In the current work, three different types of vapor cloud explosion experiments are simulated. The purpose of the simulations is twofold: firstly, to evaluate a recently developed CFD model and secondly to analyze the involved phenomena with the help of the simulation results. The proposed model, which has been implemented in the ADREA-HF CFD code, utilizes the RANS method using the Kato and Launder modification of k-e model. Combustion is modelled by taking into account the main mechanisms that contribute to the phenomenon such as chemistry, turbulence generated from the obstacles in front of the flame front, flame instabilities and turbulence generated by the flame-front itself. The CFD model is evaluated against different types of explosions in different geometries and with various fuels. Uniform premixed fuel-air mixture is considered in all cases. A large scale vented deflagration experiment in a 10 m length enclosure is firstly simulated using methane as fuel. The external explosion effect is apparent in this case. Then, a hydrogen deflagration experiment in a 78.5 m tunnel is simulated. Four mock-up cars are placed in the premixed region. Finally, propane and methane explosions inside a 1.5 m tube with obstacles and intense turbulence are simulated. Two different obstacle configurations are studied. The model predicts the overpressure values satisfactorily in all the examined cases. The factors that contribute to the pressure rise in each stage of each experiment are discussed based on the simulation results.

Turbulence and flow disturbances occurring at the aircraft wing-fuselage junction cause a deterioration of its aerodynamic performance and an increase in the aircraft's drag force. However, the aircraft junction regions are currently designed empirically due to the lack of knowledge of detailed junction flow dynamics, which consequently leads to less efficient flow management and poorer aerodynamic characteristics. The main objective of the present mathematical- modelling study is research on an aerodynamically enhanced design approach, concerning the improvement of the characteristics of the junction flow and its vortices.

Morphing offers an attractive alternative compared to conventional hinged, multi-element high lift devices. In the present work, morphed shapes of a NACA 64A010 airfoil are optimized for maximum lift characteristics. Deformed shapes of the leading and trailing edge are represented through Bezier curves derived from locally defined control points. The optimization process employs the fast Foil2w in-house viscous-inviscid interaction solver for the calculation of aerodynamic characteristics. Transitional flow results indicate that combined leading and trailing edge morphing may increase maximum lift in the order of 100%. A 60–80% increase is achieved when morphing is applied to leading edge only—the so-called droop nose—while a 45% increase is obtained with trailing edge morphing. Out of the stochastic optimization algorithms tested, the Genetic Algorithm, the Evolution Strategies, and the Particle Swarm Optimizer, the latter performs best. It produces the designs of maximum lift increase with the lowest computational cost. For the optimum morphed designs, verification simulations using the high fidelity MaPFlow CFD solver ensure that the high lift requirements set by the optimization process are met. Although the deformed droop nose increases drag, the aerodynamic performance is improved ensuring the overall effectiveness of the airfoil design during take-off and landing.

The purpose of this research work is to perform accurate numerical computations of supersonic flow in a converging nozzle and specifically to study Mach-disks. The latter process has been widely studied over the last years. In the present study numerical simulations are performed for transient supersonic flow, tracing the transition from a Mach reflection to a regular one. This has been done by enforcing the walls of a converging nozzle to come closer together, changing the deflection angle with time. Viscosity was taken into account and the full Navier- Stokes have been solved. The results obtained clearly show the gradual extinction of the Mach disk and the eventual wave intersection to a single point

M odelling is an important and useful tool for predicting the behaviour and the impact of pollutants on the local ecosystem parameters. More specifically, simulation and computational methods can be used for estimating the environmental impact on marine ecosystems. The paper presents a three-dimensional general deterministic model, developed to simulate and study the time-dependent behaviour of 137Cs in marine environments. The model capabilities are demonstrated by applying it at the northeast region of the island of Lemnos, in the NE Aegean Sea, Greece. Full Navier-Stokes equations for transient, three-dimensional turbulent flow, heat and mass transfer are solved numerically. The solution method is the finite-volume method and the general CFD code in which the present model is implemented is Phoenics.

During the design of an aircraft, a significant parameter that is taken into consideration is aerodynamic heating. Aerodynamically induced heating affects both the structure of the aircraft and its vulnerability to heat-seeking missiles in modern warfare. As a result, the ability to calculate efficiently the heating produced as well as the pressure distribution in such flows is crucial. Therefore, in this present study, the PHOENICS CFD code as modified by DRA Farnborough in order to calculate heat transfer and pressure measurement on a 5-inch hemispherical concave nose at a Mach number of 2.0 is evaluated and customized in order to produce faster and more accurate results. Apart from numerical alterations, different turbulence models are being examined as well as different discretization schemes. Numerical solutions show improvement up to 6% in comparison with the original model, both in terms of convergence rate and in terms of agreement with the available experimental data. With the new modeling suggested in the present work, the significance of both the discretization scheme and the choice of the turbulence modeling is demonstrated for the flows under consideration. The use of a high-order discretization scheme is suggested for more acute areas of the body modeled in order to improve results further.

