Structure of turbulent bubbly jets—I. Methods and centerline properties

The structure of dilute bubbly turbulent round jets, injected vertically upward in still water, was studied both theoretically and experimentally. All measurements were nonintrusive, including mean and fluctuating phase velocities, bubble number intensities, bubble-size distributions and calibration of the motion of individual bubbles. Predictions from three analyses were compared with measurements: (1) locally homogeneous flow analysis, where velocity differences between the phases were neglected; (2) deterministic separated flow analysis, where relative velocity was considered but bubble/turbulence interactions were ignored; and (3) stochasic separated flow analysis, where both relative velocity and bubble/turbulence interactions were considered using random-walk methods. This paper describes theoretical and experimental methods, flow structure near the source and mean properties along the jet axis. Effects of relative velocity were important almost everywhere in the flow; therefore, only the separated flow models yielded satisfactory predictions of bubble velocities along the axis. A companion paper treats mean and fluctuating properties in other regions of the flow.     

An analysis model of pulsatile blood flow in arteries

IntroductionTheperiodicallypulsatilebloodflowinthearterycausesthecircumferentialandaxialmotionoftheelasticbloodvesselandinturntheoscillationofthevesselaffectsthatofthebloodflow .Womersley[1]resolvedsuccessfullythisfluid_solidcouplingproblembysolvingbothlinearNavier_Stokesequationsandthemotionequationsofthethin_walledelastictubeandgainedtheexpressionsofthebloodflowvelocitiesandthevasculardisplacements.Histheoryhasbeenthebasisforthequantitativeanalysisoftherelationshipofthearterialstructureandi…     

Structure of turbulent bubbly jets—II. Phase property profiles

This is the second part of a two-part study reporting structure measurements in bubbly turbulent round jets in a still environment. Measurements are compared with three theoretical approaches: (1) locally homogeneous flow analysis, where velocity differences between the phases were neglected; (2) deterministic separated flow analysis, where relative velocity was considered but bubble/turbulence interactions were ignored; and (3) stochastic separated flow analysis, where both relative velocity and bubble/turbulence interactions were considered using random-walk methods. This part of the study considers measurements and predictions of mean and fluctuating phase velocities and mean bubble number intensities at several axial stations. Locally homogeneous flow analysis was not very satisfactory since effects of relative velocity were important for present test conditions. Deterministic separated flow analysis was also ineffective, since neglecting turbulent dispersion caused the width of the bubble-containing region to be underestimated. In contrast, the stochastic separated flow analysis yielded reasonably good predictions.     

Transition in the flow between conical cylinders

Flow visualization and the polarographic method were used to study the transition to turbulence of the flow in a concentric annulus formed by conical cylinders of the same apex angle. The inner cone was rotated and the outer one kept stationary. Observations and spectral investigation revealed different flow states in the transition as the Taylor number was increased. The flow states include Taylor vortices, helical flow, wavy spirals, fluctuations regime and turbulence. In this system the turbulence occurs when all the structures observed before disappear. Therefore a complete chaotic motion is obtained. Measurements of the mean and fluctuating wall velocity gradient complete the transition information. Received: 9 October 1997/Accepted: 3 February 2000     

动脉狭窄对血液流速的影响

为了定量计算动脉局部狭窄对动脉管中血液流动速度的影响，本文分别对狭窄区域内定常流和非定常流动进行了求解，得出了狭窄区域内定常流和脉动流的速度表达式。本文将均匀段的流速形经Ｆｏｕｒｉｅｒ分解成定常和脉动两部分，然后分别计算出狭窄区域内对应的定常和脉动流速，经Ｆｏｕｒｉｅｒ合成还原成流速时域波形，同时针对各种情况将不同狭窄对不同的流速波形的作了分析比较。     

Turbulence averaging within spark ignition engines

 This paper examines velocity averaging within Spark-Ignition (SI) engines, a non-stationary system. Comparison is made between the mean and turbulence velocities found from (a) Ensemble, (b) Cyclic and (c) Wavelet-based averaging. The various methods of extracting turbulence within this flow system result in qualitatively similar average velocities; however, there are significant differences in the turbulence velocities and spectral content of the flow field based on the definition used. The differing interpretation of turbulence results in a subjectivity to the physical understanding of the flows. The experience in extracting coherent structures in stationary turbulence suggests that wavelet analysis offers a unique insight that has applicability for engine studies. Received: 25 February 1998 / Accepted: 12 August 1998     

Dynamic structures of bubble-driven liquid flows in a cylindrical tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a cylindrical tank were investigated. The time-resolved particle image velocimetry technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 1 to 5?L/min at 0.5?MPa, and the corresponding range of bubble-based Reynolds number (Re) ranged from 8,300 to 21,100. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition analysis technique. With increasing Re, mean velocity fields driven by the rising bubbles are almost same, but turbulence is dramatically enhanced. Both spatial and temporal modes were quite different with respect to the air flow rates. Three most dominant spatial structures are recirculating flow, bubble-induced motion, and sloshing of free surface, the bigger the latter the higher Re. We found the frequency of sloshing motion from flow visualization and the FFT analysis of temporal modes.     

