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1.
In this paper.we discuss three fundamental problemsof the theory of suspended sand particles based on thetheory of fluid dynamics.They are:(1)the relative ve-locity of sand particles to the surrounding fluid in theturbulent flow.(2) the variation of the velocity fluc-tuation of the fluid when the suspended sand particlesexist,(3)the profile of the vertical distribution ofthe concentration of the suspended sand particles in two-dimensional uniform steady channel flow and the modifi-cation of usual diffusion theory.The results of our.theoretical analysis are somewhat different from the cor-responding expressions in the textbooks of hydraulicsand river mechanics.  相似文献   

2.
Combined heat and mass transfer on free, forced, and mixed convection flow along a porous wedge with magnetic effect in the presence of chemical reaction is investigated. The flow field characteristics are analyzed by the Runge-Kutta-Gill scheme with the shooting method as well as the local non-similarity method up to the third level of truncation, which are used to reduce the governing partial differential equations into nine ordinary differential equations. The governing boundary layer equations are converted to a dimensionless form by Falkner-Skan transformations. Because of the effect of suction/injection on the wall of the wedge with buoyancy force and variable wall temperature, the flow field is locally non-similar. Numerical calculations up to the third order level of truncation are carried out as a special case for different values of dimensionless parameters. Effects of the magnetic field strength in the presence of chemical reaction with variable wall temperature and concentration on the dimensionless velocity, temperature and concentration profiles are shown graphically.  相似文献   

3.
In the research of windblown sand movement,the lift-off and incident velocities of saltating sand particles play a significant role in bridging the spatial and temporal scales from single sand particle’s motion to windblown sand flux.In this paper,we achieved wind tunnel measurements of the movement of sand particles near sand bed through improving the wind tunnel experimental scheme of paticle image velocimetry(PIV) and data processing method.And then the influence of observation height on the probability distributions of lift-off and incident velocities of sand particles was analyzed.The results demonstrate that the observation height has no obvious influence on the distribution pattern of the lift-off and incident velocities of sand particles,i.e.,the probability distribution of horizontal and vertical velocities of lift-off and incident sand particles follow a Gaussian distribution and a negative exponential distribution,respectively.However,it influences the center of the Gaussian distribution,the decay constant and the amplitude of the negative exponential distribution.  相似文献   

4.
A Lorentz force flowmeter is a noncontact electromagnetic flow-measuring device based on exposing a flowing electrically conducting liquid to a magnetic field and measuring the force acting on the magnet system. The measured Lorentz force is proportional to the flow rate via a calibration coefficient which depends on the velocity distribution and magnetic field in liquid. In this paper, the influence of different velocity profiles on the calibration coefficient is investigated by using numerical simulations. The Lorentz forces are computed for laminar flows in closed and open rectangular channels, and the results are compared with the simplified case of a solid conductor moving at a constant velocity. The numerical computations demonstrate that calibration coefficients for solid bodies are always higher than for liquid metals. Moreover, it can be found that for some parameters the solid-body calibration coefficient is almost twice as high as for a liquid metal. These differences are explained by analyzing the patterns of the induced eddy currents and the spatial distributions of the Lorentz force density. The result provides a first step for evaluating the influence of the laminar velocity profiles on the calibration function of a Lorentz force flowmeter.  相似文献   

5.
An analysis is carried out to study the steady flow and heat transfer charac- teristics from a continuous flat surface moving in a parallel free stream of an electrically conducting non-Newtonian viscoelastic fluid.The flow is subjected to a transverse uni- form magnetic field.The constitutive equation of the fluid is modeled by that for a second grade fluid.Numerical results are obtained for the distribution of velocity and temperature profiles.The effects of various physical parameters like viscoelastic param- eter,magnetic parameter and Prandtl number on various momentum and heat transfer characteristics are discussed in detail and shown graphically.  相似文献   

6.
This paper discusses the effect of thermophoretic particle deposition on the transient natural convection laminar flow along a vertical flat surface,which is immersed in an optically dense gray fluid in the presence of thermal radiation.In the analysis,the radiative heat flux term is expressed by adopting the Rosseland diffusion approximation.The governing equations are reduced to a set of parabolic partial differential equations.Then,these equations are solved numerically with a finite-difference scheme in the entire time regime.The asymptotic solutions are also obtained for sufficiently small and large time.The obtained asymptotic solutions are then compared with the numerical solutions,and they are found in excellent agreement.Moreover,the effects of different physical parameters,i.e.,the thermal radiation parameter,the surface temperature parameter,and the thermophoretic parameter,on the transient surface shear stress,the rate of surface heat transfer,and the rate of species concentration,as well as the transient velocity,temperature,and concentration profiles are shown graphically for a fluid(i.e.,air) with the Prandtl number of 0.7 at 20 C and 1.013 × 10 5 Pa.  相似文献   

7.
This work deals with the influence of thermal radiation on the problem of the mixed convection thin film flow and heat transfer of a micropolar fluid past a moving infinite vertical porous flat plate with a slip velocity.The fluid viscosity and the thermal conductivity are assumed to be the functions of temperature.The equations governing the flow are solved numerically by the Chebyshev spectral method for some representative value of various parameters.In comparison with the previously published work,the excellent agreement is shown.The effects of various parameters on the velocity,the microrotation velocity,and the temperature profiles,as well as the skin-friction coefficient and the Nusselt number,are plotted and discussed.  相似文献   

