微管道内湍流转捩的实验研究

研究旨在确定微管道内流动从层流到湍流转捩的临界雷诺数。利用微观粒子图像测速技术(Micro-PIV)研究了去离子水在内径为230μm的圆形截面玻璃微管道内的流场结构,得到了从层流到充分发展湍流各流动状态下的轴向平均速度分布和湍流度分布,实验雷诺数为1020~3145,同时研究了微管道内的流动阻力特性。平均速度场和脉动速度场的实验结果表明微管道内从层流到湍流的转捩发生在Re=1800~1900左右,与流动阻力的测量结果一致,与宏观流动比较,并未发现微管道内的流动转捩有明显提前。实验结果还显示,当Re>2700时,微管道内的平均流速分布和相对湍流度分布呈现典型的充分发展湍流状态特征。     

Lattice Boltzmann simulation of flow past a non-spherical particle

Lattice Boltzmann method was used to predict the fluid-particle interaction for arbitrary shaped particles. In order to validate the reliability of the present approach, simulation of flow past a single stationary spherical, cylindrical or cubic particle is conducted in a wide range of Reynolds number (0.1 < Re_p < 3000). The results indicate that the drag coefficient is closely related to the particle shape, especially at high Reynolds numbers. The voxel resolution of spherical particle plays a key role in accurately predicting the drag coefficient at high Reynolds numbers. For non-spherical particles, the drag coefficient is more influenced by the particle morphology at moderate or high Reynolds numbers than at low ones. The inclination angle has an important impact on the pressure drag force due to the change of projected area. The simulated drag coefficient agrees well with the experimental data or empirical correlation for both spherical and non-spherical particles.     

WALL DEPOSITION OF SMALL SUSPENDED PARTICLES IN A TURBULENT CHANNEL FLOW

The diffusion of small suspended particles in a turbulent channel flow is studied by solving the transport advection-diffusion equation. The mean flowfield in the channel is simulated using a two-equation k-ε turbulence model. Deposition velocity is evaluated at different sections in the channel for different particle sizes and flow Reynolds numbers. The effects of turbulence dispersion and Brownian diffusion on particle deposition velocity are discussed. The variation of particle deposition velocity with particle diameter, density and flow Reynolds number are analyzed. The wall deposition velocities for different size particles are compared with those obtained by other models.     

Contribution of the acoustic-wake effect to the attenuation of sound in dilute suspensions of rigid particles

This work deals with the attenuation of sound in dilute suspensions of rigid particles when the hydrodynamic mechanism of the acoustic-wake effect is considered. The study extends a previous model by considering asymmetric flow-field conditions around the particles, caused by other neighbor particles along the direction of propagation of the acoustic field. The attenuation coefficient is derived from the force equation for a spherical particle in an incompressible fluid that is oscillating at low frequencies and amplitudes within the range of Reynolds numbers of 0.1相似文献   

Large eddy simulation of particle deposition and resuspension in turbulent duct flows

Particle deposition and resuspension in a horizontal, fully developed turbulent square duct flow at four flow bulk Reynolds numbers (10,320, 83k, 215k and 250k) is simulated by applying large eddy simulation coupled with Lagrangian particle tracking technique. Forces acting on particles includes drag, lift, buoyancy and gravity. Four particle sizes are considered with the diameters of 5?μm, 50?μm, 100?μm and 500?μm. Results obtained for the fluid phase are in good agreement with the available experimental and numerical data. Predictions for particles show that particle size, flow Reynolds number and the duct (celling, floor and vertical) walls play important roles in near-wall particle deposition and resuspension. For the smallest particle (5?μm), the particle deposition rates in duct ceiling, floor and vertical walls are found to be similar with each other and all increase with the flow Reynolds number, while the particle resuspension tends to occur in the middle wall regions and corners of the duct with less influenced by the flow Reynolds number. The ceiling deposition rate gradually decreases with particle size while the floor and vertical wall deposition rates both increase with particle size. The ceiling particle deposition rate increases with Reynolds number while the floor deposition rate decreases with it. The vertical deposition rate for the small particles (5–50?μm) increases with the flow Reynolds number obviously, while for the large particles (100–500?μm) that becomes insensitive. In addition, the flow Reynolds number is found to have an obvious effect on particle resuspension while the effect of particle size on particle resuspension decreases with Reynolds number. Eventually, a dynamic analysis was conducted for particles deposition and resuspension in turbulent duct flows.     

