Scaling law for a subcritical transition in plane Poiseuille flow

The scaling law for the threshold amplitude of perturbations to initiate a transition in subcritical plane Poiseuille flow as a function of the Reynolds number is demonstrated experimentally. The disturbances are introduced through an almost streamwise independent slot drilled at the bottom wall of a horizontal air-channel flow. Following the two stages of transition, linear transient growth and nonlinear secondary instability, it is found that the normalized critical injection rate (v0) scales with the Reynolds number (R) as v0 approximately R-3/2. This scaling law agrees with the theoretical predictions of Chapman [J. Fluid Mech. 451, 34 (2002).].     

Application of the wavelet transform in the laminar turbulent transition for a flow in a mixed convection phenomenon

The aim of this paper is to show the effect of secondary flows caused by natural convection on the laminar-turbulent hydrodynamic transition. It is not a question of measuring a critical threshold value of Reynolds number of transition but only to estimate the degree of turbulence in the transition regime, i.e. weak turbulence in the case of superposition (mixed convection) or not (forced convection) of secondary flows on the forced flow. This is possible thanks to the application of the wavelet transform. The calculation of the H?lder exponent, associated with the maximum value of the singularity spectrum for two configurations, vertical (forced convection) and horizontal (mixed convection) allows the degree of turbulence to be measured in both cases. The variation of the H?lder exponent versus the Reynolds number has enabled it to be shown that the secondary flows stabilise the main flow and stifle the beginnings of the turbulence during the regime of transition to turbulence; these kinds of results have also been shown in literature. Generally, large-sized secondary flows (for example Dean's flows) stabilise the turbulence. Our work confirms this, through an experiment carried out in identical conditions for mixed convection (horizontal flow) and forced convection (vertical flow). Received 30 March 1998 and Received in final form 28 April 1999     

Dry microfoams: formation and flow in a confined channel

We present an experimental investigation of the agglomeration of microbubbles into a 2D microfoam and its flow in a rectangular microchannel. Using a flow-focusing method, we produce the foam in situ on a microfluidic chip for a large range of liquid fractions, down to a few percent in liquid. We can monitor the transition from separated bubbles to the desired microfoam, in which bubbles are closely packed and separated by thin films. We find that bubble formation frequency is limited by the liquid flow rate, whatever the gas pressure. The formation frequency creates a modulation of the foam flow, rapidly damped along the channel. The average foam flow rate depends non-linearly on the applied gas pressure, displaying a threshold pressure due to capillarity. Strong discontinuities in the flow rate appear when the number of bubbles in the channel width changes, reflecting the discrete nature of the foam topology. We also produce an ultra flat foam, reducing the channel height from 250 μm to 8 μm, resulting in a height to diameter ratio of 0.02; we notice a marked change in bubble shape during the flow.     

Statistical analysis of the transition to turbulence in plane Couette flow

We argue on general grounds that the transition to turbulence in plane Couette flow is best studied experimentally at a statistical level. We present such a statistical analysis of experimental data guided by a parallel investigation of a simple coupled map lattice model for spatiotemporal intermittency. We confirm that this generic type of spatiotemporal chaos is relevant in the context of plane Couette flow, where the linear stability of the laminar regime at all Reynolds numbers insures the necessary local subcriticality. Using large ensembles of similar experiments, we show the existence of a well-defined threshold Reynolds number above which a unique, turbulent, intermittent attractor coexists with the laminar flow. Furthermore, our data reveals that this transition to spatiotemporal intermittency is discontinuous, i.e. akin to a first-order phase transition. Received: 10 April 1998 / Revised: 22 June 1998 / Accepted: 24 June 1998     

固液双相电流变系统流动过程的相转变特性

考虑电流变液多粒子近程相互作用，利用等效平板电导模型，研究了电流变液流动过程相转变点的特性，并设计实验观察了电流变液中的这种相转变现象.研究结果表明，电流变液在一定压力梯度作用下发生流动，此时为双相流；当外加电场达到某一值时，电流变液中颗粒不流动，由固液双相流转变为液体单相流动，发生场控相转变，理论模拟结果与实验观察结果基本相符.阈值电场随外加压力梯度的增加而加大，随颗粒浓度的增加而减小. 关键词： 电流变液 颗粒流 相转变     

Effect of acceleration threshold on the phase transition in a cellular automaton traffic flow model

In this paper, we incorporate new parameters into a cellular automaton traffic flow model proposed in our previous paper [Jin et al. 2010 J. Stat. Mech. 2010 P03018]. Through these parameters, we adjust the anticipated velocity and the acceleration threshold separately. It turns out that the flow rate of synchronized flow mainly changes with the anticipated velocity, and the F → S phase transition feature mainly changes with the acceleration threshold. Therefore, we conclude that the acceleration threshold is the major factor affecting the F → S phase transition.     

