DRAG FORCE IN DENSE GAS—PARTICLE TWO—PHASE FLOW

Numerical simulations of flow over a stationary particle in a dense gas-particle two-phase flow have been carried out for small Reynolds numbers (less than 100). In order to study the influence of the particles interaction on the drag force, three particle arrangements have been tested: a single particle, two particles placed in the flow direction and many particles located regularly in the flow field. The Navier-Stokes equations are discretized in the three-dimensional space using finite volume method. For the first and second cases, the numerical results agree reasonably well with the data in literature. For the third case, i.e., the multiparticle case, the influence of the particle volume fraction and Reynolds numbers on the drag force has been investigated. The results show that the computational values of the drag ratio agree approximately with the published results at higher Reynolds numbers (from 34.2 to 68.4), but there is a large difference between them at small Reynolds numbers. The project supported by the Special Funds for Major State Basis Research Projects in China (G19990222).     

Experiments in Turbulent Pipe Flow with Polymer Additives at Maximum Drag Reduction

In this paper we report on (two-component) LDV experiments in a fully developed turbulent pipe flow with a drag-reducing polymer (partially hydrolyzed polyacrylamide) dissolved in water. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity at the wall is approximately 10000. We have used polymer solutions with three different concentrations which have been chosen such that maximum drag reduction occurs. The amount of drag reduction found is 60–70%. Our experimental results are compared with results obtained with water and with a very dilute solution which exhibits only a small amount of drag reduction. We have focused on the observation of turbulence statistics (mean velocities and turbulence intensities) and on the various contributions to the total shear stress. The latter consists of a turbulent, a solvent (viscous) and a polymeric part. The polymers are found to contribute significantly to the total stress. With respect to the mean velocity profile we find a thickening of the buffer layer and an increase in the slope of the logarithmic profile. With respect to the turbulence statistics we find for the streamwise velocity fluctuations an increase of the root mean square at low polymer concentration but a return to values comparable to those for water at higher concentrations. The root mean square of the normal velocity fluctuations shows a strong decrease. Also the Reynolds (turbulent) shear stress and the correlation coefficient between the stream wise and the normal components are drastically reduced over the entire pipe diameter. In all cases the Reynolds stress stays definitely non-zero at maximum drag reduction. The consequence of the drop of the Reynolds stress is a large polymer stress, which can be 60% of the total stress. The kinetic-energy balance of the mean flow shows a large transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energy of the turbulence suggests a possibly negative polymeric dissipation of turbulent energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of a shock with a sphere suspended in a vertical shock tube

Shock wave interaction with a sphere is one of the benchmark tests in shock dynamics. However, unlike wind tunnel experiments, unsteady drag force on a sphere installed in a shock tube have not been measured quantitatively. This paper presents an experimental and numerical study of the unsteady drag force acting on a 80 mm diameter sphere which was vertically suspended in a 300 mm x 300 mm vertical shock tube and loaded with a planar shock wave of M _s = 1.22 in air. The drag force history on the sphere was measured by an accelerometer installed in it. Accelerometer output signals were subjected to deconvolution data processing, producing a drag history comparable to that obtained by solving numerically the Navier-Stokes equations. A good agreement was obtained between the measured and computed drag force histories. In order to interpret the interaction of shock wave over the sphere, high speed video recordings and double exposure holographic interferometric observations were also conducted. It was found that the maximum drag force appeared not at the time instant when the shock arrived at the equator of the sphere, but at some earlier time before the transition of the reflected shock wave from regular to Mach reflection took place. A negative value of the drag force was observed, even though for a very short duration of time, when the Mach stem of the transmitted shock wave relfected and focused at the rear stagnation point of the sphere.Received: 31 March 2003, Accepted: 7 July 2003, Published online: 2 September 2003     

等边布置三圆柱绕流的数值分析

用有限元方法对于等边三角形布置的三个相同直径的二维圆柱绕流问题进行了数值模拟，分别求得了在不同间距比下的流场分布和各圆柱的升、阻力系数以及斯托哈罗数。计算结果表明，小间距比情形下三圆柱之间的干扰是严重的，流动并不对称于中心轴线，而是偏向下游的某个圆柱。数值计算结果与有关文献和实验进行了对比，结果令人满意。     

圆柱绕流电磁控制影响因素的实验研究

实验与数值模拟表明,利用电介质溶液中圆柱体侧表面附近分布的电磁场产生电磁力可有效改变圆柱流体边界层,控制圆柱绕流.本文对圆柱绕流电磁控制过程中的主要影响因素进行了实验研究,电磁包覆在圆柱表面后部时,其控制效果与全部包覆相当,而包覆在其它位置时,消涡效果较差;电磁极板窄的圆柱绕流控制效果较极板宽的消涡与减阻效果好.另外,电磁作用参数N愈大,消涡减阻的效果愈好.     