8.
The steady, laminar, incompressible flow and heat transfer of a viscous fluid between two circular cylinders for two different types of thermal boundary conditions are investigated. The governing Navier-Stokes and thermal equations of the flow are reduced to a nonlinear system of ordinary differential equations. The equations are solved analyt- ically using the homotopy analysis method (HAM). Convergence of the HAM solutions is discussed in detail. These solutions are then compared with recently obtained numericM and perturbative solutions. Plots of the velocity and temperature profiles are provided for various values of the relevant parameters.  相似文献   

9.
The aim of this work is to study the effect of non-uniform single and double slot suction/injection into a steady mixed convection boundary layer flow over a vertical cone, while the axis of the cone is inline with the flow. The governing boundary layer equations are transformed into a non-dimensional form by a group of non-similar trans- formations. The resulting coupled non-linear partial differential equations are solved nu- merically by employing the quasi-linearization technique and an implicit finite-difference scheme. Numerical computations are performed for different values of the dimensionless parameters to display the velocity and temperature profiles graphically. Also, numerical results are presented for the skin friction and heat transfer coefficients. Results indicate that the skin friction and heat transfer coefficients increase with non-uniform slot suction, but the effect of non-uniform slot injection is just opposite.  相似文献   

10.
In order to investigate the thermal shock and the heat conduction property of a target under multi-pulsed laser radiation, analytic expressions of both temperature and thermal stress fields in the target are deduced on the basis of the non-Fourier conduction law and the thermo-elastic theory. Taking a stainless steel target as an example, we can solve the analytic expressions under appropriate boundary conditions by using the finite difference method and MATLAB software, and then reveal the evolution law of both surplus temperaturt, and thermal stress in the target. The results indicate that the temperature curves in the target irradiated by a multi-pulsed laser take on a delayed character in different sections away from the boundary, which is only affected by its relaxation time. The front of the stress wave is very steep in the non-Fourier numerical solutions, which presents an obvious thermal shock, so it is necessary to consider the non-Fourier effect of semi-infinite body under the high energy laser radiation.  相似文献   

11.
Simultaneous PIV and PTV measurements of wind and sand particle velocities   总被引:1,自引:0,他引:1  
Wind-blown sand is a typical example of two-phase particle-laden flows. Owing to lack of simultaneous measured data of the wind and wind-blown sand, interactions between them have not yet been fully understood. In this study, natural sand of 100–125 μm taken from Taklimakan Desert was tested at the freestream wind speed of 8.3 m/s in an atmospheric boundary layer wind tunnel. The captured flow images containing both saltating sand and small wind tracer particles, were separated by using a digital phase mask technique. The 2-D PIV (particle imaging velocimetry) and PTV (particle tracking velocimetry) techniques were employed to extract simultaneously the wind velocity field and the velocity field of dispersed sand particles, respectively. Comparison of the mean streamwise wind velocity profile and the turbulence statistics with and without sand transportation reveal a significant influence of sand movement on the wind field, especially in the dense saltating sand layer (y/δ < 0.1). The ensemble-averaged streamwise velocity profile of sand particles was also evaluated to investigate the velocity lag between the sand and the wind. This study would be helpful in improving the understanding of interactions between the wind and the wind-blown sand.  相似文献   

12.
风沙运动的沙粒带电机理及其影响的研究进展   总被引:13,自引:0,他引:13  
土壤风蚀以及沙尘暴是人类面临的重大自然环境灾害之一,它直接关系着人类的生存环境和生活质量.许多国家和政府以及科学家们对此一直十分关注并开展了大量研究.由于大气和地表条件的多样性以及地表沙粒的运动与风场作用的相互耦合,导致风沙运动问题研究的复杂性.除此之外,伴随着对风沙运动机理的研究和对沙尘暴的观测,人们发现在风沙跃移运动和沙尘暴中的沙粒带有电荷并形成风沙电场.沙粒的运动不仅导致沙粒带电和风沙电场形成,而且也直接受到沙粒带电和风沙电场的影响.为了弄清风沙电的起因以及揭示其影响规律,研究人员对此进行了野外观测、实验室测量和理论推测等.本文将就这一问题重点介绍有关风沙运动中沙粒带电的机理、风沙电场分布规律、沙粒带电对风沙运动的微宏观物理量以及沙尘暴中沙粒带电对电磁波散射的影响等方面的实验和理论研究的基本概况和进展以及作者等人在此方面的研究工作.主要包含内容有:沙粒带电及其电场的基本实验测量,粒带电对风沙运动的影响,沙尘暴对电磁波衰减的影响研究以及有关风沙起电的机理研究等. 同时,对这一研究领域中目前关注与存在的主要问题给予评述.图6参81  相似文献   

