Search

Abstract The representation of turbulent fluxes during oceanic convective events is important to capture the evolution of the oceanic mixed layer. To improve the accuracy of turbulent fluxes, we examine the possibility of adding a non‐local component in their expression in addition to the usual downgradient part. To do so, we extend the k–ε algebraic second‐moment closure by relaxing the assumption on the equilibrium of the temperature variance θ′2‾. With this additional evolution equation for the temperature variance, we obtain a k–ε–θ′2‾ model (the “kεt” model) which includes a non‐local term for the temperature flux. We validate this new model against Large Eddy Simulations (LES) in three test cases: free convection (FC), wind‐driven mixing, and diurnal cycle (DC). For wind‐driven mixing, kεt is equivalent to k–ε. However, in the presence of a buoyancy flux (FC and DC), we find that the vertical profile of temperature of the LES is better captured by kεt than k–ε. Particularly, the non‐local term increases the fraction of the mixed layer that is stably stratified. For FC, this fraction is near 50% for both kεt and the LES, whereas the k–ε value is 20%. We show that this improvement is due to a better representation of the temperature variance in the inner part of the mixed layer. This better representation is mainly caused by the diffusion of temperature variance, which is described by kεt and not by k–ε.

The present research work deals with the numerical simulation of double cavity scramjet combustor by using two equation standard k - ε turbulence model and finite-rate/eddy-dissipation reaction models which is again coupled with Reynolds-Averaged Navier-Stokes (RANS) equations to investigate the influence of variation of inlet boundary condition of air as well as H 2 fuel on the combustion flow-field of scramjet engine subsequently. At the same time, the validations of the current computational approach have been completed against a standard experimental data which is available in the literature. An acceptable similarity is observed between present numerical approach with the experimentally attained schlieren photograph and the pressure distribution curve. In the present work, 8 different cases are studied. Among them, first four cases are investigated for the variation of inlet boundary condition of air and the remaining four cases are studied for the variation of inlet boundary condition of H 2 fuel. The obtained results show that the formation of high-pressure region around the cavities for case 3 and case 4 actually helps to push the greater amount of air to the cavity region where it is mixed with adequate amount of H 2 for proper and stable combustion whereas for case 6, it is observed that most of the combustion phenomena closely fitted into a small space of the combustor and mainly occurs near the cavity region. [ABSTRACT FROM AUTHOR]

In the present research work, computational simulation of the double cavity scramjet combustor have been performed by using the two-dimensional compressible Reynolds-Averaged Navier–Stokes (RANS) equations coupled with two equation standard k–ɛ turbulence model as well as the finite-rate/eddy-dissipation reaction model. All the simulations are carried out using ANSYS 14-FLUENT code. Additionally, the computational results of the present double cavity scramjet combustor have been compared with experimental results for validation purpose which is taken from the literature. The computational outcomes are in satisfactory agreement with the experimentally obtained shadowgraph image and pressure variation curve. However, due to numerical calculation, the pressure variation curve obtained computationally is under-predicted in 5 locations. Further, analyses have been carried out to investigate the effect of variation of hydrogen injection pressure as well as the variation of air inlet temperature on the flow-field characteristics of scramjet engine keeping the Mach number constant. The obtained results show that the increase in hydrogen injection pressure is followed by the generation of larger vortex structure near the cavity regions which in turn helps to carry the injectant and also enhance the air/fuel mixing whereas the increase in the inlet temperature of air is characterised by the shifting of incident oblique shock in the downstream of the H 2 injection location. Again for T 0 = 1500 K, the combustion phenomena remains limited to the cavity region and spreads very little towards the downstream of the combustor. [ABSTRACT FROM AUTHOR]

The two equation standard k–ɛ turbulence model and the two-dimensional compressible Reynolds-Averaged Navier–Stokes (RANS) equations have been used to computationally simulate the double cavity scramjet combustor. Here all the simulations are performed by using ANSYS 14-FLUENT code. At the same time, the validation of the present numerical simulation for double cavity has been performed by comparing its result with the available experimental data which is in accordance with the literature. The results are in good agreement with the schlieren image and the pressure distribution curve obtained experimentally. However, the pressure distribution curve obtained numerically is under-predicted in 5 locations by numerical calculation. Further, investigations on the variations of the effects of the length-to-depth ratio of cavity and Mach number on the combustion characteristics has been carried out. The present results show that there is an optimal length-to-depth ratio for the cavity for which the performance of combustor significantly improves and also efficient combustion takes place within the combustor region. Also, the shifting of the location of incident oblique shock took place in the downstream of the H 2 inlet when the Mach number value increases. But after achieving a critical Mach number range of 2–2.5, the further increase in Mach number results in lower combustion efficiency which may deteriorate the performance of combustor. [ABSTRACT FROM AUTHOR]