Ill-posed problems in rheology

Experimental data are always noisy and often incomplete. This leads to ambiguities if one wants to infer from the data some functions, which are related to the measured quantity through an integral equation of the first kind. In rheology many of such so-called ill-posed problems appear. Two techniques to treat such problems, the regularization method and the maximum entropy method, are applied to the determination of the relaxation spectrum from data of small oscillatory shear flow. With simulated data from a reference spectrum it is discussed how the inferred spectrum depends on the region, in which data are available. It turns out that information about the asymptotic behavior of the measured quantity can be of great help in determining the full spectrum also from incomplete data.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Unsteady drag on a sphere by shock wave loading

The dynamic drag coefficient of a sphere by shock wave loading is investigated numerically and experimentally. The diameter of the sphere is varied from 8 m to 80 mm in numerical simulation. The axisymmetric Navier-Stokes equations are solved on a fine grid, and the grid convergence of the drag coefficient is achieved. The numerical result is validated by comparing the experimental data of a 80 mm sphere, measured by the accelerometer in a vertical shock tube. It is found that the sphere experiences in the early interaction one order higher drag than in the steady state. A transient negative drag, mainly resulting from the focusing of shock wave on the rear side of the sphere, is observed only for high Reynolds number flows, and the drag becomes positive because of increased skin friction for low Reynolds number flows.Received: 10 March 2004, Accepted: 24 May 2004, Published online: 20 August 2004[/PUBLISHED]M. Sun: Send offprints requests to     

纳米颗粒多相流体动力学研究及应用

纳米颗粒多相流研究是目前多相流研究中新的研究方向及重点发展领域. 为探索纳米尺度多相流相间作用机理及内部存在机制,采用理论分析及数值计算手段,对一般动力学方程的封闭处理、颗粒碰撞率宏观模型的有效构建、颗粒凝并系统动力学演变特性的机理分析、非稀相问题碰撞率的求取、双变量问题求解方法的建立以及一些实际应用进行了系统研究,提出了新的针对纳米尺度颗粒动力学演变的一般动力学方程求解方法,并将其应用于实际工业过程问题的研究. 该文对上述研究工作进行了综述.     

Particle and drop dynamics in the flow behind a shock wave

The results of an investigation of the dynamics of hard particles and liquid drops in the flow behind a transmitted shock wave are presented. From the equation of motion of a particle in the shock wave, relations for the displacement, velocity and acceleration as functions of time and certain velocity-relaxation parameters taking into account the properties of the gas and the aerodynamic drag of the particles are obtained for unsteady flow around the particles at an acceleration of 10³–10⁴ m/s². It is shown that the velocity-relaxation parameters are universal. Approaches to finding the aerodynamic drag of freely-accelerating bodies from the dynamics of their acceleration after being suddenly exposed to the flow are considered. It is established that under these conditions the drop dynamics observed can be well described in terms of the same velocity-relaxation parameters with account for linear growth of the transverse drop size. All the kinematic functions obtained are confirmed experimentally.     

Recent advances in studies on multiphase and reacting flows in China

The recent developments and advances of studies on multiphase and reacting flows, including gas-solid, gas-liquid, liquid-solid and reacting flows, in China are reviewed. Special emphasis is laid on the fundamental studies and numerical models. Some important experimental results are also reported. But measurement techniques are not covered.     

Maximum entropy principle in relativistic radiation hydrodynamics II: Compton and double Compton scattering

Recently [1], a procedure has been proposed in order to close the set of the moment equations of relativistic radiative fluid dynamics. In particular explicit expressions for the moments of the bremsstrahlung and Thomson scattering source terms have been given. In this work, as anticipated in [1], we shall treat in a systematic way Comptonization and double Compton scattering too. Numerical results relative to the Compton cooling of hot electrons are shown. Received November 14, 2001 / Published online June 4, 2002 RID="a" ID="a" e-mail: mascali@dmi.unict.it Communicated by Ingo Müller, Berlin     

Nanoparticle-laden flows via moment method: A review

The study of nanoparticle-laden multiphase flow has received much attention due to its occurrence in a wide range of industrial and natural phenomena. Many of these flows are multi-dimensional multi-species systems involving strong mass, momentum and energy transfer between carrying phase and dispersed particle phase. The purpose of the present paper is to survey some advances on our researches in this field over the last 5 years. The research includes the closure for particle general dynamic equation; the fundamental interaction between particle dynamics and flow coherent structures; theoretical analysis on nanoparticle collision rate; and the application of theoretical works in some specific problems.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.