Rotating-disk-type flow over loose boundaries

Rotating-disk-type flow of a liquid over a loose boundary, such as a layer of sand, is investigated. For this flow the formation of a new large-scale spiral pattern has been discovered. The new pattern is reminiscent of the Type-I spiral-vortex structures which characterize the laminar–turbulent transition region of boundary layers over rigid rotating disks. Flow visualizations reveal that the new pattern and the Type-I spiral vortices co-exist in the loose-boundary flow. The research investigating the origin of the new large-scale pattern is reviewed. Then photographs from flow visualizations are analysed to obtain estimates for the critical Reynolds number for which Type-I spiral vortices first appear for the loose-boundary flow and for the critical Reynolds numbers for the laminar–turbulent transition of the boundary layer. The results suggest that Type-I vortices appear at much lower Reynolds numbers over loose boundaries in comparison with flow over rigid rotating disks and that transition also appears to be advanced to much lower Reynolds numbers. The discussion of the results suggests that advanced transition arises from disturbances introduced into the flow after the loose boundary has been mobilized and not from disturbances associated with the roughness that the surfaces of the granular layer represents to the flow while grains are at rest.     

SCALING LAWS FOR PARTICLE BREAKUP IN NOZZLE GENERATED SHOCKS

Conditions for the onset of particle breakup in normal shock waves have been investigated. A normalized particle drag behind the shock has been determined in terms of gas stagnation conditions and particle diameter for a range of gas Mach numbers 1 ≤ M₁ ≤ 5 by introducing appropriately defined particle Knudsen and Reynolds numbers into analytical expressions for the drag coefficient. Numerical computations of the particle drag, normalized with gas stagnation pressure and particle area, indicate a peak at a gas Mach number M₁ ≳ 2.2, The magnitude of the peak was found to decrease with increasing particle diameter and reservoir gas density.

Criteria for the onset of agglomerate breakup were defined in terms of a modified Weber number for the adhesion mechanisms due to Van der Waals forces, electrostatic attraction and adsorbed surface films. These results indicate that larger more closely packed agglomerates made up of smaller constituent particles have a greater tendency to resist breakup for a given set of stagnation conditions and shock Mach number.     

Wall effects on separation of flow past a circular cylinder

Abstract

The nature of the flow past a circular cylinder has been a classical problem, raising many questions concerning the various wake phenomena that have been observed. This study focused on the critical Reynolds number for the onset of the recirculation region for a flow restricted in a channel. The influence of the bounded walls is examined. The trend is that larger critical Reynolds numbers are accomplished with larger values of blockage ratio (defined as the ratio of cylinder diameter to the channel width). Furthermore, as the blockage ratio tends to zero, the trend seems to imply that the critical Reynolds number approaches the experimental value for flows in an unbounded domain.     

Non-stokesian sedimentation as applied to the analysis of the interaction of particles in a suspension

For the rate of sedimentation of a particle, a simple interpolation formula including the limiting cases of high and low Reynolds numbers is suggested. It is seen that coarse particles settling by the Newton law entrain fine fractions of a suspension much more weakly than analogous particles settling according to the Stokes law. The influence of non-Stokesian sedimentation on the effect of acceleration of particles in a polydisperse suspension has been analyzed. In particular, it is found that the rate of sedimentation of the finest particles of a polydisperse suspension is equal to the Stokesian rate for a particle with an effective diameter that depends on the suspension parameters. An expression for this effective diameter is given.     

Integral light-scattering and absorption characteristics of large, nonspherical particles

We obtain and analyze simple analytical formulas for asymmetry parameters and absorption cross sections of large, nonspherical particles. The formulas are based on the asymptotic properties of these characteristics at strong and weak absorption of radiation inside particles. The absorption cross section depends on parameter phi, which determines the value of the light-absorption cross section for weakly absorbing particles. It is larger for nonspherical scatterers. The asymmetry parameter depends on two parameters. The first is the asymmetry parameter g(0) of a nonspherical, transparent particle with the same shape as an absorbing one. The second parameter, beta, determines the strength of the influence of light absorption on the value of the asymmetry parameter. Parameter beta is larger for nonspherical particles. One can find these three parameters (phi, g(0), and beta) using a ray-tracing code (RTC) for nonabsorbing and weakly absorbing particles. The RTC can then be used to check the accuracy of the equations at any absorption for hexagonal cylinders and spheroids. It is found that the error of computing the absorption cross section and 1 - g (g is the asymmetry parameter) is less than 20% at the refractive index of particles n = 1.333. Values for asymmetry parameters of large, nonabsorbing, spheroidal particles with different aspect ratios are tabulated for the first time to our knowledge. They do not depend on the size of particles and can serve as an independent check of the accuracy of T-matrix codes for large parameters.     

