Lateral Lift Force Research Articles

Particle focusing in a wavy channel

It is known that inertial lift forces can lead to particle focusing in channel flows; yet oscillatory straining effects have also been suggested as a mechanism for particle focusing in wavy channels. To explore the synergy between these two mechanisms, we analytically and experimentally investigate the focusing behaviour of rigid neutrally buoyant particles in a wavy channel. We decompose the particle-free channel flow into a primary Poiseuille flow and secondary eddies induced by the waviness. We calculate the perturbation of the particle on the particle-free flow and the resulting lateral lift force exerted on the particle using the method of matched asymptotic expansions. This yields a zeroth-order lift force arising from the Poiseuille flow and a first-order lift force due to the waviness of the channel. We further incorporate the inertial lift force into the Maxey–Riley equation and simulate particle trajectories in wavy channels. The interactions between the zeroth-order lift force and the particle-free flow largely determine the focusing locations. Experiments in wavy channels with varying amplitudes at channel Reynolds numbers ranging from 5 to 250 are consistent with the predictions of the focusing locations, which are mainly governed by the channel Reynolds number as well as the competition between the inertial lift and the oscillatory straining effects.

Numerical study of the trailing vehicle length on train aerodynamics under crosswind

Limited by the wind tunnel size, a one-and-a-half train model is generally adopted during the test to represent the realistic long tandem. Thus, the reasonable arrangement of the trailing vehicle length is of significance for experimental accuracy. In this work, the aerodynamic performance of trains with different trailing vehicle lengths subjected to crosswind was studied using the improved detached eddy simulation method combined with the shear–stress–transport k–ω turbulence model. The 1/8th scaled high-speed trains with five trailing lengths were proposed, and the aerodynamic differences were evaluated at yaw angles ranging from β = 0° to β = 60°. The numerical method was validated by the previous wind tunnel test. Results show that the aerodynamics and flow patterns of the La = 0.50 and the benchmark are highly relevant. The downstream dummy vehicle length has the greatest effect on aerodynamic coefficients at a higher yaw angle (β > 30°), especially at β = 60°, whereas the effects become insignificant at a lower yaw angle (β < 30°). For various La lengths, notable discrepancy appears at the leeward and top side where large vortexes shed off from the roof. The larger contributions to the lateral force and lift force coefficients are mainly due to these areas. A suitable length of La = 0.50 is therefore recommended to obtain more accurate aerodynamics of a long train set.

Numerical Simulation of Flow around A Cylinder in Hyperbolic Flow Field Induced by Internal Solitary Waves

To study the flow field and hydrodynamic characteristics of hyperbolic flow around a cylinder caused by an internal solitary wave, the hyperbolic flow around the cylinder induced by the inner solitary wave, inlet condition is the lower deep horizontal velocity induced by the inner solitary wave and the two-dimensional Navier-Stokes equation is used as the flow field control equation according to the applicability conditions of the eKdV inner solitary wave theory. A numerical simulation method for the nonlinear action of internal solitary wave-induced hyperbolic flow on a horizontal cylinder is established. A series of calculation results show that the numerical simulation results of horizontal force on the cylinder are in good agreement with experimental results. The wakefield of hyperbolic flow around a cylinder has distinct characteristics. In the process of flow around the cylinder, the vortex does not fall off alternately and the lateral lift force is neglected. The force of internal solitary wave-induced hyperbolic flow on the cylinder is non-centrosymmetric, and with the increase of maximum Reynolds number and characteristic period, the horizontal resistance of the cylinder becomes unidirectional. Therefore, the maximum Reynolds number and the characteristic period affect the horizontal force of internal solitary wave-induced hyperbolic flow on a two-dimensional cylinder.

Fundamental research on the dynamic safety of a high-speed train under strong wind conditions

New engineering phenomena in China have renewed interest in research on the dynamic risk boundaries and characteristics of train overturning, derailment, and the interaction mechanism with aerodynamic loads in windy conditions. This study focuses on this challenge, the train aerodynamic loads characteristics, dynamic response and safety risk characteristics, and the contribution rate of different aerodynamic loads to train safety risks in different wind scenarios were investigated by computational fluid dynamics and multi-body dynamics simulation. The results reveal that when a train is completely exposed to strong winds with or without a windbreak, the safety risk assessment should focus on the overturning coefficient of the first wheelset of the first bogie of the head car. When the train is gradually exposed to strong winds, the probability of the train derailing risk is greatly increased. The train safety risk assessment should not only focus on the overturning coefficient, and more attention should be paid to the derailment coefficient of the first wheelset of the first bogie of the head car. In addition, there may be a risk of misjudgment in assessing the crosswind safety of trains from the aerodynamic point of view only by lateral force, lift force, and roll moment.