The present work is a numerical simulation of a turbulent free jet issuing from an axisymmetric orifice into quiescent air environment. The numerical simulation was carried out by solving the Reynolds Averaged Navier-Stokes equations using OpenFOAM. The standard two-equation k- ɛ eddy viscosity turbulence model was used to simulate the turbulent flow field in a three dimensional cylindrical domain. The numerical predictions are compared with experimental data in order to assess the capability/limitations of the turbulence model to reproduce the physics involved and the code using jet case examined in this work. The standard k -ɛ model predictions in terms of centre line mean velocity decay, spread rate, entrainment, self-similarity, turbulence intensities and Reynolds stress, are found to reproduce the physics of the jet flow and agree approximately with experimental data. New information such as evolution of turbulent kinetic energy budget, length scales and time scales is provided. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model that simulates the wind fields, turbulence fields, and dispersion of gaseous substances in urban areas on building to city block scales. A Computational Fluid Dynamics (CFD) approach using the steady-state, Reynolds-Averaged Navier-Stokes (RANS) equations with the standard k-ɛ turbulence model within control volumes of non-uniform cuboid shapes has been employed. Dispersion field is computed by solving an unsteady transport equation of passive scalar. Another approach based on Gaussian plume model is used to correct the turbulent Schmidt number of tracer, in order to improve the dispersion simulation. The experimental data from a wind tunnel under neutral conditions are used to validate the numerical results of velocity, turbulence, and dispersion fields. The numerical results show a reasonable agreement with the wind tunnel data. The deviation of concentration between the simulation with corrected turbulent Schmidt number and the wind tunnel experiments may arise from 1) imperfect point sources, 2) heterogeneous turbulent diffusivity, and 3) the constant turbulent Schmidt assumption used in the model. [ABSTRACT FROM AUTHOR]

In this study, turbulent flows and morphological changes in an open-channel that has alternate vegetated zones are numerically simulated by using a three-dimensional model. The Reynolds-averaged Navier-Stokes Equations are solved with the k-ɛ model. The simulated flow structures compared well with the measured data in the literature. Through numerical experiments, the evolution of the mean flow and turbulence structures for different vegetation densities is presented. The vegetation was found to curve the flow, and the meandering flow pattern becomes more definite with increasing vegetation density. When the channel morphological changes are simulated with a series of floods, the initial rectangular channel becomes a compound channel and the flow pattern changes from a straight stream to a meandering stream. The evolution of vegetation zones after the floods indicates that vegetation zones are expanded in the leeward of the initial vegetation zones, and that the size of the vegetation zone is increased about 2.5 times. [ABSTRACT FROM AUTHOR]

A vertical two-dimensional numerical model has been applied to solving the Reynolds Averaged Navier-Stokes (RANS) equations in the simulation of current and wave propagation through vegetated and non-vegetated waters. The k-ɛ model is used for turbulence closure of RANS equations. The effect of vegetation is simulated by adding the drag force of vegetation in the flow momentum equations and turbulence model. To solve the modified N-S equations, the finite difference method is used with the staggered grid system to solver equations. The Youngs' fractional volume of fluid (VOF) is applied tracking the free surface with second-order accuracy. The model has been tested by simulating dam break wave, pure current with vegetation, solitary wave runup on vegetated and non-vegetated channel, regular and random waves over a vegetated field. The model reasonably well reproduces these experimental observations, the modeling approach presented herein should be useful in simulating nearshore processes in coastal domains with vegetation effects. [ABSTRACT FROM AUTHOR]

Abstract: Numerical predictions of the wind energy distribution over terrains are essential for the appropriate selection of a suitable site for a wind power plant. This paper presents three-dimension numerical simulations of turbulence flow over sinusoidal hills. AMG in HYPRE (High Performance Preconditioner Library) is introduced in this work and applied on solving the FVM-based discrete systems of the governing equations for turbulent flow. The three-dimensional non-orthogonal collocated grid systems are generated and the AMG-based algorithm is established. Turbulence is modelled with the standard model and flow parameters of Governing equations are solving by AMG method. The numerical results were obtained from the simulations of flow over three types of three-dimensional sinusoidal hills. Comparisons have been made by virtue of experimental data with the computational data. The numerical results agree with the experimental results, the numerical methods are effective. [Copyright &y& Elsevier]

