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Iztok Tiselj - One of the best experts on this subject based on the ideXlab platform.
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Slug Modeling with 1D two-Fluid Model
Kerntechnik, 2012Co-Authors: Iztok Tiselj, C. Samuel MartinAbstract:Abstract Simulations of condensation-induced water hammer with one-dimensional two-Fluid Model requires explicit Modeling of slug formation, slug propagation, and in some cases slug decay. Stratified flow correlations that are more or less well known in 1D two-Fluid Models, are crucial for accurate description of the initial phase of the slug formation and slug propagation. Slug formation means transition to other flow regime that requires different set of correlations. To use such two-Fluid Model for condensation induced water hammer simulations, a single slug must be explicitly recognized and captured. In the present work two cases of condensation-induced water hammer simulations performed with WAHA code, are described and discussed: injection of cold liquid into horizontal pipe filled with steam and injection of hot steam into horizontal pipe partially filled with cold liquid.
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Numerical simulations of basic interfacial instabilities with incompressible two-Fluid Model
Nuclear Engineering and Design, 2011Co-Authors: Luka Štrubelj, Iztok TiseljAbstract:Abstract The incompressible two-Fluid Model for stratified flow was improved. The interface of the stratified two-phase flow was successfully recognized and sharpened within the two-Fluid Model. After the advection step of volume fraction the numerical diffusion of the interface was reduced in such a way that the thickness of the interface is kept constant during the simulation. The surface tension force was implemented in the system of the two-Fluid Model equations. The two basic instabilities of stratified flows: Rayleigh–Taylor and Kelvin–Helmholtz instability were used to validate the proposed two-Fluid Model. The proposed two-Fluid Model with interface sharpening presents a step towards the simulations of mixed flows, where locally dispersed flow or stratified flow will be simulated with appropriated subModels within the two-Fluid Model.
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Two-Fluid Model with interface sharpening
International Journal for Numerical Methods in Engineering, 2010Co-Authors: Luka Štrubelj, Iztok TiseljAbstract:Two-Fluid Models are applicable for simulations of all types of two-phase flows ranging from separated flows with large characteristic interfacial length scales to highly dispersed flows with very small characteristic interfacial length scales. The main drawback of the two-Fluid Model, when used for simulations of stratified flows, is the numerical diffusion of the interface. Stratified flows can be easily and more accurately solved with interface tracking methods; however, these methods are limited to the flows, that do not develop into dispersed types of flows. The present paper describes a new approach, where the advantage of the two-Fluid Model is combined with the conservative level set method for interface tracking. The advection step of the volume fraction transport equation is followed by the interface sharpening, which preserves the thickness of the interface during the simulation. The proposed two-Fluid Model with interface sharpening was found to be more accurate than the existing two-Fluid Models. The mixed flow with both: stratified and dispersed flow, is simulated with the coupled Model in this paper. In the coupled Model, the dispersed two-Fluid Model and two-Fluid Model with interface sharpening are used locally, depending on the parameter which recognizes the flow regime. Copyright © 2010 John Wiley & Sons, Ltd.
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Numerical Simulation of Rayleigh-Taylor Instability With Two-Fluid Model and Interface Sharpening
Volume 1: Symposia Parts A and B, 2008Co-Authors: Luka Štrubelj, Iztok TiseljAbstract:The free surface flows are successfully Modeled with one of the existing free surface Models, such as: level set method, volume of Fluid method, front tracking method, two-Fluid Model (two momentum equations) with modified interphase force and some others. The main disadvantage of the two-Fluid Model used for simulations of free surface flows is numerical diffusion of the interface, which can be significantly reduced as presented in this paper. The interface is sharpened with the conservative level set method, where after the advection step of volume fraction the numerical diffusion of the interface is reduced in such a way that the thickness of the interface is kept constant during the simulation. The reduction of the interface diffusion can also be called interface sharpening. In the present paper the two-Fluid Model with interface sharpening is validated with Rayleigh-Taylor instability. Under assumptions of isothermal and incompressible flow of two immiscible Fluids, we simulated a system with the Fluid of higher density located above the Fluid of smaller density in two dimensions. Due to the gravity in the system, the Fluid with a higher density moves below the Fluid with a smaller density. The initial condition is not a flat interface between the Fluids, but a cosine wave with small amplitude, which develops into a mushroom-like structure. Mushroom-like structure in simulation of Rayleigh-Taylor instability later develops into small droplets as result of numerical dispersion of interface (interface sharpening) or to narrow trails with interface diffusion (no interface sharpening). The results of the two-Fluid Model with interface sharpening are compared to two-Fluid Model without interface sharpening and single-Fluid-Model with/without interface sharpening. The analytic solution of amplitude growth can be found for small amplitudes and was also compared to simulation.Copyright © 2008 by ASME
Wahid Farhat - One of the best experts on this subject based on the ideXlab platform.