This paper presents the mathematical modelling and numerical simulation of the turbulent, two-phase flow of liquid and gas in a gas-induced agitated stirred-tank reactor, using Computational Fluid Dynamics (CFD) techniques. The reactor used as an application demonstration of the developed model is the ozone-induced one, first designed and modeled by Yang et al. (1999). A three-dimensional (3D), transient, Euler-Euler two-phase flow model is developed and used to investigate the turbulent flow and mixing of liquid and bubbles in the stirred-tank reactor, applying the sliding mesh approach. Turbulence is simulated by means of several available models, the Renormalization Group (RNG) k-ε model being the one finally recommended as the most appropriate of the ones studied, for the present application. Two-way coupling between the two phases is modeled by means of appropriate inter-phase interaction relations. The study focused on bubbles of one size group (mean aerodynamic diameter of 2.5E-03m), but it is easily extended to any number of sizes. It is concluded that the predicted overall flow field pattern and the mixing of both phases around the two blades of the simulated baffled stirred vessel, as well as inside and outside of the main tube of the reactor, are physically plausible, appear reasonably accurate, and are, therefore, satisfying.

A toxic gas release, e.g. H2S, from pipeline accidents or sour wells, although improbable, may lead to serious consequences for the health of people and the environment. Such incidents might also jeopardize occupants of nearby indoor environments via infiltration of toxic contaminants. Despite that risk, there is still a lack of data and of comparative studies concerning the appropriate models and mitigation methods.The purpose of this work is, therefore, the improved modelling of toxic gas building infiltration, by a combination of tools and approaches. Using a realistic release scenario, the present comprehensive analysis demonstrates the need to consider the detailed building characteristics and meteorology. Significant deviations are observed between simple and advanced building ingress models. Furthermore, the consequences assessment leads to contradicting conclusions depending on the employed dose-response approaches. The proposed methodology could serve as a guide for the improvement of relevant risk-assessment tools and of future studies. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

In the present work, CFD simulations of a large scale open deflagration experiment are performed. Stoichiometric hydrogen-air mixture occupies a 20 m hemisphere. Two combustion models are compared and evaluated against the experiment: the Eddy Dissipation Concept model and a multi-physics combustion model which calculates turbulent burning velocity based on Yakhot's equation. Sensitivity analysis on the value of fractal dimension of the latter model is performed. A semi-empirical relation which estimates the fractal dimension is also tested. The effect of the turbulence model on the results is examined. LES approach and k-epsilon models are used. The multi-physics combustion model with constant fractal dimension value equal to 2.3, using the RNG LES turbulence model achieves the best agreement with the experiment. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Prediction of particles distribution in the smaller-scale atmospheric environment, such as the indoor atmosphere, is of major importance for the comfort and the well-being of its occupants. The objective of this study is to investigate the airflow and particles transport, as well as the particles concentration evolution indoors, using Computational Fluid Dynamics (CFD) techniques. A three-dimensional, Euler-Euler two-phase flow model for the investigation of the indoor aerosol is developed, within a CFD general-purpose computer program (PHOENICS), and is validated against experimental measurements from the literature, for an ordinary case of indoor dilute aerosol. Turbulent flow is simulated by Large Eddy Simulation (LES) and the results are compared with those obtained applying the Reynolds-averaged Navier-Stokes (RANS) equations together with the ReNormalisation Group (RNG) k-E model. wo-way coupling between the two phases is modelled by means of appropriate interphase interactions. This study focused on particles of one size group (mean aerodynamic diameter of 10 m) but the numerical method described can equally well be applied for a broader size range. It is concluded that for the very dilute aerosols considered here, simpler models (such as single-phase and drift flux) do as well in predicting the important parameters of the flow, as the more complex ones.

In the present work, CFD simulations of hydrogen deflagration in a real scale vented room are performed. Two ignition points were simulated: at the wall opposite to the vent (back ignition) and at the center of the chamber (center ignition). The overpressure time series and flame front velocities are compared with the experimental results. The combustion model is based on the turbulent flame speed concept. The turbulent flame speed is calculated based on a modification of Yakhot's equation, in order to account for all the main physical mechanisms which appear in hydrogen deflagrations. Special attention is given to the simulation of Rayleigh-Taylor instability. This instability occurs at the vent area and results in sudden explosion of the mixture that has been pushed outside the chamber at the initial stage of the explosion. The importance of this external explosion to the generated overpressures inside the chamber is highlighted. The agreement between experimental and computational results is satisfactory in both back ignition and center ignition cases.