13.
风沙影响下铁路道碴变形模量的离散元数值分析   总被引:1,自引:0,他引:1  
针对风沙区有碴铁路道床的结构特性,对其在细沙贯入下的有效变形模量研究有助于理解风沙影响下的道床动力特性。采用离散单元模型对道碴碎石和细沙颗粒进行数值建模,并对不同含沙率γ下的有效变形模量进行了数值分析。结果表明,在低含沙率下(γ<30%)沙石混合体的变形模量基本保持不变,在高含沙率下(γ>30%),变形模量随含沙率的增加呈线性降低,以上变形模量随含沙量的变化规律与试验结果相一致。基于离散单元模型的数值模拟,在细观尺度上对沙石混合体的力链强度、空间分布及配位数进行了分析,揭示了沙石混合体有效变形模量随含沙量变化的内在机理。本文工作对风沙区有碴铁路道床力学行为的研究具有一定的借鉴意义,有助于促进风沙影响下有碴铁路道床的结构设计和沙害治理。  相似文献   

14.
Two-phase measurements of saltating turbulent boundary layer flow   总被引:2,自引:0,他引:2  
Mean wind and particle speed measurements as functions of heigh were made for a saltating turbulent boundary layer flow. An exponential dependence of particle flux with height above the surface was found independent of windspeed and particle size. Particle-speed distributions as functions of height above the surface were measured for experiments conducted at two ambient pressures: atmospheric pressure and approximately 1% atmospheric pressure. The wind was shown to be a more efficient mover of particles at atmospheric pressure.High-speed motion pictures of saltating ground walnut shells (of diameter 500–1500 μm and density 1.1 g/cm3) were taken in an environmental wind tunnel to simulate the planetary boundary layer. These experiments verify the existence and magnitude of particle spin rates proposed by White & Schulz (1977). There was remarkable agreement between numerical trajectory solutions, including the spin effect, and the filmed trajectories. An observation was made that not all particles spin exclusively in the vertical longitudinal plane (in the direction of flow). At low pressures (0.6 kPa) the effect of spin forces on the particle's trajectory was shown to have little influence and was verified by a theoretical force ration balance of spin to drag force.  相似文献   

15.
IntroductionDuringthetimeofbedsurfacesedimenttransportation,sedimentparticlesofbedloadmakethecollectivemotionofvariousdifferentformsontheriverbedsurface,thustheformofbedsurfacechangesconstantly .Thiskindofcollectivemotionofsedimentparticlesonthebedsurf…  相似文献   

16.
The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ 2-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed.  相似文献   

17.
The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ2-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed. The project supported by the National Natural Science Foundation of China (10532040) and the Hundred Talents Project, the Knowledge Innovation Project of Chinese Academy of Sciences (KZCX2-304). The English text was polished by Keren Wang.  相似文献   

18.
地表温度对颗粒跃移轨迹的影响   总被引:2,自引:0,他引:2  
郑晓静  岳高伟 《应用力学学报》2005,22(2):207-211,i006
为了研究由地表温度变化引起的向上的垂向气流对沙粒跃移运动的影响,本文给出了考虑近地表温度变化和水平来流风场作用下的沙粒的跃移运动。在定量给出不同时刻的近地表温度和垂向风速的基础上,计算了由于太阳辐射所引起的近地表层垂向气流对沙粒跃移运动的影响,发现:垂向风速在午后可达到1.5m/s并使得沙粒跃移轨迹的最大高度和长度分别增加55.56%和73.68%;同时,与不考虑温度效应的情况不同的是,沙粒跃移轨道最大高度将随粒径变化。  相似文献   

19.
The unsteady free convection flow over an infinite vertical porous plate, which moves with time-dependent velocity in an ambient fluid, has been studied. The effects of the magnetic field and Hall current are included in the analysis. The buoyancy forces arise due to both the thermal and mass diffusion. The partial differential equations governing the flow have been solved numerically using both the implicit finite difference scheme and the difference-differential method. For the steady case, analytical solutions have also been obtained. The effect of time variation on the skin friction, heat transfer and mass transfer is very significant. Suction increases the skin friction coefficient in the primary flow, and also the Nusselt and Sherwood numbers, but the skin friction coefficient in the secondary flow is reduced. The effect of injection is opposite to that of suction. The buoyancy force, injection and the Hall parameter induce an overshoot in the velocity profiles in the primary flow which changes the velocity gradient from a negative to a positive value, but the magnetic field and suction reduce this velocity overshoot.  相似文献   

20.
采用基于CFD(Computational Fluid Dynamics)的数值方法研究了定常吸气方法对平板风致静力的影响。研究对象选为一处于均匀来流风场中的平板模型,在该平板靠近来流端设置定常吸气入口,通过改变定常吸气系数和吸气模式计算该平板模型的风致静力三分力,进而获取定常吸气对平板风致静力作用的影响。风场数值模拟采用RSM(Reynolds Stress Equation Model)湍流模型。计算结果表明,在定常吸气作用下平板的阻力系数显著降低,扭矩系数增大;平板迎风端面的压力随着吸气能量的增强而减小,背风端面的压力基本不变。数值结果表明:定常吸气可以有效地改善平板绕流边界层结构,降低风致阻力。  相似文献   

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