EXPERIMENTAL FLOW STUDY WITHIN A SELF OSCILLATING COLLAPSIBLE TUBE

The flow field within a self-excited flexible tube was studied by employing flow visualization, velocity and pressure measurements. Under low positive tansmural pressures at the tube inlet (of the order of 50 mm H₂O) the tube was set to an oscillatory motion, the initiation of which was due to a flow asymmetry. Namely, although initially the flow was separated from both sides of the formed divergent nozzle close to the tube outlet, at an arbitrary instant this became attached to one side, but stayed separated in the remaining part of the nozzle. When this flow asymmetry occurred, the walls approached each other and the tube neck formed there by started oscillating streamwise, setting the tube to an oscillatory motion. During the downstream motion of the tube neck, it was shrunk, causing a twofold increase of the local velocities compared to the inlet ones, which remained almost constant. On the contrary, when moving upstream, the tube neck expanded, causing a flow reversal in this area and a flow deceleration in the remaining part of the tube. The pressure signals upstream and downstream of the tube exhibited a phase difference, the latter leading, taking an order of magnitude higher values than the first one.     

Turbulence structure and heat and mass transfer mechanism at a gas-liquid interface in a wind-wave tunnel

Turbulence structure and heat and mass transfer mechanism across a wavy sheared gas-liquid interface are fluid-mechanically investigated in a wind-wave tunnel. Heat and mass transfer velocities are reported and the relationship between the scalar transfer velocities and the turbulence structure is discussed. In addition, three-dimensional direct numerical simulation is carried out to investigate the flow structure over a rigid-wavy wall similar to that over the wave. The results show that the organized motion in the air flow intermittently appears on the front side of the wave crest, and its structure is rather similar to the flow structure over the rigid-wavy wall. The organized motion in the air flow induces the organized motion in the water flow and the organized motion renews the air-water interface. The scalar transfer across a wavy sheared gas-liquid interface is controlled by the organized surface-renewal motion in the water flow     

Study of laminar–turbulent flow transition under pulsatile conditions in a constricted channel

In this paper, direct numerical simulation is performed to investigate a pulsatile flow in a constricted channel to gain physical insights into laminar–turbulent–laminar flow transitions. An in-house computer code is used to conduct numerical simulations based on available high-performance shared memory parallel computing facilities. The Womersley number tested is fixed to 10.5 and the Reynolds number varies from 500 to 2000. The influences of the degree of stenosis and pulsatile conditions on flow transitions and structures are investigated. In the region upstream of the stenosis, the flow pattern is primarily laminar. Immediately after the stenosis, the flow recirculates under an adverse streamwise pressure gradient, and the flow pattern transitions from laminar to turbulent. In the region far downstream of the stenosis, the flow becomes re-laminarised. The physical characteristics of the flow field have been thoroughly analysed in terms of the mean streamwise velocity, turbulence kinetic energy, viscous wall shear stresses, wall pressure and turbulence kinetic energy spectra.     

Planar velocity measurements of a two-dimensional compressible wake

The present study describes the application of particle image velocimetry (PIV) to investigate the compressible flow in the wake of a two-dimensional blunt base at a freestream Mach number M_X=2. The first part of the study addresses specific issues related to the application of PIV to supersonic wind tunnel flows, such as the seeding particle flow-tracing fidelity and the measurement spatial resolution. The seeding particle response is assessed through a planar oblique shock wave experiment. The measurement spatial resolution is enhanced by means of an advanced image-interrogation algorithm. In the second part, the experimental results are presented. The PIV measurements yield the spatial distribution of mean velocity and turbulence. The mean velocity distribution clearly reveals the main flow features such as expansion fans, separated shear layers, flow recirculation, reattachment, recompression and wake development. The turbulence distribution shows the growth of turbulent fluctuations in the separated shear layers up to the reattachment location. Increased velocity fluctuations are also present downstream of reattachment outside of the wake due to unsteady flow reattachment and recompression. The instantaneous velocity field is analyzed seeking coherent flow structures in the redeveloping wake. The instantaneous planar velocity and vorticity measurements return evidence of large-scale turbulent structures detected as spatially coherent vorticity fluctuations. The velocity pattern consistently shows large masses of fluid in vortical motion. The overall instantaneous wake flow is organized as a double row of counter-rotating structures. The single structures show vorticity contours of roughly elliptical shape in agreement with previous studies based on spatial correlation of planar light scattering. Peak vorticity is found to be five times higher than the mean vorticity value, suggesting that wake turbulence is dominated by the activity of large-scale structures. The unsteady behavior of the reattachment phenomenon is studied. Based on the instantaneous flow topology, the reattachment is observed to fluctuate mostly in the streamwise direction suggesting that the unsteady separation is dominated by a pumping-like motion.     