The risks entailed by an accidental spill of Liquefied Natural Gas (LNG) should be indentified and evaluated, in order to design measures for prevention and mitigation in LNG terminals. For this purpose, simulations are considered a useful tool to study LNG spills and to understand the mechanisms that influence the vapor dispersion. In the present study, the ADREA-HF CFD code is employed to simulate the TEEX1 experiment. The experiment was carried out at the Brayton Fire Training Field, which is affiliated with the Texas A&M University system and involves LNG release and dispersion over water surface in open-obstructed environment. In the simulation the source was modeled as a two-phase jet enabling the prediction of both the vapor dispersion and the liquid pool spreading. The conservation equations for the mixture are solved along with the mass fraction for natural gas. Due to the low prevailing temperatures during the spill ambient humidity condenses and this might affect the vapor dispersion. This effect was examined in this work by solving an additional conservation equation for the water mass fraction. Two different models were tested: the hydrodynamic equilibrium model which assumes kinetic equilibrium between the phases and the non hydrodynamic equilibrium model, in order to assess the effect of slip velocity on the prediction. The slip velocity is defined as the difference between the liquid phase and the vapor phase and is calculated using the algebraic slip model. Constant droplet diameter of three different sizes and a lognormal distribution of the droplet diameter were applied and the results are discussed and compared with the measurements.

In this paper CFD modeling techniques are used to simulate deflagration in homogenous, near stoichiometric hydrogen air mixture in a model of a tunnel. The tunnel is 78.5 m long. Hydrogen air mixture is located in a 10 m long region in the middle of the tunnel. Two cases are studied: one with a complete empty tunnel and one with the presence of four vehicles near the center of the tunnel. The combustion model is based on the turbulent flame speed concept. The turbulent flame speed is a modification of Yakhot's equation, in order to account for additional physical mechanisms. A sensitivity analysis for the parameter of the combustion model and for the mesh resolution was made. The agreement between experimental and computational results concerning the value of the maximum pressure, and the time it appears, was satisfactory in both empty and non-empty tunnel case. The sensitivity analysis for the parameter of the combustion model showed that even small changes in it can have impact on the simulating results, whereas the sensitivity analysis of the mesh resolution did not reveal any significant differences. Finally, the effect of the turbulence model is examined (LES and RANS type of model). The only significant difference in the results between LES and BANS model was the arrival time of the pressure peak. A delay in the arrival time in the case of the BANS model was observed. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

This paper reviews the problems and successes of computing turbulent flow. Most of the flow phenomena that are important to modern technology involve turbulence. The review is concerned with methods for turbulent flow computer predictions and their applications, and describes several of them. These computational methods are aimed at simulating either as much detail of the turbulent motion as possible by current computer power or, more commonly, its overall effect on the mean-flow behaviour. The methods are still being developed and some of the most recent concepts involved are discussed.Some success has been achieved with two-equation models for relatively simple hydrodynamic phenomena; indeed, routine design work has been undertaken during the last three decades in several applications of engineering practise, for which extensive studies have optimised these models.Failures are still common for many applications particularly those that involve strong curvature, intermittency, strong buoyancy influences, low-Reynolds-number effects, rapid compression or expansion, strong swirl, and kinetically-influenced chemical reaction. New conceptual developments are needed in these areas, probably along the lines of actually calculating the principal manifestation of turbulence, e.g. intermittency. A start has been made in this direction in the form of 'multi-fluid' models, and full simulations.The turbulence modelling approaches presented here are, Reynolds-Averaged Navier-Stokes (RANS), two-fluid models, Very Large Eddy Simulation (VLES), Unsteady Reynolds-Averaged Navier-Stokes (URANS), Detached Eddy Simulation (DES) and some interesting, relatively recent, hybrid LES/RANS techniques.A large number of relatively recent studies are considered, together with reference to the numerical experiments existing on the subject.The authors hope that they provide the interested reader with most of the appropriate sources of turbulence modelling, exhibiting either as much detail as it is possible, by means of bibliography, or illustrating some of the most recent developments on the numerical modelling of turbulent flows. Thus, the potential user has the appropriate information, for him to select the suitable turbulence model for his own case of interest. (C) 2014 Elsevier Inc. All rights reserved.