Force Balance of Particles in Water During Ultrafiltration Membrane Process

Membrane filtration is vital in our daily lives to extract fresh water from contaminated water. However, membrane filtration is suffering from fouling, which is induced by accumulations of particles on membrane surface. Due to the hydraulic field and electrical field (from functional groups of membranes) near membrane surface, particle will be exerted by different forces. The intermolecular forces such as the Lateral lift force, Brownian force, shear-induced force, Van Der Waal force and electrostatic repulsion force may affect the fouling process, and the water flux inside the system cab exert permeance drag force to the particles near the surface of membrane. In order to figure out the forces that may affect the filtration process, a model was established about the filtration process. In the model, the velocities of the each forces are listed and calculated in this paper. From the analysis, the relations between the particle velocities, which represents the filtration efficiency, and the forces, such as lift or shear forces, are well illustrated. This article shows the importance of potentially understanding the fouling process and providing insight of fouling mitigation for the membrane filtration process.

Lift and drag force on a spherical particle in a viscoelastic shear flow

We present a comprehensive 3D numerical study of particles with imposed velocities relative to the local bulk flow (termed “slip velocities”) in a viscoelastic shear flow. We consider the force on a spherical particle sedimenting, a spherical bubble rising, and a spherical neutral squirmer swimming in an imposed viscoelastic shear flow. We demonstrate that any particle moving with a slip velocity in the flow or gradient direction of the shear flow experience a lateral lift force. We calculate and compare the magnitude and direction of the lift force in all situations. At small Deborah (De) and Weissenberg (Wi) numbers, our results show good agreement with an existing perturbation theory for rigid particles (Einarsson and Mehlig, 2017 [1]) and new perturbation theories for drops and for squirmers respectively. Our simulations extend these results to higher De and Wi regimes. Through our simulations, we uncover the physical mechanism of the lateral force on all particles. For rigid particles, we find the lift force arises from an imbalance in polymer stress on either side of the particle, which in turn is due to the imbalance of polymer stretch surrounding the particle. If this lift force is not balanced by an external force, a lateral drift velocity arises. We further consider the implication of this lateral drift for rigid particles hydrodynamically forced in a viscoelastic Poiseuille flow, where particles can migrate either toward the channel center plane or toward the wall, depending on whether the direction of the applied force is in the direction aligned or opposite to the direction of the Poiseuille flow, respectively. We study both the migration of a single particle as well as a suspension of particles in channel flow. Even with the addition of hydrodynamic interactions, we show that particles forced in the direction of the Poiseuille flow migrate towards the channel center.

Influence of the bubble size distribution on the bubble column flow regime

The role of the bubble size dependent lateral lift force on the flow regime is experimentally investigated in a tall bubble column. In the experiments, only the initial bubble size distribution was modified by using a gas sparger with different injection needles and varying the flow through specific injection needles. The gas flow rate was kept unchanged in all experiments. Depending on the bubble size distribution homogeneous, transitional or heterogeneous flow regimes were observed. The flow regime is in good agreement with the predictions from the stability criterion obtained by a previously conducted linear stability analysis for poly-dispersed flow. It was found that as a rule of thumb the following criterion can be used.If the majority of the gas volume is transported by bubbles smaller than the critical diameter at which the lift force changes its sign, the flow is stabilized leading to a homogeneous flow regime. If most of the gas volume is transported by bubbles larger than that diameter the flow is de-stabilized leading a heterogeneous flow regime. If the fraction of gas transported by small and large bubbles is about the same, the initial conditions remain dominant throughout the column height.