Abstract: This paper evaluates three-dimensional turbulent flow and convective heat transfer through mixing split vane in a single-phase and steady-state sub-channel of AP1000 nuclear reactor core by using general computational fluid dynamics code, Fluent6.3.26.Realizable K-ɛ model is used as a turbulent closure model, and the sub-channel is analyzed by numerical simulation. The results show that the mixing split vanes, which affect strongly the heat transfer in the sub-channel, are lateral flow, swirl and turbulent intensity between the sub-channels, and give reasonable agreements with experimental equations and AP1000 thermal hydraulic design. [Copyright &y& Elsevier]

Abstract: CFD modelling has proven to be a powerful tool for the design of UV reactors. However, the validation of the hydraulics predicted by the CFD model remains a point of attention. Using standard turbulence models such as the k–ɛ model, the CFD model often wrongly predicts local flow features around a UV lamp. Therefore, more advanced modelling approaches such as the LES model were considered. The modelling approaches were explored for a single cross-flow UV lamp. It is shown by means of comparison with experimental data that the LES model predicts the flow around a UV lamp more precisely than the k–ɛ model. The impact of differences in resolved velocity fields on the predicted disinfection was also investigated. Depending on the local geometry (presence and positioning of baffles) the disinfection results were completely different for the different modelling approaches. [ABSTRACT FROM AUTHOR]

Abstract: For fusion reactors, molten salt is one of the candidates for coolant materials. Molten salt is a high-Prandtl-number fluid; thus, it is necessary to enhance the heat transfer coefficient. It is proposed that rods are inserted into a duct to enhance the heat transfer coefficient. The flow field behind the rod in the duct is visualized to compare experimental data with simulation results. The trends and distributions in the numerical simulation are the same as those in the experiment, and furthermore, the magnitudes of the time and space scales in the numerical simulation are of the same order as those in the experiment. Thermohydraulic numerical analysis confirmed that the heat transfer coefficient is improved by inserting the rod when the fluid is a high-Prandtl-number fluid and the flow field is in the turbulent region. However, it is necessary for the rods to be arranged in the streamwise direction. [Copyright &y& Elsevier]

Abstract: This work presents a new CFD model for the hydrodesulfurization process in the petroleum industry. The new model considers the loss of effectiveness of the catalyst by coupling a model that considers the physical structure of the catalyst. The hydrodesulfurization process takes place in a pseudo-three-phase fluidized-bed slurry bubble column reactor with liquid and gas flowing upwards. This reactor presents a transient behavior for the catalytic effectiveness due to the obstruction of the catalyst pores, caused mainly by parallel reactions of hydrodemetalization acting on the catalyst. The emphasis of this work is the introduction of a kinetic model for the reactivity reduction of the catalyst inside slurry bubble column reactors. The hydrodynamic model is simple and adopts an Eulerian–Eulerian approach, where the momentum and mass conservation equations are used to describe the two-phase fluid dynamic fields. The k–ɛ model is used to account for turbulence. The kinetic model considers the reactions controlled by diffusion and occurring on the catalyst pores, which are obstructed along the process by the hydrodemetalization by-products. The model presented in this work allows for an assessment of the influence of the operational conditions of the reactor, such as liquid and gas feed velocities, as well as catalyst properties. The estimation of the yield of the process takes into account the transient activity of the catalyst. [Copyright &y& Elsevier]

Abstract: A single-phase of gas, three-dimensional turbulent flow velocity distribution model –k–ɛ model, which considers the effects of impulse high-voltage electrostatic fields, was founded. The k–ɛ model was dispersed by control-volume method and solved by Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. The axial-, the tangential-, and the radial-velocity distributions of flow field inside cyclone with impulse electrostatic excitation on the various operating parameters were obtained by computer numerical simulation, and verified partly by experiment. It was concluded that shape of curves of velocity distributions of flow field inside cyclone impulse electrostatic precipitation (CIESP) was similar to that of cyclone with no impulse electrostatic field while the values of the axial-, the tangential-, and the radial-velocity of airflow had distinctly changed in impulse electrostatic field, in particular, the radial-velocity of airflow. Compared to cyclone with no impulse electrostatic field, the radial-velocity of airflow in CIESP was influenced strongly by the electric field at same inlet gas rate. [Copyright &y& Elsevier]

