翼型绕流干涉噪声的实验与数值研究

来流湍流干扰噪声在风力机叶片气动总噪声级中占有重要地位．选取圆柱/翼型干涉模型从实验和数值两方面研究此类干涉发声现象．实验中通过对翼型表面非定常载荷的测量,重点研究了圆柱位置和翼型攻角的影响,选取的翼型包括两个NACA系列翼型（NACA0012和NACA0018）和两个风力机翼型（s809和s825）,同时利用PIV（particle image velocimetry）技术对低攻角状态下翼型的前缘流场进行了研究．实验结果表明翼型表面非定常压力与圆柱涡脱落存在一定相关性．与此同时采用非定常Reynolds平均(URANS)方法对圆柱/NACA0012翼型的干涉流场进行了非定常数值模拟,并将得到的翼型表面压力频谱与实验结果进行了对比．     

小攻角高超声速钝锥边界层中不同扰动对转捩的影响

为了研究上游不同扰动对转捩位置的影响,针对来流Ma=6,攻角1°,半锥角5°的钝锥边界层的转捩进行了数值模拟．首先研究了边界层中小扰动的演化,与流动稳定性理论进行了对比,结果表明:在所考虑的流场中,流动稳定性线性理论可以对扰动的增长率做出一个较好的预测．在此基础上,研究了不同扰动对转捩位置的影响．计算给出了在两种不同频率分布的扰动情况下,转捩位置沿周向的分布．结果表明,转捩位置沿周向分布与入口扰动的幅值和频率有关．某子午面的转捩位置由该处的最不稳定波在入口的幅值决定．根据大多数风洞中背景扰动的特性,解释了小攻角圆锥转捩实验中背风面先转捩,迎风面后转捩的现象．同时,还解释了在背风面附近转捩位置“凹陷”的现象．     

考虑接触角滞后性多孔介质内非混相驱替研究

接触角滞后性表现为前进和后退接触角不同,其是润湿表面上两相流动中的重要现象.该文采用改进的伪势格子Boltzmann(LB)两相模型,并与几何润湿边界条件相结合,研究了接触角滞后性、毛细数以及几何结构对多孔介质内不混溶驱替过程的影响.数值结果表明:单渗透性多孔介质内相同毛细数下,保持后退角一定,驱替效率随着前进角的增大而增大;疏水和中性接触角滞后性窗口中,驱替效率随滞后性窗口大小增大而减小.在亲水接触角滞后性窗口中,接触角滞后性大小作用不明显;同等窗口大小下,所有选取的亲水滞后性窗口驱替效率大于中性滞后性窗口,中性滞后性窗口驱替效率大于疏水滞后性窗口.单渗透性多孔介质内相同接触角滞后性条件下,毛细数C_a越大,驱替相在多孔介质内的指进现象越明显,驱替效率越小.另外,双渗透多孔介质中驱替相更易在高渗透性区域流动并率先突破边界,驱替效率较单渗透性显著下降.     

基于不变流形的登月轨道设计

研究了基于三体问题的不变流形设计低成本登月轨道的问题．考虑了黄道面和白道面之间夹角不为零的三维情况,将太阳-地球-月亮-卫星组成的四体问题分解成由太阳-地球-卫星和地球-月亮-卫星组成的非共面的两个限制三体问题．给出了这两个三体系统Halo轨道不变流形与两轨道面相交处进行小的变轨来设计低成本探月轨道的一般方法．比较结果表明用该方法设计的轨道比传统的Hohmann变轨节省约20%的燃料．从轨道能量的角度分析了用流形设计轨道比Hohmann变轨节省燃料的原因,并给出了理论表达式．该方法对于深空探测轨道设计的能量分析具有普遍的适用性,可为设计提供一个选择参数的标准．     

Lattice Boltzmann Study on the Motion of Dual Droplets in Microchannels With Contact Angle Hysteresis北大核心CSCD

接触角滞后表现为流体在非理想固体表面上运动时前进接触角和后退接触角不同,是两相流体在润湿表面上流动的重要现象.该文采用改进的伪势格子Boltzmann(LB)多组分模型,并与几何润湿边界条件相结合,研究了两个液滴在具有接触角滞后性微通道表面上的运动行为,主要研究了通道内特征数、通道表面性质以及液滴初始参数的影响.研究结果表明：毛细数的增大有助于液滴的移动,然而并不利于液滴的排出,且毛细数的增加对上游液滴的影响大于其对下游液滴的影响;另一方面,接触角滞后性窗口越大,液滴运动和形变更迟缓,但形变程度更明显,两液滴更早地发生合并,但更晚地排出管道;液滴间距的增加使液滴的运动行为在不同阶段表现为不同的模式,但都导致通道中残留小液滴,使得液滴排出通道的时间增加.研究结果还表明：上游液滴和下游液滴的相对尺寸差距越大,越不利于液滴排出管道.     

