

The mixing phenomena for two fluid streams in pressure-driven rectangular microchannels are analyzed and directly compared with the measurements of mixing intensity for a wide range of aspect ratio (width/depth = 1–20). In the analysis, the three-dimensional transport equation for species mixing was solved using the spectral method in a dimensionless fashion covering a large regime of the normalized downstream distance. The analysis reveals the details of non-uniform mixing process, which originates from the top and bottom walls of the channel and stretches out toward the center of the channel, and its transition to uniformity. Employing different length scales for the non-uniform and uniform mixing regimes, the growth of mixing intensity can be expressed in a simple relationship for various aspect ratios in the large range. The mixing experiments were carried out on silicon- and poly(methyl methacrylate) (PMMA)-based T-type micromixers utilizing fluids of pH indicator (in silicon channel) and fluorescent dye (in PMMA channel) to evaluate the mixing intensity based on flow visualization images. Using conventional microscopes, the experiments demonstrate the mixing intensity as a power law of the stream velocity for all the microfluidic channels tested. The variations of measured mixing intensity with the normalized downstream distance are found in favorable agreement with the numerical simulations. The comparison between the experiments and simulations tells the capabilities and limitations on the use of conventional microscopes to measure the mixing performance.     

Manifold method coupled velocity and pressure for Navier-Stokes equations and direct numerical solution of unsteady incompressible viscous flow

Numerical manifold method (NMM) application to direct numerical solution for unsteady incompressible viscous flow Navier-Stokes (N-S) equations was discussed in this paper, and numerical manifold schemes for N-S equations were derived based on Galerkin weighted residuals method as well. Mixed covers with linear polynomial function for velocity and constant function for pressure was employed in finite element cover system. The patch test demonstrated that mixed covers manifold elements meet the stability conditions and can be applied to solve N-S equations coupled velocity and pressure variables directly. The numerical schemes with mixed covers have also been proved to be unconditionally stable. As applications, mixed cover 4-node rectangular manifold element has been used to simulate the unsteady incompressible viscous flow in typical driven cavity and flow around a square cylinder in a horizontal channel. High accurate results obtained from much less calculational variables and very large time steps are in very good agreement with the compact finite difference solutions from very fine element meshes and very less time steps in references. Numerical tests illustrate that NMM is an effective and high order accurate numerical method for unsteady incompressible viscous flow N-S equations.     

LSFD method for accurate vibration modes and modal stress-resultants of freely vibrating plates that model VLFS

The Ritz method and some finite element formulations fail to furnish accurate modal stress-resultants for vibrating plates with free edges, even though the natural frequencies and mode shapes are accurately obtained. For example, by using the Ritz method, it was found that the modal twisting moments and shear forces violate the natural boundary conditions and that they contain erroneous “oscillations”. This paper presents the least squares finite difference (LSFD) method for solving the freely vibrating plate problem. It will be shown herein that the modal stress-resultants obtained by the LSFD method satisfy the natural boundary conditions at the free edges without any oscillations.     

Spectral and Haar wavelet collocation method for the solution of heat generation and viscous dissipation in micro-polar nanofluid for MHD stagnation point flow

The aim and significance of paper presents, the semi-numerical investigation of magnetohydrodynamic flow of micropolar nanofluid with stagnation point is carried out under the influence of viscous dissipation and heat generation. The micropolar nanofluids are electrically conducting non-Newtonian fluids. The important applications of these fluids are observed in many research areas viz. bioengineering, biofuels and biomedical sectors etc. The appropriate similarity transformations are used to transform the governing equations into system of coupled nonlinear ordinary differential equations and are solved by using shifted Chebyshev collocation method and Haar wavelet collocation method. The variations in velocity, angular velocity, temperature and concentration profiles under the impact of various physical parameters, characterizing the flow field are discussed and are presented via graphs and tables. Temperature enhancement occurs with increment in each parameter except for Prandtl number. The concentration near the surface decreases with increment in the values of parameters and gradually it increases, except for Prandtl number and Schmidt number. The reverse trend of heat transfer occurs​ near a surface, when the dominance of stream velocity over stretching velocity is observed.     

Performance analysis of single‐phase,multiphase, and multicomponent lattice‐Boltzmann fluid flow simulations on GPU clusters

The lattice‐Boltzmann method is well suited for implementation in single‐instruction multiple‐data (SIMD) environments provided by general purpose graphics processing units (GPGPUs). This paper discusses the integration of these GPGPU programs with OpenMP to create lattice‐Boltzmann applications for multi‐GPU clusters. In addition to the standard single‐phase single‐component lattice‐Boltzmann method, the performances of more complex multiphase, multicomponent models are also examined. The contributions of various GPU lattice‐Boltzmann parameters to the performance are examined and quantified with a statistical model of the performance using Analysis of Variance (ANOVA). By examining single‐ and multi‐GPU lattice‐Boltzmann simulations with ANOVA, we show that all the lattice‐Boltzmann simulations primarily depend on effects corresponding to simulation geometry and decomposition, and not on the architectural aspects of GPU. Additionally, using ANOVA we confirm that the metrics of Efficiency and Utilization are not suitable for memory‐bandwidth‐dependent codes. Copyright © 2010 John Wiley & Sons, Ltd.     

固体火箭发动机喷管两相流场与结构温度场一体化数值模拟与软件实现

为准确、高效地进行固体火箭发动机的喷管结构设计与性能分析,考虑两相流场和结构温度场之间的相互影响,采用有限差分方法求解二维轴对称两相欧拉方程组,进行喷管气体-颗粒两相无黏流动数值模拟;采用有限元法求解二维轴对称瞬态导热微分方程,进行喷管复合结构温度场数值模拟;根据面向对象编程思想,采用VC+ +以人机交互的工作方式实现喷管内型面和结构设计,并完成有限差分网格和有限元网格的自动划分和显示;通过数值模拟方法与面向对象软件设计方法的有效结合,实现二维轴对称喷管两相无黏流场和复合结构温度场的一体化数值模拟. 数值模拟结果表明,该方法有助于在统一的软件平台上充分利用计算机辅助技术完成喷管性能与结构的综合评估,可以用于固体火箭发动机喷管的工程设计.