Abstract: The present study deals with the numerical simulation of flow patterns and mixing behaviour in Kenics static mixer over a wide range of Reynolds number. Three different sets of Kenics mixer (aspect ratio=1.5) comprised of 3, 9 and 25 elements each have been characterized. The Reynolds number was varied in the range of 1 to 25,000 (i.e., from laminar to turbulent flow regime). The numerical approach takes into account the aspects of the fluid flow at higher Reynolds number values including circumferential velocity profiles at different cross-sections within the Kenics mixer, which were neglected in previous studies. It was observed that cross-sectional mixing in the turbulent flow regime takes place up to 30% of each element length at element-to-element transition; beyond that velocity profiles were uniform. The experiments were also carried out to measure the circumferential and axial velocity profiles and pressure drop in three different Kenics Mixers using air as fluid. The pressure drop per unit element (ΔP/η) was found to be independent of the number of Kenics mixing elements used in the system. The total pressure drop across Kenics mixer obtained by CFD simulations were compared with the experimental pressure drop values and correlations available in the literature. The numerical results were found in good agreement with the experimental as well as the results reported in the literature. A new pressure drop correlation in the Kenics static mixer has been developed. [Copyright &y& Elsevier]

Abstract: The three-dimensional flow and mixing characteristics of multiple and multi-set three-dimensional confined turbulent round opposing jets in a novel in-line mixer were examined numerically using the standard k–ɛ turbulence model. The model was validated with the available experimental results in a Tee mixer [D.Y. Tang, A.J. Dukat, J.J. Ou, Dynamics of fluid mixing induced at a T-junction, experimental characterization and fluid dynamic computation of temperature distribution in space, Ind. Eng. Chem. Res. 32 (1993) 1727–1733] in which a side stream normally impinges a main stream. Good agreement between the simulated and experimental results is obtained. The effects of multiple opposing jet injection and multi-set opposing jet injections with/without offset on mixing are discussed for this novel opposing jet configuration. [Copyright &y& Elsevier]

Abstract: Computational fluid dynamics (CFD) codes are increasingly being used in the liquefied natural gas (LNG) industry to predict natural gas dispersion distances. This paper addresses several issues regarding the use of CFD for LNG dispersion such as specification of the domain, grid, boundary and initial conditions. A description of the k–ɛ model is presented, along with modifications required for atmospheric flows. Validation issues pertaining to the experimental data from the Burro, Coyote, and Falcon series of LNG dispersion experiments are also discussed. A description of the atmosphere is provided as well as discussion on the inclusion of the Coriolis force to model very large LNG spills. [Copyright &y& Elsevier]

Abstract: The main objective of this work is to improve the conversion inside bubble column reactors by varying the way in which feed is input inside these systems. Bubble column reactors are usually cylindrical in shape, and the reactor feed is at the bottom. The whole idea lies on the fact that the fluid dynamic fields inside the reactor affect its yield. Since the flow pattern in these reactors depend on the radial feed velocities profiles and radial feed holdup profiles at the bottom, several radial feed profiles for these variables were tested in order to analyze how they affect the reactor conversion. The mass flow input for both phases is the same in all situations. This in-house model uses an Eulerian–Eulerian approach to simulate a set of idealized radial profiles feed input situations, that will produce different fluid dynamic fields inside the reactor, and their effect on reactor conversion. The fluid dynamics and concentrations fields, for each phase, is estimated using mass and momentum conservation equations. Turbulence is taken into account using the k–ɛ model. The petroleum thermal hydrocraking is modeled using a pseudo-component model. [Copyright &y& Elsevier]

Abstract: This paper presents a continuous sensitivity equation method (SEM) for incompressible turbulent heat transfer. The k–ɛ model of turbulence with wall functions is used to model turbulence. Heat transfer is handled using an eddy conductivity. The SEM formulation accounts for complex parameter dependence and is suitable for a wide range of problems. It is applied to turbulent flow over a heated backward facing step and to turbulent flow over a heated flat plate. Several uses of the SEM are demonstrated: fast computation of nearby flows, identification of key parameters controlling the flow, and uncertainty analysis. Sensitivities are used to cascade input data uncertainties through the CFD code to yield uncertainty estimates of the skin friction and Stanton number. The SEM also provides detailed information about the contribution of each parameter to the global uncertainty. [Copyright &y& Elsevier]

In a Very-Large-Eddy Simulation (VLES), the filterwidth-wavenumber can be outside the inertial range, and simple subgrid models have to be replaced by more complicated (‘RANS-like’) models which can describe the transport of the biggest eddies. One could approach this by using a RANS model in these regions, and modify the lengthscale in the model for the LES-regions[1–3]. The problem with these approaches is that these models are specifically calibrated for RANS computations, and therefore not suitable to describe inertial range quantities. We investigated the construction of subgrid viscosity and transport equations without any calibrated constants, but these are calculated directly form the Navier-Stokes equation by means of a Renormalization Group (RG) procedure. This leads to filterwidth dependent transport equations and effective viscosity with the right limiting behaviour (DNS and RANS limits). [ABSTRACT FROM AUTHOR]