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Validation of Two-Fluid Model of Urban Traffic for Arterial Streets
Transportation Research Record, 2004Co-Authors: Elizabeth G. Jones, Wahid FarhatAbstract:A series of vehicular traffic experiments conducted in Omaha, Nebraska, shows that the two-Fluid Model of urban traffic is valid not only at the network scale but also at the arterial street scale. The data collected support the validity of the two assumptions of the two-Fluid Model for traffic on arterial streets. The two-Fluid parameters are effective in assessing the quality of traffic between different arterial streets, over time on the same arterial street, and on portions of an arterial street. The scalability of the two-Fluid Model also permits systemwide effects of localized changes to a traffic network to be assessed.
Vinayak Dixit - One of the best experts on this subject based on the ideXlab platform.
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Integrity of estimates of the two-Fluid Model and gender impacts
Transportation Research Part C-emerging Technologies, 2014Co-Authors: Anurag Pande, Vinayak Dixit, Katherine Spansel, Brian WolshonAbstract:This paper summarizes a research study to develop a methodology for utilizing naturalistic Global Positioning System (GPS) driving data for two-Fluid Model estimation. The two-Fluid vehicular traffic flow Model describes traffic flow on a street network as a mix of stopped and running vehicles. The parameters of the Model essentially represent 'free flow' travel time and the level of interaction among vehicles. These parameters have traditionally been used to evaluate roadway networks and corridors with partially limited access. However, the two-Fluid Model has been found to be a direct result of driver behavior, and can also be used to represent behavioral aspects of driver populations, e.g., aggressiveness, passiveness, etc. Through these behavioral aspects they can also be related to safety on roadways. Due to which the two-Fluid Model can be considered to be a safety footprint for a particular road or individual driver. Due to which it is critical to understand factors that influence the two-Fluid Model. In this study, two-Fluid Models were estimated using naturalistic driving data collected with GPS data loggers in San Luis Obispo (SLO), California. Linear referencing in ArcMap was used to link the GPS data with roadway characteristic data for each element of the roadway network. The linear referencing methodology is the key to relate the GPS driving data with the elements of roadway network. This study explores two fundamental questions: (1) how sensitive are the estimates of the two Fluid parameters to various samples? This question is fundamentally important to help define the integrity of the two-Fluid Model for planning and operational purposes. To this end we use a random sampling approach to address this question. (2) Are there behavioral differences across gender? This provides important behavioral insights on driving behavior across gender. Significant differences were observed between male and female drivers, with female drivers being more aggressive.
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Behavioural foundations of two-Fluid Model for urban traffic
Transportation Research Part C-emerging Technologies, 2013Co-Authors: Vinayak DixitAbstract:Traditionally traffic flow Models have been based on analogous physical phenomena. Though these Models have been successful in representing traffic flow, there is a need to provide a systematic behavioural explanation for their existence. One such Model is the two-Fluid Model which is analogous to the principles of Bose–Einstein condensation of particles at low temperatures. The Model has been extensively used to characterize the quality of traffic on urban networks and arterial streets. The two parameters of the Model essentially represent ‘free flow’ travel time and level of interaction among vehicles. Though the studies have found the parameters of the two-Fluid Model to be significantly correlated with driver behaviour (aggressive/conservative) and crash rates, no systematic behavioural explanation has been found. This paper proposes a behavioural framework based on individual trade-off behaviour to explain the two-Fluid Model phenomenon. The two-Fluid Model is derived based on a driver’s attempt to maximize his quality of travel, by travelling fast while maintaining safety. Contrary to earlier assumptions the proposed framework shows the two parameters to be correlated. The theoretical framework was tested using two-Fluid Model data from various cities. The data was also used to estimate the effects of geometric factors on the perception of likelihood of a crash and the severity of the crash that affect the two-Fluid Model. Increase in the fraction of one-way streets was found to reduce the driver’s perception of likelihood to crash. While reduction in the fraction of one way streets and increase in average number of lanes per street, signal density and fraction of actuated signals increased the perceived level of severity of a crash.