低雷诺数下管状非球形颗粒群的阻力系数

以红豆、黑米、铺路石、垫片、大瓷管、中瓷管作为非球形颗粒物料,以硅油作为流态化介质,通过测量液固散式流化床的流速与空隙率的关系,测量了非球形颗粒群在层流区的阻力系数。根据阻力系数和散式流态化Richarson和Zaki指数的实验结果,经研究和分析,提出了低雷诺数时非球形颗粒群阻力系数的计算公式为CD=24(εnψ0.83Re)-1。     

Study of sedimentation of non-cohesive particles via CFD–DEM simulations

The sedimentation process of granular materials exists ubiquitously in nature and many fields which involve the solid–liquid separation. This paper employs the coupled computational fluid dynamics and discrete element method (CFD–DEM) to investigate the sedimentation process of non-cohesive particles, including the hindered settling stage and the deposition stage. Firstly, the coupled CFD–DEM model for sedimentation is validated by the hindered settling velocity at different solid volume concentrations of suspension \(\phi _{0} \), i.e., \(\phi _0 =\) 0.05–0.6. Two typical modes of sedimentation are also presented by the concentration profiles and the equal-concentration lines. Then, the comparisons between mono- and poly-dispersed particle system are detailed. In the sedimentation of the poly-dispersed particle system, the segregation phenomenon is simulated. Furthermore, this segregation effect reduces with the increase of the initial solid concentration of suspension. From the simulations, the contact force between every pair of particles can be obtained, hence we demonstrate the “effective stress principle” from the view of the particle contact force by giving the correspondence between the particle contact force and the “effective stress”, which is a critical concept of soil mechanics. Moreover, the deposition stage can be simulated by CFD–DEM method, therefore the solid concentrations of sediment bed \(\phi _{\mathrm{max}} \) on different conditions are studied. Based on the simulation results of \(\phi _{\mathrm{max}} \) and the theory of sedimentation, this paper also discusses a method to calculate the critical time when sedimentation ends of two typical modes of sedimentation.     

水线-雷诺数效应-斜拉索振动关系的试验研究

通过对不同位置粘贴有水线的斜拉索模型进行测力和测振风洞试验,分析了不同雷诺数下的阻力系数、升力系数和振幅等参数,研究了水线影响雷诺数效应及通过影响雷诺数效应导致振动的机理,研究结果表明:水线能影响力系数随雷诺数的变化规律,力系数的大小和变化规律与振动有着十分密切的关系;在较低的水线位置,随着雷诺数的上升,伴随着阻力系数的减小,升力系数大幅增大,发生类似临界雷诺数的效应,力系数随水线位置的变化规律导致驰振系数为负,在这种情况下发生大幅振动,用临界雷诺数效应和驰振均可解释;在较高的雷诺数下,伴随着阻力系数的减小,升力系数大幅变化,每个水线位置都发生了不同程度的振动,该振动的机理可能与临界雷诺数效应导致的振动机理类似。     

Transition to centrifugal particle motion in rotating drums

The dynamics and the transition to the centrifugal regime are studied analytically and numerically for particles in rotating drum. The importance of the particle-wall friction coefficient is demonstrated by studying first the motion of one non-rotating particle where three different regimes are found in the transition to the centrifugal motion. When a few rotating particles are considered, they behave similarly to one non-rotating particle in the low friction limit. A critical particle number is necessary to reach the centrifugal regime for which an analytic expression is derived in the limit of negligible inter-particle friction.     

AN IMPROVED SINGLE DROPLET COLLECTION EFFICIENCY FOR THE INTERMEDIATE FLOW REGIME

An improved angle droplet collection efficiency model for the intermediate flow regime is presented in this paper, taking into account both inertial impaction and interception mechanisms. This model uses the equations of motion that has been derived by performing a force balance on a particle interacting with the flow field of a spherical collector. The fluid flow field around the collector is assumed to be the approximate solution as developed by Hamielec and Johnson for Reynolds numbers ranging between 10 and 80 and Tomotika and Aoi for Reynolds numbers less than 1.0.