土壤非光滑表面电渗粘附的试验优化技术研究

阎备战等．土壤非光滑表面电渗粘附的试验优化技术研究．数理统计与管理，１９９８，１７（２），１７～２１．本文应用正交多项式试验优化技术，对非光滑表面电渗原理进行了试验设计，探讨了非光滑表面电渗技术的两个重要影响因素对土壤粘附产生的影响作用，揭示了一定条件下直流电渗电压与电渗时间对粘附力的影响规律。     

基于上限理论的孔隙水压力作用下隧道洞口段含裂缝仰坡稳定性分析

研究了孔隙水压力作用下隧道洞口段含裂缝仰坡的稳定性。采用极限分析上限法,构建了坡顶含竖向裂缝的对数螺旋转动破坏机制,推导了可反映边坡临界坡高的稳定系数计算公式,将计算结果与未考虑孔隙水压力作用下的含裂缝边坡稳定性极限分析结果进行对比,验证了所提研究方法的合理性。通过算例分析,研究了坡顶裂缝最不利位置分布及仰坡整体安全系数。结果表明:坡顶裂缝开裂深度、土体内摩擦角、坡角越大及水位分布越浅,裂缝位置越靠近坡顶边缘处;孔隙水压力系数、坡顶裂缝开裂深度越大,仰坡稳定性系数越小;坡顶裂缝越深、孔隙水压力系数越大、边坡越陡,越不利于仰坡稳定;而坡内水位分布越低,越有利于仰坡稳定。     

角的转化变形策略分析

角的变形有四种：1．将一个角拆成两个角的和或差;2．对角进行换元;3．将所求式的角凑成条件式中两已知角的和或差;4．用未知角表示已知角．变形的目的是充分用已知条件,简化解题过程．     

应用正交多项式设计法研究煤的粘附规律

丛茜等．应用正交多项式设计法研究煤的粘附规律．本文对影响煤与工作部件表面法向粘附力的二个主要因素：煤的含水量及所受正压力,进行了正交多项式回归设计,建立了显著的回归方程,并揭示了煤的粘附规律     

太阳低层大气中的磁重联

向量磁场的观测,为确定太阳活动区的磁场拓扑,提供了决定性的约束,从而,为证认可能的物理过程,提供了观测基础,从向量磁场观测和理论探讨两方面证实,由视向磁图观测发现的磁对消现象,反映了两个独立的磁环系统的相互作用,主要是发生在太阳低层大气的磁重联过程,这一重联过程可能通过向上层大气输运磁通量和磁螺度,影响上层大气的磁活动,并可能导致日冕内的快重联,提供太阳耀斑所需的能量.     

Modelling lumped-parameter sorption kinetics and diffusion dynamics of odour-causing VOCs to dust particles

An analytical algorithm is presented for fast simulation of the adsorption kinetics and diffusion dynamics of odour-causing volatile organic compounds (VOC-odour) which originate in the stored swine manure to airborne dust particles in a ventilated airspace. The model is an extension to the well-known lumped-parameter model (LPM) that incorporates a Langmuir–Hinshelwood (LH) kinetic concept dependent on VOC-odour concentration with diffusion limitation. The basic idea behind the model implementation is to couple the calculations of the two major processes in the VOC-odour/dust particle system: VOC-odour diffusion based on the homogeneous surface diffusion model (HSDM) and surface reaction based on the LH kinetics in an LPM scheme. The LPM employs Laplace transforms and gamma distributions of the rate coefficient to produce a lumped-parameter gamma model (LPGM) for kinetic equation of VOC-odour adsorption to airborne dust particles, whereas the HSDM incorporates the age and size distributions of airborne dust for evaluating the dust-borne VOC-odour dynamics. The integrate assessment of VOC-odour sorption kinetics and diffusion dynamics allows to relate the adsorption rate coefficient, reaction order, and surface effective diffusivity in a complex VOC-odour/dust particle system. The LPGM fitted well with the data obtained numerically from HSDM and successfully determined the adsorption rate coefficient and reaction order for each sorption process.     