A Second Generation Multiphase-CFD Framework Toward Predictive Modeling of DNB

Building on the strong belief that the advancement and consistent adoption of cutting-edge simulation tools is critical to the future of nuclear power, three-dimensional thermal-hydraulic methods in the form of computational fluid dynamics (CFD) have made enormous advancement and promise to transform the way we approach the design of more efficient and reliable systems. The success of these methods hinges on the accuracy and predictive ability of the underlying models, which must, at the same time, limit the computational cost and allow optimal scalability. A large effort at the Massachusetts Institute of Technology has been devoted to the development of a second-generation of multiphase-CFD (M-CFD) closures and to leveraging the continuous progression in the experimental techniques. Among the numerous objectives, the central challenge that has driven the overall approach is the prediction of departure from nucleate boiling. This work focuses on deriving the fundamental meso-scale mechanisms from the CFD-grade experiments and incorporates them in the M-CFD framework as subgrid-scale models. A more complete representation of lateral lift force and near-wall effects are proposed, in combination with direct numerical simulation–driven understanding of bubble-induced turbulence effects. The improved description of the multiphase flow distribution is coupled to a novel representation of boiling heat transfer, which aims at introducing all the physical mechanisms that are encountered at the boiling surface. Starting from the improved representation at the wall, this work concentrates on the micro-hydrodynamics of the thin liquid film on the heated surface, which governs the critical heat flux limit.

Aerodynamic Effects of Unsteady Blade-Tower Interaction

In this study, a 2D model as the horizontal cross-sections for HAWT of NREL phase VI is investigated to figure out the flickering of aerodynamic force with six different geometries by pressure-based Navier-Stokes solver and k-ω SST turbulence model. All six cases presented have zero yaw angle and zero pitch angle. Computational domain is divided as sliding mesh and stagnation domains to simulate the blade motion. In this article the dynamic response of tower interference is performed by lateral force, lift force, pressure coefficient and velocity contour by locations of the blade. The aerodynamic response is arranged in an overshoot function for the lift force coefficient of the airfoil to account for the aerodynamic coupling with the tower. This work present in this study gives an insight into rotor structure aerodynamic and aeroelastic interaction.

Deposition of particles in the supersonic flow past a wedge

In our previous research, we find three particle motion patterns when submicron particles enter a supersonic laminar boundary layer over a flat plate. In these three patterns the deposition pattern is the most intriguing one because it is the lateral lift force directed to the wall that results in the particle deposition, hence the lateral deposition. In this paper, we investigate the particle motion in the supersonic flow past a wedge instead of a flat plate. The inertial deposition mechanism is introduced when the wedge replaces the flat plate. The inertial deposition is essential for the whole deposition process. Therefore, we build a two dimensional Eulerian Lagrangian numerical model to investigate this process. Several factors including particle initial position, wedge angle, initial Mach number and particle size are studied. It turns out that the higher particle has a more important lateral deposition mechanism, and as the wedge angle, initial Mach number or particle size increase, the importance of inertial deposition increases. Finally, as the three particle motion still exists in the wedge cases, we also propose a non-dimensional number to describe these patterns.

Scaling law for bubbles rising near vertical walls

This paper examines the rising motion of a layer of gas bubbles next to a vertical wall in a liquid in the presence of an upward flow parallel to the wall to help with the understanding of the fluid dynamics in a bubbly upflow in vertical channels. Only the region near the wall is simulated with an average pressure gradient applied to the domain that balances the weight of the liquid phase. The upward flow is created by the rising motion of the bubbles. The bubbles are kept near the wall by the lateral lift force acting on them as a result of rising in the shear layer near the wall. The rise velocity of the bubbles sliding on the wall and the average rise velocity of the liquid depend on three dimensionless parameters, Archimedes number, Ar, Eötvös number, Eo, and the average volume fraction of bubbles on the wall. In the limit of small Eo, bubbles are nearly spherical and the dependency on Eo becomes negligible. In this limit, the scaling of the liquid Reynolds number with Archimedes number and the void fraction is presented. A scaling argument is presented based on viscous dissipation analysis that matches the numerical findings. Viscous dissipation rates are found to be high in a thin film region between the bubble and the wall. A scaling of the viscous dissipation and steady state film thickness between the bubble and the wall with Archimedes number is presented.

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2m (180mm i.d.), 0.5m (476mm i.d.) and 0.8m (760mm i.d.) at high superficial gas velocities (0.12–0.62m·s−1) and high solid concentrations (0–30vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.