Numerical study of physiological turbulent flows through series arterial stenoses

A numerical investigation on the characteristics of transitional turbulent flow over series bell‐shape stenoses for a physiological pulsatile flow is presented in the present study. The flow behaviours for the physiological pulsatile flow are studied by considering the effects of the Reynolds number, Womersley number, constriction ratio and spacing ratio of the stenoses on the pulsatile turbulent flow fields. Especially, the mutual influences between the double stenoses under different flow conditions are considered. The numerical results show that the variation of these flow parameters puts significant impacts on the flow developments in the arteries with series stenoses. The double stenoses lead to the higher peak turbulence disturbance and the greater area with comparatively high turbulence intensity distal to the stenoses in comparison with the single stenosis. The analysis shows that for the physiological pulsatile flow, the downstream stenosis usually does not have perceptible influences on the upstream flow fields. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of particles suspended in a fluid flow on the decay of isotropic turbulence

Dynamic equations have been obtained for the two-point double correlations of the fluctuation velocities of a fluid and the particles suspended in it at low volume concentrations of the solid phase. In the case of uniform isotropic turbulence these equations can be considerably simplified. The final period of decay of isotropic turbulence has been studied in detail. At this stage in the case of high-inertia particles the inhomogeneous-fluid turbulence is similar to the turbulence of a homogeneous fluid (without particles) in the sense that the presence of the particles affects only the fluctuation energy but leaves unchanged the spatial scales of turbulence and the spatial energy spectrum function. The suspended particles lead to exponential damping of the turbulent pulsations.Little theoretical information is available on the hydrodynamics of a suspension of fine particles in a turbulent liquid or gas. Research has been mainly confined to the behavior of the individual particles in a given turbulence field [1]. The problem of the turbulent motion of the mixture as a whole has been examined by Barenblatt [2], who derived the equations of motion of the mixture, using Kolmogorov's hypothesis to close them. Hinze [3] has also attempted to derive equations for turbulent pulsations of the mixture. However, as Murray showed [4], Hinze' s equations contradict Newton' s third law.The effect of suspended particles on the turbulence of a two-phase flow is governed by the noncorrespondence of the local velocities of the particles and the medium. The forces of resistance to the motion of the particles relative to the fluid lead to additional dissipation of fluctuation energy and decay of turbulence [2]. On the other hand, if the averaged velocities of particles and medium do not correspond, the suspended particles may also have a destabilizing effect [5, 6], causing energy transfer from the averaged to the pulsating motion. Below we shall consider the case where the averaged velocities of the two phases coincide, i.e., we shall deal only with the first of the two above-mentioned effects.The authors thank G.I. Barenblatt for his useful advice.     

Particle image velocimetry measurements of the separated flow behind a rearward facing step

Particle image velocimetry (PIV) was used to measure mean flow and turbulence characteristics in the separated flow behind a rearward facing step. The confidence limits characterising estimates of mean velocity and turbulence intensity obtained by PIV are discussed. The relevance of background turbulence levels and sample size in determining these limits is considered.     

Autoregressive spectral estimation in transitional pulsatile flow

An autoregressive spectral estimation technique is applied to describe the frequency content of velocity disturbances created by pulsatile flow through a constricted tube. The Reynolds number and frequency parameter are such that transitional phenomena, including vortex formation and coherent disturbances, as well as turbulence, are created during various phases of the pulsatile cycle. Although under some circumstances Fourier methods of spectral estimation suffer from poor frequency resolution and large variance under these unsteady flow conditions, the autoregressive technique is shown to be capable of identifying essential flow disturbance features with good resolution and considerably smaller statistical variation. This method should be particularly useful in analyzing energy spectra of flow disturbance variables under unsteady mean flow conditions or when a limited amount of data is available.     