Solitary waves in self-gravitating dusty plasma

The electrostatic dust acoustic wave is studied in a uniform gravitating dusty plasma, when the self-gravitation does alter the linear modes given by the dispersion relation. It is observed that instead of a single nonlinear equation we get two coupled KdV-like systems. It is observed that such a system can sustain solitary-wave-like excitations under suitable conditions on the plasma parameters. Such an analysis is very important in ascertaining the fate of a grain orbiting around a magnetized planet in space, where there is a competition between magnetic and gravitational forces. And this in turn gives rise to the question of magnetogravitational instability, which plays an important role in star formation. It is observed that the gravitational soliton's amplitude, being proportional to the Jeans frequency, is always smaller compared to the electrostatic counterpart.     

Efficient implementation of the Barnes-Hut octree algorithm for Monte Carlo simulations of charged systems

Computer simulation with Monte Carlo is an important tool to investigate the function and equilibrium properties of many biological and soft matter materials solvable in solvents.The appropriate treatment of long-range electrostatic interaction is essential for these charged systems,but remains a challenging problem for large-scale simulations.We develop an efficient Barnes-Hut treecode algorithm for electrostatic evaluation in Monte Carlo simulations of Coulomb many-body systems.The algorithm is based on a divide-and-conquer strategy and fast update of the octree data structure in each trial move through a local adjustment procedure.We test the accuracy of the tree algorithm,and use it to perform computer simulations of electric double layer near a spherical interface.It is shown that the computational cost of the Monte Carlo method with treecode acceleration scales as log N in each move.For a typical system with ten thousand particles,by using the new algorithm,the speed has been improved by two orders of magnitude from the direct summation.     

Numerical modeling of interaction between surface radiation and natural convection of atmospheric aerosol in presence of transverse magnetic field

After the era of industrialization, technology is developing daily since the last century. Urbanization, communication, and transportation have grown rapidly and simultaneously deforestation and volcanic eruptions take place on a large scale. As result every moment tons of foreign particles like soot, dust, ash, and bio-fuel contaminants are released into the atmosphere. These contaminants mix with air and various green house gases, form a blanket structure in atmosphere. This mixture of ultrafine particle suspension with atmospheric air is known as aerosol. In the present study, numerical simulations of hydrodynamic single cell buoyant convection of atmospheric aerosol sample enclosed within a gray enclosure in the presence of a transverse magnetic field and surface radiation is addressed. Flow of the aerosol over deserts and industrial belts is a practical example of such a condition, where the thermal radiation emanating from the surface, affects the flow mechanism of the aerosol transport. The emphasis of the present study is only on carbon-black solid particles of a size in the nanometer range present in atmospheric air. The aerosol is treated as nanofluid for the numerical simulation. A comprehensive study on the controlling parameters that affect the flow and heat transfer characteristics are delineated. The governing equations are solved using modified MAC method and SIMPLER algorithm has been used to solve pressure velocity coupling employing relaxation technique. The transport equation for surface radiation is solved using the net radiation method. The cross string method is used to evaluate the view factor. The most striking result is that the heat transfer rate increases with increase in the volume fraction of the carbon-black particles, which has an adverse effect on both the climate and living creatures. The results are presented in tabular and graphical form. The heat transfer and flow characteristics are depicted in the form of isotherms and streamlines revealing the physics of this complex phenomenon.     

A study of the vestigial records of cosmic rays in lunar rocks using a thick section technique

An experimental technique has been developed for systematic measurements of fossil tracks along selected planes cut from grains and rocks. With controlled etching, the technique allows successive revelation of tracks in different minerals in the same section, a typical sequence being olivine, anorthite, clinopyroxene. It thus becomes possible to study precisely the cosmic ray track density variations over dimensions much greater than those of individual crystals. The technique also provides accurate information on the relative recording characteristics of different minerals present in a rock and cosmic ray tracks can be studied with a minimum interference of tracks due to spontaneous fission of uranium and transuranic elements. Continuous chains of sections, each section measuring approximately 1 cm., have been cut along several different planes in fifteen rocks from Mare Tranquillitatis, Oceanus Procellarum and Fra Mauro region. The cosmic ray track measurements from these sections have provided dramatic evidence for a number of processes affecting lunar rocks. The statistical, and non-uniform nature of erosion by micrometeorite bombardment can be seen in sections intersecting exposed surface which show regions of very steep track density gradients interspersed with eroded regions having lower track densities. The thick section technique permits determination of the energy spectrum of VH nuclei from track density gradients that extend over distances limited only by the dimensions of the rock, and, more important, in samples of identical orientation. The latter is particularly important in higher energy regions (deeper within the rock) where variations in crystal orientation cause track density differences of the same order as real changes in the gradient. Also in the near surface regions of rocks where low energy particles produce steep track density gradient, the thick section method has proved indispensable since it permits accurate depth determinations not possible in the spot sampling procedure. In this paper the technique of studying track profiles in thick sections is described. Although developed primarily for studying lunar samples, the thick section technique is also useful for similar studies in meteorites, particularly for gas-rich meteorites containing irradiated grains. In contrast to single grain studies, thick sections preserve the grain boundaries and permit accurate depth—density measurements. In addition thick section studies have revealed occasional large uniformly irradiated lithic fragments which would not have been possible to discover by spot sampling methods.     