Numerical investigation on lateral migration and lift force of single bubble in simple shear flow in low viscosity fluid using volume of fluid method

Two phase flow systems have been widely used in industrial engineering. Phase distribution characteristics are vital to the safety operation and optimization design of two phase flow systems. Lift force has been known as perpendicular to the bubbles’ moving direction, which is one of the mechanisms of interfacial momentum transfer. While most widely used lift force correlations, such as the correlation of Tomiyama et al. (2002), were obtained by experimentally tracking single bubble trajectories in high viscosity glycerol–water mixture, the applicability of these models into low viscosity fluid, such as water in nuclear engineering system, needs to be further evaluated. In the present paper, bubble in low viscosity fluid in shear flow was investigated in a full 3D numerical simulation and the volume of fluid (VOF) method was applied to capture the interface. The fluid parameter: fluid viscosity, bubble parameter: diameter and external flow parameters: shear stress magnitude and liquid velocity were examined. Comparing with bubble in high viscosity shear flow and bubble in low viscosity still flow, relative large bubble in low viscosity shear flow keep an oscillation way towards the moving wall and experienced a shape deformation process. The oscillation amplitude increased as the viscosity of fluid decreased. Small bubble migrated to the static wall in a line with larger migration velocity than that in high viscosity fluid and no deformation occurred. The shear stress triggered the oscillation behaviors while it had no direct influence with the behavior. The liquid velocity had no effect on lift coefficient in the present research range of 0<V<0.186m/s.

Transition between regimes of a vertical channel bubbly upflow due to bubble deformability

The effect of the deformability of viscous bubbles on the flow rate of bubbly upflow in a vertical channel is examined using direct numerical simulations. A sharp transition between two different flow regimes has been observed. At large bubble deformability, characterized by large Eötvös number (Eo), the flow rate is close to the single phase flow rate, with adjusted pressure gradient, and the bubbles are almost uniformly distributed in the middle of the channel. On the other hand, at low Eo the bubbles are concentrated near channel walls and flow rates are much smaller than the single phase flow. The transition from high flow rate to low flow rate occurs rather abruptly. It is found that the transition occurs when the less deformable bubbles enter the viscous sublayer due to the lateral lift force on the bubbles. This leads to an increase in the viscous dissipation near the wall which leads to a decrease in the flow rate.

An analysis of the transient forces acting on Savonius rotors with different aspect ratios

The present study deals with the analysis of the transient forces of a Savonius vertical axis wind turbine (VAWT) using computational fluid dynamics (CFD). Unsteady, Reynolds Averaged Navier–Stokes CFD simulations were conducted using two different rotors and two simulation methodologies. The first rotor had an aspect ratio of 1.1 and simulations were conducted by incorporating the dynamic equations of the rigid-rotor motion using a one degree-of-freedom (1-DOF) module to evaluate the rotor's free motion. The second rotor had an aspect ratio of 0.7 and simulations were conducted by specifying a fixed rotational velocity. The predicted torque and power curves were validated using experimental data. The transient forces from the two rotor blades, the resultant forces, and the longitudinal drag and lateral lift coefficients were evaluated using the two complementary data sets. The lateral force coefficient had a large contribution to the rotor torque at low azimuth angles and at low tip speed ratios (λ). At tip speed ratios above 0.6, the effect of the longitudinal drag force on the rotor operation increased and the lateral lift force contribution decreased. The resultant force angle was shown to become more acute due to the increase in the longitudinal force with increasing λ.

List of Participants

The current paper presents the prediction results of a bubbly flow under plunging jet conditions using multiphase mono- and poly-dispersed approaches. The models consider interfacial momentum transfer terms arising from drag, lift, and turbulent dispersion force for the different bubble sizes. The turbulence is modeled by an extended k–ɛ model which accounts for bubble induced turbulence. Furthermore in case of a poly-dispersed air–water flow the bubble size distribution, bubble break-up and coalescence processes as well as different gas velocities in dependency on the bubble diameter are taken into account using the Inhomogeneous MUSIG model. This model is a generalized inhomogeneous multiple size group model based on the Eulerian modeling framework which was developed in the framework of a cooperative work between ANSYS-CFX and Forschungszentrum Dresden-Rossendorf (FZD). The latter is now implemented into the CFD code CFX.According to the correlation on the lateral lift force obtained by Tomiyama (1998); this force changes its sign in dependence on the bubble size. Consequently the entrained small bubbles are trapped below the jet. They can escape from the bubble plume only by turbulent fluctuations or by coalescence. If the size of the bubbles generated by coalescence exceeds the size at which the lift force changes its sign these large bubbles go out from the plume and rise to the surface.A turbulent model based on an additional source term for turbulence kinetic energy and turbulence eddy dissipation equation is compared to the common concept for modeling the turbulence quantities proposed by Sato et al. (1981). It has been found that the large bubble distribution is slightly affected by the turbulence modeling which affects particularly the bubble coalescence and break-up process.