Supersonic flow in ablative discharge capillaries

The Mach number equation for plasma flow in ablative discharge capillaries was derived from a set of 1-D flow equations which incorporate the ablation and Ohmic dissipation processes. By examining this equation it is concluded that, while in the case of a capillary with a constant cross section the flow is subsonic and the sound velocity can be reached only at the open end, there is a possibility to reach supersonic plasma velocities inside a capillary with a monotonically increasing cross-sectional area.     

Transition to turbulence in a shear flow

We analyze the properties of a 19-dimensional Galerkin approximation to a parallel shear flow. The laminar flow with a sinusoidal shape is stable for all Reynolds numbers Re. For sufficiently large Re additional stationary flows occur; they are all unstable. The lifetimes of finite amplitude perturbations shows a fractal dependence on amplitude and Reynolds number. These findings are in accord with observations on plane Couette flow and suggest a universality of this transition scenario in shear flows.     

单毛细管中赫切尔-巴尔克莱流体的分形分析

运用分形理论给出了非牛顿流体中的赫切尔-巴尔克莱流体在单毛细管中的渗流分形模型. 此模型将赫切尔-巴尔克莱流体的流量、流速、启动压力梯度和有效渗 透率与流体的流变特性、毛细管的结构参数联系起来, 并且不含经验常数, 每个参数都具有明确的物理意义, 所得分形模型更能体现出赫切尔-巴尔克莱流体流动的内在物理机理.     

Modelling two-phase flow in porous media at the pore scale using the volume-of-fluid method

We present a stable numerical scheme for modelling multiphase flow in porous media, where the characteristic size of the flow domain is of the order of microns to millimetres. The numerical method is developed for efficient modelling of multiphase flow in porous media with complex interface motion and irregular solid boundaries. The Navier–Stokes equations are discretised using a finite volume approach, while the volume-of-fluid method is used to capture the location of interfaces. Capillary forces are computed using a semi-sharp surface force model, in which the transition area for capillary pressure is effectively limited to one grid block. This new formulation along with two new filtering methods, developed for correcting capillary forces, permits simulations at very low capillary numbers and avoids non-physical velocities. Capillary forces are implemented using a semi-implicit formulation, which allows larger time step sizes at low capillary numbers. We verify the accuracy and stability of the numerical method on several test cases, which indicate the potential of the method to predict multiphase flow processes.     

开口角度对二维颗粒流稀疏流-密集流转变的影响

用计算机模拟的方法研究了开口角度对二维颗粒流稀疏流—密集流转变的影响.在固定入口流量和固定颗粒数两种条件下,均发现当开口角度大于零时,开口角度的增大可以提高颗粒流由稀疏流向密集流转变的最大出口流量.在稀疏流状态下,出口流量与开口角度无关;而在密集流状态下,出口流量随开口角度的增大而增大.进一步的计算还发现增加开口角度可以提高颗粒流出开口的流动速度,且最大出口流量与颗粒的流动速度呈线性关系. 关键词： 颗粒物质 颗粒流 分子动力学模拟     

Lattice-Boltzmann accuracy in pore-scale flow simulation

We investigate the possibility of using nominally second-order-accurate techniques for resolving flow about solid boundaries as a means of improving accuracy and reducing grid resolution requirements in pore-scale simulations. An LBGK method is used to calculate flow in several geometries of increasing complexity, using a first-order accurate and two nominally second-order-accurate methods for no-slip boundaries. The geometries include uniform flow past an isolated sphere, quadratic flow past a sphere near a wall, flow through a BCC array of spheres, and through a randomly packed bed of spheres. The packed bed flows are also used to compare hydrodynamic dispersion results. The results confirm second-order-accurate behavior where Navier–Stokes flows are clearly developed. However 3D pore-scale simulations involve a trade-off between resolution of the flow and the number of pore spaces, and there is a resolution threshold, below which certain flow features, such as recirculation, are not resolved. We conjecture that most simulations will tend to operate near this threshold because of the competing demands for resolution and statistical accuracy. We consider local flow features and the velocity distribution, in addition to hydraulic permeability and drag, to provide a fuller understanding of accuracy near this threshold.     