Simulation of the temperature and wear behaviour of a disc brake

Aerosols are defined in the simplest form as a mixture of solid or liquid particles suspended in a gas [1]. Brake dust is an aerosol, generated as a result of erosion between brake disc and brake pad. The aerosols can cause visual disorder, heart and lung diseases, short term acute symptoms like asthma, bronchitis and long term chronic irritation and inflammation of the respiratory track, which can lead to cancer. Therefore, it is important to know which factors are affecting the erosion rate of brakes. The main objective of this work is to measure the erosion rate of brake discs and brake pads, depending on temperature changes and deformation by performing experiments and simulations at varying velocities and load conditions. The general purpose, commercially available finite element solver Abaqus is used for performing the thermomechanically coupled simulations. Archad's Wear Law is used for the wear calculation. Experiments are conducted by using a pin on disc test bench. Temperature changes are measured by using thermocouples and erosion rates by measuring the total volume loss. At the end, relationships between erosion rates and temperature changes for different brake disc velocities and load conditions are investigated and experimental and simulation results are compared and discussed. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The depth of a steep evaporating grain boundary groove: Application of comparison theorems

The nonlinear Mullins diffusion equation for the development of a surface groove by evaporation-condensation is reconsidered. Comparison theorems for differential equations are employed to obtain upper and lower bounds on the profile of the groove itself and on the growth rate of the groove at the grain boundary. It is shown that as the dihedral angle at the grain boundary approaches zero, the groove growth rate diverges logarithmically. This phenomenon is in disparity with the finite growth rate found in the development of grooves by surface diffusion.     

Three dimensional electromagnetic scattering T-matrix computations

The infinite T-matrix method is a powerful tool for electromagnetic scattering simulations, particularly when one is interested in changes in orientation of the scatterer with respect to the incident wave or changes of configuration of multiple scatterers and random particles, because it avoids the need to solve the fully reconfigured systems. The truncated T-matrix (for each scatterer in an ensemble) is often computed using the null-field method. The main disadvantage of the null-field based T-matrix computation is its numerical instability for particles that deviate from a sphere. For large and/or highly non-spherical particles, the null-field method based truncated T-matrix computations can become slowly convergent or even divergent. In this work, we describe an electromagnetic scattering surface integral formulation for T-matrix computations that avoids the numerical instability. The new method is based on a recently developed high-order surface integral equation algorithm for far field computations using basis functions that are tangential on a chosen non-spherical obstacle. The main focus of this work is on the mathematical details required to apply the high-order algorithm to compute a truncated T-matrix that describes the scattering properties of a chosen perfect conductor in a homogeneous medium. We numerically demonstrate the stability and convergence of the T-matrix computations for various perfect conductors using plane wave incident radiation at several low to medium frequencies and simulation of the associated radar cross of the obstacles.     

Effect of magnetic field dependent viscosity on ferroconvection in the presence of dust particles

This paper deals with the theoretical investigation of the effect of magnetic field dependent (MFD) viscosity on the thermal convection in a ferromagnetic fluid in the presence of dust particles. For a flat ferromagnetic fluid layer contained between two free boundaries, the exact solution is obtained using a linear stability analysis and a normal mode analysis method. For the case of stationary convection, dust particles always have a destabilizing effect, whereas the MFD viscosity has a stabilizing effect on the onset of convection. In the absence of MFD viscosity, the destabilizing effect of magnetization is depicted but in the presence of MFD viscosity, non-buoyancy magnetization may have a destabilizing or a stabilizing effect on the onset of convection. The critical wave number and critical magnetic thermal Rayleigh number for the onset of stationary convection are also determined numerically for sufficiently large values of buoyancy magnetization parameter M ₁. Graphs have been plotted by giving numerical values to the parameters to depict the stability characteristics. It is observed that the critical magnetic thermal Rayleigh number is reduced solely because the heat capacity of clean fluid is supplemented by that of the dust particles. The principle of exchange of stabilities is found to hold true for the ferromagnetic fluid heated from below in the absence of dust particles. The oscillatory modes are introduced due to the presence of the dust particles, which were non-existent in their absence. A sufficient condition for the non-existence of overstability is also obtained.