The results of this work indicate that the collection efficiencies calculated by using potential flow conditions may have overestimated the overall collection of particulate matter. It was identified that the transition from intermediate to potential flow occurs when the Reynolds number is about 80. The interpolation scheme for the single droplet collection efficiency proposed in this work can be used from Stokes flow to potential flow conditions including intermediate flow regime.     

A lattice Boltzmann method for simulating transport and agglomeration of resolved particles

The main purpose of this contribution is to present a lattice Boltzmann method for modelling the transport, collision and agglomeration of freely moving spherical particles and agglomerates. In order to take the hydrodynamic interaction between fluid and particles into account, the particle surface is fully resolved by the numerical grid using a curved no-slip boundary condition. In addition to various test cases with sedimenting single particles and particle pairs, a comparison with a finite element simulation is performed to evaluate the LBM-based treatment of flow-induced particle forces for gap widths smaller than the resolution limit of the fluid. Furthermore, the influence of viscous forces on the motion of approaching particles is analysed. As a final step, first results on the transient agglomeration inside a poly-sized particle cluster settling under gravity are presented for demonstrating the applicability of the code to more complex problems such as agglomeration in turbulent two-phase flows. The obtained agglomerate morphologies are characterised by various structural parameters such as a convex hull-based porosity and the radius of gyration. In the simulations, the observed particle Reynolds numbers are in the range [0.2, 84.8].     

The linear and nonlinear stability of thread-annular flow

The surgical technique of thread injection of medical implants is modelled by the axial pressure-gradient-driven flow between concentric cylinders with a moving core. The linear stability of the flow to both axisymmetric and asymmetric perturbations is analysed asymptotically at large Reynolds number, and computationally at finite Reynolds number. The existence of multiple regions of instability is predicted and their dependence upon radius ratio and thread velocity is determined. A discrepancy in critical Reynolds numbers and cut-off velocity is found to exist between experimental results and the predictions of the linear theory. In order to account for this discrepancy, the high Reynolds number, nonlinear stability properties of the flow are analysed and a nonlinear, equilibrium critical layer structure is found, which leads to an enhanced correction to the basic flow. The predictions of the nonlinear theory are found to be in good agreement with the experimental data.     

Fluid–solid transition in unsteady,homogeneous, granular shear flows

Discrete element numerical simulations of unsteady, homogeneous flows have been performed by shearing a fixed volume of identical, soft, frictional spheres. A constant, global, shear rate was instantly applied to particles that are initially at rest, non interacting, and randomly distributed. The granular material exhibits either large or small fluctuations in the evolving pressure, depending whether the average number of contacts per particle (coordination number) is less or larger than a critical value. When the coordination number is less than the critical value, the amplitude of the pressure fluctuations is dependent on the shear rate, whereas, it is rate-independent in the opposite case, signatures, according to the case, of fluid-like and solid-like behaviour. The same critical coordination number has been previously found to represent the minimum value at which rate-independent components of stresses develop in steady, simple shearing and the jamming transition in isotropic random packings. The observed complex behaviour of the measured pressure in the fluid–solid transition can be predicted with a constitutive model involving the coordination number, the particle stiffness and the intensity of particle agitation.     

等离子喷涂中雷诺数对熔滴扁平化行为的影响

采用狭缝法收集不同工艺条件(雷诺数)下的氧化钇部分稳定的二氧化锆(YSZ)扁平粒子, 采用扫描电子显微镜(SEM)及三维激光显微镜观察形貌, 同时采用Fluent流体力学软件模拟不同雷诺数下YSZ熔滴的铺展和凝固过程, 研究雷诺数对扁平粒子形貌的影响规律。实验及计算结果表明: 当欧氏数大于0.2时, 雷诺数对YSZ熔滴的扁平化行为具有重要的影响作用, 熔滴飞溅的临界雷诺数为450±20, 即当雷诺数小于450±20时, 熔滴呈圆盘状且不发生飞溅, 反之熔滴发生飞溅。对于粒径相同的YSZ熔滴, 超音速等离子喷涂(SAPS)获得的熔滴扁平率约为普通等离子喷涂(APS)工艺的1.3倍。