Numerical modeling of a two-dimensional vertical turbulent two-phase jet

The results of numerically modeling two-dimensional two-phase flow of the “gas-solid particles” type in a vertical turbulent jet are presented for three cases of its configuration, namely, descending, ascending, and without account of gravity. Both flow phases are modeled on the basis of the Navier-Stokes equations averaged within the framework of the Reynolds approximation and closed by an extended k-? turbulence model. The averaged two-phase flow parameters (particle and gas velocities, particle concentration, turbulent kinetic energy, and its dissipation) are described using the model of mutually-penetrating continua. The model developed allows for both the direct effect of turbulence on the motion of disperse-phase particles and the inverse effect of the particles on turbulence leading to either an increase or a decrease in the turbulent kinetic energy of the gas. The model takes account for gravity, viscous drag, and the Saffman lift. The system of equations is solved using a difference method. The calculated results are in good agreement with the corresponding experimental data which confirms the effect of solid particles on the mean and turbulent characteristics of gas jets.     

Isothermal velocity and turbulence measurements downstream of a model multilobed turbofan mixer

An isothermal experimental investigation of the three dimensional flow field downstream of a model multilobed turbofan forced mixer is presented. LDA measurements of the three mean velocities and corresponding turbulence intensities were obtained in the downstream duct where the turbine (primary) and fan (secondary) streams emerging from the lobes mix together. The flow development in the near field was quantified by measuring the cross plane velocities. These were found to consist of large radial flows, of order 15% of the mean axial velocities at the lobe inlet, with changing sign depending on location. The cross-plane flow is consistent with a large scale axial vortex pair (per lobe) which persists throughout the downstream duct and enhances mixing in this region. Turbulence generation and anisotropy of the turbulence structure were evident mainly in the shear layers formed as the fan and turbine streams emanated from the lobe trailing edge. Spatial uniformity in the mean and turbulent fields was measured as little as five heights downstream of the mixer exit, indicating the rapid mixing achievable in these systems.     

剪切水-气界面下湍流猝发特征的实验研究

利用氢气泡时间线-脉线组合示踪技术定量地考察剪切水-气界面下的湍流猝发现象,分析 猝发事件的信号特征,重点探讨猝发与湍能产生之间的联系. 在猝发过程中,水面近区的瞬 时流速和Reynolds切应力出现较大幅度的脉动,它们在时间和空间垂直方向上表现出高度 的相干性,这是猝发事件的一个显著特征. 在猝发期,猝发事件涉及的空间区域内Reynolds 切应力和湍流脉动强度明显比平均值和非猝发期的情况大. 其结果表明：在所考察的实验 条件下,猝发是剪切水-气界面附近湍流产生的主要过程.     

Self-consistency and the use of correlated stochastic separated flow models for prediction of particle-laden flows

Computational fluid dynamics (CFD) is being used increasingly in the design and analysis of particle-laden flows. A significant challenge of this work is in correctly predicting the interaction of the fluid turbulence with the particulate phase. Typically, Lagrangian tracking is used to calculate the particle trajectories with stochastic treatments used to provide an instantaneous turbulent flow field. The stochastic calculations are based on the mean velocities and turbulence quantities calculated by the CFD solver. The current work examines the correlated stochastic separated flow (SSF) model used to synthesize the instantaneous fluid velocity field. Two functional forms of the Eulerian spatial correlation are considered: exponential, and Frenkiel with loop parameter m equal to unity. It is well known that the use of a Frenkiel function is incorrect due to the Markovian nature of the model. Nonetheless, a literature review indicates that the Frenkiel function is still being used in the CFD community. In order to illustrate the implications of this, numerical predictions are compared to Taylor's analytical result for fluid particle dispersion in homogeneous isotropic turbulence. Excellent predictions are obtained with the exponential correlation and recommendations on timestep requirements are made. In contrast, predictions from the Frenkiel model are in poor agreement with Taylor's solution. This poor agreement results from an inconsistency between the effective correlation of fluid velocities arising from the model and the original intended correlation.     

Dynamics of fibres in a turbulent flow field – A particle-level simulation technique

A particle-level simulation technique has been developed for modelling the flow of fibres in a turbulent flow field. A single fibre is conceived here as a chain of segments, thus enabling the model fibre to have all the degrees of freedom (translation, rotation, bending and twisting) needed to realistically reproduce the dynamics of real fibres. Equations of motion are solved for each segment, accounting for the interaction forces with the fluid, the contact forces with other fibres and the forces that maintain integrity of the fibre.The motion of the fluid is resolved as a combination of 3D mean flow velocities obtained from a CFD code and fluctuating turbulent velocities derived from the Langevin equation. A case of homogeneous turbulence is treated in this paper.The results obtained show that fibre flocs in air-fibre flows can be created even when attractive forces are not present. In such a case, contacts between fibres, properties of an individual fibre (such as flexibility and equilibrium shapes) and properties of the flow of the carrying fluid are shown to govern the physics behind formation and breaking up of fibre flocs. Highly irregular fibre shapes and stiff fibres lead to strong flocculation.The modelling framework applied in this work aims at making possible a numerical model applicable for designing processes involving transport of fibres by air at industrial scale.