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Relationship of Two-Fluid Model Parameters with Safety on Arterials
2011Co-Authors: Vinayak Dixit, Anurag Pande, Mohamed Abdel-aty, Essam RadwanAbstract:The two-Fluid Model for vehicular traffic flow explains the traffic on arterials as a mix of stopped and running vehicles. It describes the relationship between the vehicles’ running speed and the fraction of running vehicles. Two parameters of the Model essentially represent ‘free flow’ travel time and level of interaction among vehicles, respectively, and may be used to evaluate urban roadway networks and urban corridors with partially limited access. These parameters are supposed to be related not only with the characteristics of the roadway but also with behavioral aspects of driver population, e.g., aggressiveness. In this study two-Fluid Models were estimated for eight arterial corridors in Orlando. The parameters of the two-Fluid Model, traditionally used to evaluate network operations, were used for the first time to estimate these parameters’ correlations with rates of crashes having different types/severity. This is considered the first attempt to use the two-Fluid Model in a traffic safety application. Significant correlations were found between two-Fluid parameters and rear-end and angle crash rates. Rate of severe crashes was also found to be significantly correlated with the Model parameter signifying inter-vehicle interactions. While there is need for further analysis, the findings suggest that the two-Fluid Model parameters may have potential as surrogate measures for traffic safety on urban arterial streets. These Models may be useful for flagging corridors with certain safety issues.
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Effects of Time of Day and Wet Pavement Condition on Two-Fluid Model
2009Co-Authors: Vinayak Dixit, Essam RadwanAbstract:The Two-Fluid Model characterizes the relationship between running time per mile and travel time per mile. The parameters of the two-Fluid Model have been widely used to quantify the performance of urban networks. Historically, the two-Fluid Model has been developed utilizing the travel time data collected for the morning and evening peak hours, assuming that there does not exist significant differences in the two-Fluid Model trends between these two peak hours. The purpose of this study is to investigate the effect of time of day and wet pavement of the two-Fluid Model trends. Travel time per mile and stop time per mile data were collect for the Orlando downtown network during the morning, afternoon and evening peak. It was found that the two Fluid trends were significantly different between the morning peak and evening peak. No significant difference was observed in the two-Fluid Model trends between the evening and midday peaks. Significant differences were also observed in the two-Fluid Model under wet and dry conditions. These results indicate that drivers interact differently with other vehicles and other network elements depending on the time of day and wet pavement conditions. These findings suggest that there is a need to evaluate the two-Fluid Model for the network separately based on time of day. This Modeling technique would be particularly useful in evaluating quality of traffic in cities with large number of modes of travel (mixed traffic, and shared lanes). The two-Fluid Model allows for a holistic evaluation of a transportation network.
Mamoru Ishii - One of the best experts on this subject based on the ideXlab platform.
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Interfacial drag force in one-dimensional two-Fluid Model
Progress in Nuclear Energy, 2012Co-Authors: Caleb S. Brooks, Takashi Hibiki, Mamoru IshiiAbstract:Abstract In the two-Fluid Model, the interfacial momentum transfer is the key closure relation for defining the degree of coupling between phases. The two approaches utilized in one-dimensional system analysis codes, namely the “drag coefficient approach” (RELAP5/MOD2 and TRAC-PF1/MOD1) and “drift-velocity approach” (RELAP5/MOD3 and TRACE V5) to describe the interfacial drag force, are discussed in detail. Shortcomings in the current and past forms of interfacial drag are identified. Considering the code drawbacks, two approaches which are conceptually consistent with the two-Fluid Model are discussed. The first approach is an improved form of the drag coefficient approach and can be easily implemented with the classical two-Fluid Model without requiring any additional development. The second formulates the most consistent and advanced description of interfacial drag force for the use with the two-Fluid Model with two-group interfacial area transport equation. The framework of the two-group interfacial drag, which Models spherical/distorted bubbles (group-1) and cap/slug/churn-turbulent bubbles (group-2) separately, is presented. Proper description of distribution parameter, drag coefficient, relative velocity, bubble size/shape characterization and interfacial area concentration is discussed, providing a comprehensive road map to advanced closure of the one-dimensional interfacial drag force.
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interfacial area transport equation and implementation into two Fluid Model
Journal of Thermal Science and Engineering Applications, 2009Co-Authors: Mamoru Ishii, Takashi HibikiAbstract:A dynamic treatment of interfacial area concentration has been studied over the last decade by employing the interfacial area transport equation. When coupled with the two-Fluid Model, the interfacial area transport equation replaces the flow regime dependent correlations for interfacial area concentration and eliminates potential artificial bifurcation or numerical oscillations stemming from these static correlations. An extensive database has been established to evaluate the Model under various two-phase flow conditions. These include adiabatic and heated conditions, vertical and horizontal flow orientations, round, rectangular, annulus, and 8 X 8 rod-bundle channel geometries, and normal-gravity and reduced-gravity conditions. Currently, a two-group interfacial area transport equation is available and applicable to comprehensive two-phase flow conditions spanning from bubbly to churn-turbulent flow regimes. A framework to couple the two-group interfacial area transport equation with the modified two-Fluid Model is established in view of multiphase computational Fluid dynamics code applications as well as reactor system analysis code applications. The present study reviews the current state-of-the-art in the development of the interfacial area transport equation, available experimental databases, and the analytical methods to incorporate the interfacial area transport equation into the two-Fluid Model.