Wind-Direction Effects on Urban-Type Flows

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.

Numerical study of a bubble plume generated by bubble entrainment from an impinging jet

The current paper presents the prediction results of a bubbly flow under plunging jet conditions using multiphase mono- and poly-dispersed approaches. The models consider interfacial momentum transfer terms arising from drag, lift, and turbulent dispersion force for the different bubble sizes. The turbulence is modeled by an extended k– ɛ model which accounts for bubble induced turbulence. Furthermore in case of a poly-dispersed air–water flow the bubble size distribution, bubble break-up and coalescence processes as well as different gas velocities in dependency on the bubble diameter are taken into account using the Inhomogeneous MUSIG model. This model is a generalized inhomogeneous multiple size group model based on the Eulerian modeling framework which was developed in the framework of a cooperative work between ANSYS-CFX and Forschungszentrum Dresden-Rossendorf (FZD). The latter is now implemented into the CFD code CFX. According to the correlation on the lateral lift force obtained by Tomiyama (1998); this force changes its sign in dependence on the bubble size. Consequently the entrained small bubbles are trapped below the jet. They can escape from the bubble plume only by turbulent fluctuations or by coalescence. If the size of the bubbles generated by coalescence exceeds the size at which the lift force changes its sign these large bubbles go out from the plume and rise to the surface. A turbulent model based on an additional source term for turbulence kinetic energy and turbulence eddy dissipation equation is compared to the common concept for modeling the turbulence quantities proposed by Sato et al. (1981). It has been found that the large bubble distribution is slightly affected by the turbulence modeling which affects particularly the bubble coalescence and break-up process.

On the role of the lateral lift force in poly-dispersed bubbly flows

An extensive experimental database comprising air–water as well as steam-water upwards vertical pipe flows for a pressure up to 6.5 MPa was used to investigate the effect of the lateral lift force on turbulent poly-dispersed flows with medium or high gas volume fraction. It was clearly shown that the lift force plays an important role also in such flows. Several effects such as bubble coalescence and breakup as well as fast rising large bubbles which push small bubbles towards the pipe wall superpose the effect of the lift force but can be separated from this effect. The critical bubble diameter, at which the lift force changes its sign, predicted by using Tomiyama’s correlation agrees well with experimental data obtained for turbulent air–water and steam-water flows with medium and high void fraction and a broad spectrum of bubbles sizes. The values for this critical bubble diameter are confirmed by the experimental data within the frame of the uncertainty of the data. Consequences of the action of the lateral lift force on flow structures in different flow situations are discussed. From the investigations it can be concluded that the lift force including the bubble size dependent change of its sign should be considered in a proper numerical 2D or 3D-simulation on flows in which bubbles in the range of several millimeters are present.

GRAVITY-DRIVEN BUBBLY FLOWS

▪ Abstract Gravity-driven bubbly flows are a specific class of flows, where all action is provided by gravity. An industrial example is formed by the so-called bubble column: a vertical cylinder filled with liquid through which bubbles flow that are introduced at the bottom of the cylinder. On the bubble scale, gravity gives rise to buoyancy of individual bubbles. On larger scales, gravity acts on nonuniformities in the spatial bubble distribution present in the bubbly mixture. The gravity-induced flow and flow structures can increase the inhomogeneity of the bubble distribution, leading to a turbulent flow. In this flow, specific scales are identified: a large-scale circulation with the liquid flowing upward in the center of the column and downward close to the wall. On the intermediate scale there are vortical structures; eddies of liquid, with a size on the order of the diameter of the column, that stir the liquid and radially transport the bubbles. On the small scale there is the local stirring of the bubbles. We describe the ideas developed over time and identify some open questions. We discuss the experimental findings on the turbulence generated, the stability of the flow, axial dispersion, and the similarities between bubble columns and air lifts. Especially for higher gas fractions, many questions still lack accurate answers. The lateral lift force in bubble swarms and the structure of the turbulence in the bubbly mixture are important examples of inadequately understood physical phenomena, providing many challenges for fundamental and applied research on bubbly flows.