Wake Entrainment Research Articles

Large Eddy Simulation of Particle Re-suspension During a Footstep

Re-suspension of particulate matter under human activity can have a significant impact on particle concentrations in indoor environments. The removal of surface-bound particulates as a person's foot contacts a substrate and the subsequent transport of such particulates through wake entrainment effects and through deposition is an important mechanism for particle dispersion. To study aspects of this event, we perform high-fidelity simulations of particle transport due to the heel-to-toe contact of a foot with a particle-laden carpet. For this purpose, an immersed boundary method is extended to account for particle transport, re-suspension, and deposition near the surface of the foot. Particle deposition is modeled as a combination of gravitational settling, Brownian diffusion, and convective impaction, while a dynamic re-suspension model is used to model particle re-suspension from the surfaces. We demonstrate the details of transient transport phenomena of re-suspended particles during a heel-to-toe foot ...

Predictions of Phase Distribution in Liquid–Liquid Two-Component Flow

Ground-based liquid–liquid two-component flow can be used to study reduced-gravity gas-liquid two-phase flows provided that the two liquids are immiscible with similar densities. In this paper, we present a numerical study of phase distribution in liquid–liquid two-component flows using the Eulerian two-fluid model in FLUENT, together with a one-group interfacial area transport equation (IATE) that takes into account fluid particle interactions, such as coalescence and disintegration. This modeling approach is expected to dynamically capture changes in the interfacial structure. We apply the FLUENT-IATE model to a water-Therminol 59® two-component vertical flow in a 25-mm inner diameter pipe, where the two liquids are immiscible with similar densities (3% difference at 20°C). This study covers bubbly (drop) flow and bubbly-to-slug flow transition regimes with area-averaged void (drop) fractions from 3 to 30%. Comparisons of the numerical results with the experimental data indicate that for bubbly flows, the predictions of the lateral phase distributions using the FLUENT-IATE model are generally more accurate than those using the model without the IATE. In addition, we demonstrate that the coalescence of fluid particles is dominated by wake entrainment and enhanced by increasing either the continuous or dispersed phase velocity. However, the predictions show disagreement with experimental data in some flow conditions for larger void fraction conditions, which fall into the bubbly-to-slug flow transition regime. We conjecture that additional fluid particle interaction mechanisms due to the change of flow regimes are possibly involved.

MUSIG modeling and evaluation of nitrogen bubble coalescence in a bottom-closed vertical tube

A MUSIG model is numerically resolved for the coalescence process of dispersed small nitrogen vapor bubbles into big bubbles in a bottom-closed vertical tube. The effects of turbulence, laminar shear as well as wake entrainment on bubble collision and coalescence are taken into account. Then photographic experiments are conducted to evaluate the model, using a high-speed CCD camera at an acquiring frequency of 1000 frames/second. Comparison of numerical results against the experimental data demonstrates that the MUSIG model can give satisfactory predictions of the bubble diameter, bubble velocity, as well as the height of Taylor Bubble Formation (TFB). The numerical results also contribute to understanding of the phenomenon observed in the experiments. However, it is also found that beyond the height of TFB, the numerical results deviate from the experimental data, which indicates that beyond that height, the MUSIG model becomes invalid.

CFD Simulation of Phase Distribution in Adiabatic Upward Bubbly Flows Using Interfacial Area Transport Equation

In the study of gas-liquid two-phase flows, one challenge is to describe the dynamic changes in flow structure, which can be considerably affected by bubble coalescence and/or disintegration in addition to bubble nucleation and condensation processes. The interfacial structure, to a first-order approximation, may be characterized by the void fraction and a geometric parameter named “interfacial area concentration,” the evolution of which can be modeled by an interfacial area transport equation (IATE). A one-group IATE has been developed for bubbly flows in the literature, accounting for three dominant mechanisms: coalescence of bubbles due to random bubble collisions driven by turbulence, coalescence of bubbles due to wake entrainment, and disintegration of bubbles caused by turbulent-eddy impact. The current study is aimed at examining the capability of a computational fluid dynamics code, namely, FLUENT, with the one-group IATE implemented, in predicting two-phase-flow phase distributions. Simulations using the Eulerian multiphase model in FLUENT 6.2.16 have been performed for adiabatic upward bubbly flows in a pipe of 50.8-mm inner diameter with a range of void fractions from 4.9 to 23.1%. The predicted phase distributions yield satisfactory agreement with available experimental data, demonstrating that FLUENT with the IATE can provide a valuable simulation tool for two-phase bubbly flows.