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Formulation of time and volume averaged two-Fluid Model considering structural materials in a control volume
Nuclear Engineering and Design, 2008Co-Authors: Takashi Hibiki, Mamoru IshiiAbstract:Abstract Time–volume averaged two-Fluid Model considering structural materials in a control volume (porous media approach) is formulated for two-phase flow analyses. The time–volume averaged two-Fluid Model is obtained rigorously by averaging local time-averaged two-Fluid Model over a control volume including structural materials. The volume porosity based Model obtained by the simple time–volume averaging is converted into the surface porosity based Model by introducing the directional surface porosity concept. The covariance approach is adopted to consider distributions of various variables in the control volume. The obtained porous media formulation is consistent with existing two-Fluid Model. Existing constitutive equations may be applicable to the porous media formulation provided they are properly time–volume averaged in the control volume. The strategy to obtain time–volume averaged constitutive equations is briefly discussed.
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One-Dimensional Two-Fluid Model
Thermo-Fluid Dynamics of Two-Phase Flow, 2006Co-Authors: Mamoru Ishii, Takashi HibikiAbstract:The two-Fluid Model is the most detailed and accurate macroscopic formulation of the thermo-Fluid dynamics of two-phase systems. In the two-Fluid Model, the field equations are expressed by the six conservation equations consisting of mass, momentum and energy equations for each phase. Since these field equations are obtained from an appropriate averaging of local instantaneous balance equations, the phasic interaction term appears in each of the averaged balance equations. These terms represent the mass, momentum and energy transfers through the interface between the phases. The existence of the interfacial transfer terms is one of the most important characteristics of the two-Fluid Model formulation. These terms determine the rate of phase changes and the degree of mechanical and thermal non-equilibrium between phases, thus they are the essential closure relations that should be Modeled accurately. However, because of considerable difficulties in terms of measurements and Modeling, reliable and accurate closure relations for the interfacial transfer terms are not fully developed. In spite of these shortcomings of two-Fluid Models, there is, however, no substitute available for Modeling accurately two-phase phenomena where two phases are weakly coupled. Examples of these are: Sudden mixing of two phases; Transient flooding and flow reversal; Transient countercurrent flow; Two-phase flow with sudden acceleration.
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modified two Fluid Model for the two group interfacial area transport equation
Annals of Nuclear Energy, 2003Co-Authors: Mamoru Ishii, Joseph M. KellyAbstract:This paper presents a modified two-Fluid Model that is ready to be applied in the approach of the two-group interfacial area transport equation. The two-group interfacial area transport equation was developed to provide a mechanistic constitutive relation for the interfacial area concentration in the two-Fluid Model. In the two-group transport equation, bubbles are categorized into two groups: spherical/distorted bubbles as Group 1 while cap/slug/churn-turbulent bubbles as Group 2. Therefore, this transport equation can be employed in the flow regimes spanning from bubbly, cap bubbly, slug to churn-turbulent flows. However, the introduction of the two groups of bubbles requires two gas velocity fields. Yet it is not practical to solve two momentum equations for the gas phase alone. In the current modified two-Fluid Model, a simplified approach is proposed. The momentum equation for the averaged velocity of both Group-1 and Group-2 bubbles is retained. By doing so, the velocity difference between Group-1 and Group-2 bubbles needs to be determined. This may be made either based on simplified momentum equations for both Group-1 and Group-2 bubbles or by a modified drift-flux Model.
Elizabeth G. Jones - One of the best experts on this subject based on the ideXlab platform.
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Validation of Two-Fluid Model of Urban Traffic for Arterial Streets
Transportation Research Record, 2004Co-Authors: Elizabeth G. Jones, Wahid FarhatAbstract:A series of vehicular traffic experiments conducted in Omaha, Nebraska, shows that the two-Fluid Model of urban traffic is valid not only at the network scale but also at the arterial street scale. The data collected support the validity of the two assumptions of the two-Fluid Model for traffic on arterial streets. The two-Fluid parameters are effective in assessing the quality of traffic between different arterial streets, over time on the same arterial street, and on portions of an arterial street. The scalability of the two-Fluid Model also permits systemwide effects of localized changes to a traffic network to be assessed.