微通道冷却器内流动和传热特性的数值模拟

为满足固体激光器用微通道冷却器的换热要求, 根据冷却器结构分别建立了二维和三维物理模型, 利用计算流体力学方法首先对比研究两者的流动特性, 然后考察雷诺数和玻片生热量对微通道流动和传热特性的影响。结果表明:对于类似大平板间的矩形微通道层流流动区域, 其流动及传热特性可直接采用二维简化模型进行模拟分析;对于重点关注的转捩区, 采用三维模型模拟分析更好;当雷诺数增大到转捩点, 流体的传热效果得到明显增强;随着雷诺数的增大, 玻片生热量对通道内最低压力需求的影响逐渐减小;不同玻片生热量对微通道流动影响不可忽略, 对努赛尔数和通道总压降基本无影响。     

Generation of a magnetic field by dynamo action in a turbulent flow of liquid sodium

We report the observation of dynamo action in the von Kármán sodium experiment, i.e., the generation of a magnetic field by a strongly turbulent swirling flow of liquid sodium. Both mean and fluctuating parts of the field are studied. The dynamo threshold corresponds to a magnetic Reynolds number R(m) approximately 30. A mean magnetic field of the order of 40 G is observed 30% above threshold at the flow lateral boundary. The rms fluctuations are larger than the corresponding mean value for two of the components. The scaling of the mean square magnetic field is compared to a prediction previously made for high Reynolds number flows.     

Numerical simulation for a two-phase porous medium flow problem with rate independent hysteresis

The paper is devoted to the numerical simulation of a multiphase flow in porous medium with a hysteretic relation between the capillary pressures and the saturations of the phases. The flow model we use is based on Darcy's law. The hysteretic relation between the capillary pressures and the saturations is described by a play-type hysteresis operator. We propose a numerical algorithm for treating the arising system of equations, discuss finite element schemes and present simulation results for the case of two phases.     

Effect of unit Reynolds number on the laminar-turbulent transition on a swept wing in supersonic flow

An experimental study of the influence of unit Reynolds number on the position of laminar-turbulent transition in swept-wing boundary layer at supersonic flow velocities was carried out. In the experiments, a swept-wing model with 3-% circular arc airfoil and 45-deg gliding angle of wing edges was used. The position of the transition was iden-tified using a hot-wire anemometer. It was found that at М = 2 and 2.5, an increase in the unit Reynolds number (Re₁) leads to a transition delay. It was shown that an increase in freestream Mach number and in the level of flow pulsations in the wind-tunnel test section leads to a less pronounced influence of Re1 on the transition position. At a high noise level due to the growth of Mach number or due to the introduction of vortical disturbances, no effect due to unit Reyn-olds number on the transition position was observed.     

Spontaneous growth of fluctuations in the viscous flow of a fluid past a soft interface

The flow induced instability in the flow past a soft material is studied in the limit of low Reynolds number where inertial effects are insignificant. A transition from laminar flow to a more complicated flow profile is observed when the strain rate of the base flow increases beyond a critical value; the transition is found to be reproducible. The experimental results are compared with theoretical predictions and quantitative agreement is found with no adjustable parameters.     

Bifurcation of a Newtonian-fluid flow in a planar channel with sudden contraction and expansion

The features of a Newtonian-fluid flow in a two-dimensional channel with sudden contraction and expansion are investigated by numerical modeling. The kinetics of the bifurcation transition from the symmetric mode to steady-state asymmetric flow on the outlet from the zone of contraction of the channel is analyzed. The linear dependence of the degree of asymmetry of flow on the Reynolds number is established.     

Study of force-dependent and time-dependent transition of secondary flow in a rotating straight channel by the lattice Boltzmann method

A numerical study using the lattice Boltzmann method has been carried out for flow through a rotating straight channel with a rectangular cross section. With different forces applied, the secondary flow exhibits two-cell states, four-cell states or six-cell states at a range of low rotational Reynolds number, however, within which only the two-cell states have been commonly reported. In addition to the force-dependent flow transition, a time-dependent flow transition of the secondary flow among two-cell states, four-cell states and six-cell states is also discovered during flow development. These newly found flow transitions and their regulations by force application have been analyzed. Based on numbers of case studies, it is found that a dimensionless number, the ratio of the driving pressure gradient to the centrifugal force, regulates such flow transitions. This study not only releases new phenomena of flow transition, but also indicates new applications in flow control, particle separation and heat transfer.     

Flow separation behind a rib in a channel with laminar flow

Visualization data and results of combined measurements of flow quantities in flow with separation past a rib at nominally laminar regime of channel flow are reported. In the separation region, the flow is found to be essentially three-dimensional and unsteady, exhibiting a distinct cellular structure and flow zones with transverse motion. It is shown that the rib-induced flow separation gives rise to low-frequency fluctuations of flow velocity and initiates the turbulence transition in the channel flow. The critical Reynolds number at which flow instability starts developing in the channel is estimated. It is shown that at Reynolds numbers higher than the critical Reynolds number the linear integral scale of flow velocity fluctuations in the channel is defined by the duct size.