Interfacial-Area Transport Equation at Reduced-Gravity Conditions

The interfacial-area transport equation is of practical importance for two-phase flow analyses at reduced-gravity conditions. In view of this, the interfacial-area transport equation, which takes the gravity effect into account, is studied in detail. The constitutive equation for the sink term of the interfacial-area concentration due to wake entrainment has been developed by considering the body acceleration due to frictional pressure loss. A comparison of the newly developed interfacial-area transport equation with various experimental data taken at normal-gravity and microgravity conditions shows a satisfactory agreement. An example computation of the newly developed interfacial-area transport equation has been performed at various gravity conditions such as 0, 1.62, 3.71, and 9.80 m/s 2 , which correspond to zero-gravity and the lunar, Martian, and Earth surface gravities, respectively. It has been revealed that the effect of the gravity on the interfacial-area transport in a two-phase flow system is more pronounced for low-liquid-flow and low-void-fraction conditions, whereas the gravity effect can be ignored for high-mixture-volumetric-flux conditions.

Benchmarking of the one-dimensional one-group interfacial area transport equation for reduced-gravity bubbly flows

In order to properly design and safely operate two-phase flow systems, especially those deployed on future space missions, it is necessary to have accurate predictive capabilities. The application of a novel predictive method, the interfacial area transport equation (IATE), to dynamically predict the change of interfacial area concentration for reduced-gravity two-phase flows is described in this paper. Fluid particle interaction mechanisms such as coalescence and breakup that are present in reduced-gravity two-phase flows have been studied experimentally as reported in a previous paper by the current authors [Vasavada et al., 2007]. These mechanisms represent the source and sink terms in the IATE and their mechanistic models are benchmarked using experimental data obtained in a 25 mm inner diameter ground-based test section wherein reduced-gravity conditions were simulated. The comparison of the predictions from the model against experimental data shows good agreement. It has been found that, in contrast to the hypothesis extended in the literature, the wake entrainment based coalescence mechanism is present in reduced-gravity two-phase flows and in some cases is more important than coalescence due to random collision. Physics based arguments are extended to support this conclusion.

多流体モデルによる気泡塔内多分散気泡流の数値計算(流体工学,流体機械)

Bubble coalescence and breakup models are implemented into the (N+2)-field model, which is a hybrid combination of a multi-fluid model and an interface tracking method to examine the applicability of available bubble coalescence and breakup models to poly-dispersed bubbly flows in bubble columns. The implemented models are a breakup model proposed by Luo & Svendsen and coalescence models proposed by Prince & Blanch and Wang et al. The validation of these models is carried out using experimental data for a coalescence-dominating flow, a breakup-dominating flow, a flow in-between, and a flow with low coalescence-breakup frequencies. Good predictions of void and bubble size distributions for the four kinds of poly-dispersed bubbly flows in bubble columns are obtained. It is also demonstrated that all the possible bubble coalescence mechanisms, i.e. bubble coalescence caused by bubble collisions due to turbulent eddies, the difference in bubble velocities and shear slip, and bubble coalescence due to wake entrainment, should be taken into account for obtaining good predictions.

Mechanistic study for the interfacial area transport phenomena in an air/water flow condition by using fine-size bubble group model

A set of number density transport equations based on the bubble size are used to predict the void fraction and the interfacial area concentration in an air/water flow conditions. As the closure relations for the number density transport equations, a coalescence due to random collisions and a breakup due to an impact of the turbulent eddies are modified based on previous studies. The bubble expansion term due to a pressure reduction and a coalescence due to a wake entrainment are modeled for the number density transport equation. In order to predict the local experimental data, a computational fluid dynamic (CFD) code coupling the two-fluid model and number density transport equations are developed in this study. As for the results of the numerical analysis, the developed model predicts well the void fraction and interfacial area concentration although some deviations between the prediction and the experiment are shown for the high void fraction conditions.