The use of hydrogen as a fuel should always be accompanied by a safety assessment concerning the case of an accidental release. To evaluate the potential hazards in a spill accident both experiments and simulations are performed. In the present work, the CFD code, ADREA-HF, is used to simulate the liquefied hydrogen (LH2) spill experiments (test 5, 6, 7) conducted by the Health Safety Laboratory (HSL). Two horizontal releases, the one along the ground and the other one at a distance above the ground, and one vertical release are examined with spill rate 60 lt/min. The main focus of this study is on the presence of humidity in the atmosphere and its effect on the vapor dispersion. When humidity is present is cooled, condenses and freezes due to the low prevailing temperature (similar to 20 K near the release), and releases heat. In addition, during the release hydrogen droplets are formed due to mechanical and flashing breakup, and water droplets and ice crystals due to humidity phase change. Therefore, two models are tested: the hydrodynamic equilibrium model, which assumes that the phases are in thermodynamic and kinematic equilibrium and the non hydrodynamic equilibrium model (slip model), which assumed that the phases are in thermodynamic equilibrium but they can obtain different velocities. The fluctuating wind direction was also taken into account, since it greatly affects the hydrogen dispersion. The computational results are compared with the experimental measurements, and it is concluded that humidity along with the slip effect influences the buoyancy of the cloud to a great extent. The best simulation case (humidity and slip effect) is consistent with the experiment for all three tests for the majority of the sensors. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

A 3-D hybrid turbulence model, simulating the transport and fate of oil spills in various waters, is used to evaluate the influence of natural dispersion on the spreading of water-in-oil emulsions formed in the water column. The model combines the Navier-Stokes equations for two-phase flows, the RNG k-ε submodel, and parameterized expressions of the basic processes affecting the fate of oil spills. The model also considers the presence of waves, the windand waveinduced surface drifts, and the influence of surface wave breaking on the oil spills. Using a stochastic probability model of breaking waves, the loss of surface wave energy into turbulence, due to breaking, is derived and the rate of natural dispersion of oil mass and that of oilwater emulsions formed in the water column is evaluated, under a variety of sea state conditions. Results in the form of oil concentration profiles with depth, graphs showing the variation of the fraction of water (mass) absorbed by the dispersed oil, at various depths and times, as well as graphs showing the oil mass balance, at the sea surface, at various times are compared with counterpart profiles, and graphs obtained from the literature, and useful conclusions are drawn.

Aeration is one of the most common stages in the liquid waste clarification process. Most aeration tanks are equipped with aeration devices that inject oxygen to enhance the biodegradation of the liquid waste. In this work an aeration process is simulated, where the air is injected from the bottom of a cylindrical tank. Two numerical models are developed and two different geometries are considered. Their results are verified against published data. Both models solve the momentum, continuity and k- å equations for the relevant phases. The first model considers liquid and gas phases as homogeneous fluid. The calculated velocity refers to the mixture of the phases. The second model assumes liquid and gas as two distinct phases. Velocity and volume fraction profiles portray the induced motion of the liquid and the extent of the aeration process. Regarding the two different geometries, the first assumes a flat free liquid surface, while the second a liquid surface free to swell according to the gas injection rate. It is shown that the second geometry gives more accurate results.

The use of hydrogen as fuel should always be accomp anied by a safety assessment, in case of an accidental release. To evaluate the potential hazar ds in a spill accident both experiments and simulations are performed. In the present work, the CFD code, ADREA-HF, is used to simulate the liquefied hydrogen (LH2) spill experiments (test 5, 6, 7) conducted by the Health and Safety Laboratory (HSL). In these tests, LH2 was spilled a t a fixed rate of 60lt/min in several directions an d for several durations. The factors that influence t h vapor dispersion under cryogenic release conditions that were examined in this study are: th air humidity, the wind direction, and the slip ef fect of droplets formed by both the cryogenic liquid and the condensation of air humidity. The numerical results were compared with the experimental measure ments, and the effect of each abovementioned factors in the vapor dispersion is being discussed.

The use of LNG (liquefied natural gas) as fuel brings up issues regarding safety and acceptable risk. The potential hazards associated with an accidental LNG spill should be evaluated, and a useful tool in LNG safety assessment is computational fluid dynamics (CFD) simulation. In this paper, the ADREA-HF code has been applied to simulate LNG dispersion in open-obstructed environment based on Falcon Series Experiments. During these experiments LNG was released and dispersed over water surface. The spill area is confined with a billboard upwind of the water pond. FA1 trial was chosen to be simulated, because its release and weather conditions (high total spill volume and release rate, low wind speed) allow the gravitational force to influence the cold, dense vapor cloud and can be considered as a benchmark for LNG dispersion in fenced area. The source was modeled with two different approaches: as vapor pool and as two phase jet and the predicted methane concentration at sensors' location was compared with the experimental one. It is verified that the source model affect to a great extent the LNG dispersion and the best case was the one modeling the source as two phase jet. However, the numerical results in the case of two phase jet source underestimate the methane concentration for most of the sensors. Finally, the paper discusses the effect of neglecting the −9.3° experimental wind direction, which leads to the symmetry assumption with respect to wind and therefore less computational costs. It was found that this effect is small in case of a jet source but large in the case of a pool source.