Theoretical prediction of flow regime transition in bubble columns by the population balance model

Theoretical prediction of flow regime transition in bubble columns was studied based on the bubble size distribution by the population balance model (PBM). Models for bubble coalescence and breakup due to different mechanisms, including coalescence due to turbulent eddies, coalescence due to different bubble rise velocities, coalescence due to bubble wake entrainment, breakup due to eddy collision and breakup due to large bubble instability, were proposed. Simulation results showed that at relatively low superficial gas velocities, bubble coalescence and breakup were relatively weak and the bubble size was small and had a narrow distribution; with an increase in the superficial gas velocity, large bubbles began to form due to bubble coalescence, resulting in a much wider bubble size distribution. The regime transition was predicted to occur when the volume fraction of small bubbles sharply decreased. The predicted transition superficial gas velocity was about 4 cm/s for the air–water system, in accordance with the values obtained from experimental approaches.

C108 微小重力環境下の気液二相流の界面輸送

In view of the great importance of two geometrical parameters such as void fraction and interfacial area concentration to the accurate two-phase flow analysis at microgravity conditions, axial developments of flow parameters such as void fraction, interfacial area concentration, bubble Sauter mean diameter, and bubble number density were measured by image-processing in bubbly flow at microgravity and low liquid Reynolds number conditions where the gravity effect on the flow parameters were pronounced. Negligible bubble breakup was observed because of weak turbulence under tested flow conditions. The velocity profile entrainment effect under microgravity was likely to be comparable to the wake entrainment effect under normal gravity in the tested flow conditions.

Three-Dimensional One-Way Bubble Tracking Method for the Prediction of Developing Bubble-Slug Flows in a Vertical Pipe (1st Report, Models and Demonstration)

A three-dimensional one-way bubble tracking method is one of the most promising numerical methods for the prediction of a developing bubble flow in a vertical pipe, provided that several constitutive models are prepared. In this study, a bubble shape, an equation of bubble motion, a liquid velocity profile, a pressure field, turbulent fluctuation and bubble coalescence are modeled based on available knowledge on bubble dynamics. Bubble shapes are classified into four types in terms of bubble equivalent diameter. A wake velocity model is introduced to simulate approaching process among bubbles due to wake entrainment. Bubble coalescence is treated as a stochastic phenomenon with the aid of coalescence probabilities that depend on the sizes of two interacting bubbles. The proposed method can predict time-spatial evolution of flow pattern in a developing bubble-slug flow.

Two-group interfacial area transport in vertical air–water flow: I. Mechanistic model

The prediction of the dynamical evolution of interfacial area concentration is one of the most challenging tasks in two-fluid model application. This paper is focused on developing theoretical models for interfacial area source and sink terms for a two-group interfacial area transport equation. Mechanistic models of major fluid particle interaction phenomena involving two bubble groups are proposed, including the shearing-off of small bubbles from slug/cap bubbles, the wake entrainment of spherical/distorted bubble group into slug/cap bubble group, the wake acceleration and coalescence between slug/cap bubbles, and the breakup of slug/cap bubbles due to turbulent eddy impacts. The existing one-group interaction terms are extended in considering the generation of cap bubbles, as well as different parametric dependences when these terms are applied to the slug flow regime. The complete set of modeling equations is closed and continuously covers the bubbly flow, slug flow, and churn-turbulent flow regimes. Prediction of the interfacial area concentration evolution using a one-dimensional two-group transport equation and evaluation with experimental results are described in a companion paper.

1016 気泡流界面輸送に及ぼす重力の影響

Axial developments of one-dimensional void fraction, bubble number density, interfacial area concentration, and Sauter mean diameter of adiabatic nitrogen-water bubbly flows in a 9-mm-diameter pipe were measured under a microgravity environment using an image-processing method. The interfacial area transport mechanism was determined based on visual observation. Marked bubble coalescence occurred when fast-moving bubbles near the channel center overtook and swept up slower-moving bubbles in the vicinity of the channel wall (velocity profile entrainment). Negligible bubble breakup was observed because of weak turbulence under tested flow conditions. Axial changes of measured interfacial area concentrations were compared with the interfacial area transport equation considering the bubble expansion and wake entrainment as observed under a normal gravity environment. The velocity profile entrainment effect under microgravity was likely to be comparable to the wake entrainment effect under normal gravity in the tested flow conditions. This apparently led to insignificant differences between measured interfacial area concentrations and those predicted by the interfacial area transport equation with the wake entrainment model under normal gravity.

Interfacial Area Transport of Bubbly Flow in a Small Diameter Pipe

In relation to the development of the interfacial area transport equation, this study focused on modeling of the interfacial area transport mechanism of vertical adiabatic air-water bubbly flows in a relatively small diameter pipe where the bubble size-to-pipe diameter ratio was relatively high and the radial motion of bubbles was restricted by the presence of the pipe wall. The sink term of the interfacial area concentration was modeled by considering wake en-trainment as a possible bubble coalescence mechanism, whereas the source term was neglected by assuming negligibly small bubble breakup for low liquid velocity conditions based on visual observation. One-dimensional interfacial area transport equation with the derived sink term was evaluated by using five datasets of vertical adiabatic air-water bubbly flows measured in a 9.0mm-diameter pipe (superficial gas velocity: 0.013–0.052 m/s, superficial liquid velocity: 0.58–1.0m/s). The modeled interfacial area transport equation could reproduce the proper trend of the axial interfacial area transport and predict the measured interfacial area concentrations within an average relative deviation of 1±11.1%. It was recognized that the present model would be promising for predicting the interfacial area transport of the examined bubbly flows.

Interfacial Area Transport of Bubbly Flow in a Small Diameter Pipe.

In relation to the development of the interfacial area transport equation, this study focused on modeling of the interfacial area transport mechanism of vertical adiabatic air-water bubbly flows in a relatively small diameter pipe where the bubble size-to-pipe diameter ratio was relatively high and the radial motion of bubbles was restricted by the presence of the pipe wall. The sink term of the interfacial area concentration was modeled by considering wake en-trainment as a possible bubble coalescence mechanism, whereas the source term was neglected by assuming negligibly small bubble breakup for low liquid velocity conditions based on visual observation. One-dimensional interfacial area transport equation with the derived sink term was evaluated by using five datasets of vertical adiabatic air-water bubbly flows measured in a 9.0mm-diameter pipe (superficial gas velocity: 0.013–0.052 m/s, superficial liquid velocity: 0.58–1.0m/s). The modeled interfacial area transport equation could reproduce the proper trend of the axial interfacial area transport and predict the measured interfacial area concentrations within an average relative deviation of 1±11.1%. It was recognized that the present model would be promising for predicting the interfacial area transport of the examined bubbly flows.

Experimental, Numerical, and Analytical Models for a Dispersion Study

Wind tunnel tests, computational fluid dynamics simulations, and Gaussian models are employed for a dispersion study in nonuniform atmospheric boundary layer conditions with releases affected by neighboring building wake entrainment and topography. A vertical stack release and a horizontal crosswind release are subjected to both qualitative (plume trajectory and geometry) and quantitative (concentration) analysis. The results of the three modeling approaches (experiments, computational fluid dynamics simulations, and Gaussian models) are compared, their relative strengths are discussed, and conclusions are presented. It is suggested that a combined numerical (computational fluid dynamics) and physical (wind tunnel) modeling approach might benefit atmospheric dispersion studies in complex, nonuniform atmospheric environments.

One-group interfacial area transport in vertical bubbly flow

In the two-fluid model, interfacial concentration is one of the important parameters. The objective of this study is to develop an interfacial area equation with the source and sink terms being properly modeled. For bubble coalescence, the random collisions between bubbles due to turbulence, and the wake entrainment process due to the relative motions of the bubbles, were included. For bubble breakup, the impact of turbulent eddies is considered. Compared with measured axial distributions of the interfacial area concentration under various flow conditions, the adjustable parameters in the source/sink terms were obtained for the simplified one-dimensional transport equation.

Kinematic, Dynamic, and Thermodynamic Analysis of a Weakly Sheared Severe Thunderstorm over Northern Alabama

A kinematic, dynamic, and thermodynamic analysis of a weakly sheared, airmass thunderstorm observed over northern Alabama is presented. Most notable is the fact that the dominant cell in this storm closely resembles the Byers and Braham model for warm-based, airmass storms. Several phenomena never documented for this storm-type are discussed. One of these is a strong and deep downdraft observed at midlevels with an associated “weak-echo” trench. Its origin appears to be related to a wake entrainment process. A midlevel inflow which causes a visible constriction in the storm cloud is also observed. This inflow results in a division of the thermal buoyancy into two maxima in the vertical: one associated with the precipitation core and the other with strong positive vertical motions in the growing cumulus turret. In addition, a downdraft separate from that seen at midlevels develops at low levels and causes a microburst outflow at the surface. This downdraft appears to be initiated by precipitation loading and intensified by negative thermal buoyancy. The Byers and Braham model of the cumulus, mature and dissipating stages, is discussed in light of these new features.