Open-source Computational Fluid Dynamics Code Research Articles

On the feasibility of overnight industrial high-fidelity simulations of CSP technologies on modern HPC systems

In the last decades, computational fluid dynamics (CFD) has become a standard design tool in many fields, such as the automotive, aeronautical, and renewable energy industries. The driving force behind this is the development of numerical techniques in conjunction with the progress of high-performance computing (HPC) systems. However, simulation time remains the most limiting factor for large-eddy simulations (LES) to be adopted in the industry. A consensus exists that, to be feasible, LES simulations should be completed overnight In this context, this work assesses the feasibility of overnight LES simulations on GPU-accelerated supercomputers with TFA, our novel in-house code, which relies on a symmetry-preserving discretisation for unstructured collocated grids that, apart from being virtually free of artificial dissipation, is shown to be unconditionally stable. The study cases will be taken from central receivers used in concentrated solar power (CSP) plants, and a comparison with open-source CFD codes will be made.

Numerical Simulation of Gas Explosion with Non-uniform Concentration Distribution by Using OpenFOAM.

Gas explosions in coal mines have occurred occasionally, which may cause casualties and economic losses. In the actual mine roadway, the gas concentration distribution is uneven because the gas density is lower than that of air. Gas explosion characteristics of uneven gas distribution with the concentration gradient in mine roadways were analyzed by using the open-source computational fluid dynamic code OpenFOAM. The flame and pressure characteristics were calculated, and the flame and shock wave propagation laws of the non-uniform gas-air mixture explosion with different concentration gradients were analyzed and compared with the uniform gas-air mixture gas. The results show that when the overall gas concentration is the same, the flame velocity and the pressure growth rate of the uniform gas explosion are lower than those of the non-uniform, but the pressure peaks of both are similar. At the same time, when the initial volume concentration is 10%, the non-uniform gas explosion has the highest flame propagation velocity and peak value. The peak explosion pressure of different concentration gradients is proportional to the initial concentration. The above studies clarified the characteristics of gas-air mixture explosions with concentration gradients and provided theoretical support for the prevention and control of gas explosion disasters.

On the application of overset meshing to numerical studies of roll damping of hulls in complex scenarios

The application of computational fluid dynamics (CFD) and overset meshing, to predict the roll response of ships is of significant interest to ocean engineering. Despite its growing popularity, it remains essential to validate the efficacy of computations using overset meshing for modelling strong vortex flows, particularly in complex geometric scenarios involving relative motion and narrow gaps, and for deriving roll damping coefficients. This study presents a new series of results in which the solvers and overset meshing methods in the open-source CFD code, OpenFOAM, were applied to compute the vortex flow from sharp edges and the roll damping of hulls, including the particularly complex scenario of a hull rolling next to a solid vertical wall. It has been demonstrated that the methods can effectively model the intricate flow physics associated with flow separation, including the shedding of primary and secondary vortices, and reliably estimate damping coefficients. Further, the study provides suitable numerical configurations for overset mesh applications and new insights to explain the discrepancies between experimental and numerical results. These new results not only contribute to the understanding of wave-induced roll response but also reinforce the value of the overset mesh technique for the prediction of related problems in fluid mechanics.

Development and validation in water of FLUNED, an open-source tool for fluid activation calculations

In nuclear fusion installations, high-energy plasma neutron activates the cooling and breeding fluids, generating a mobile and distributed source of radiation. In water-cooled fusion reactors, such as ITER and DEMO, the most concerning radioisotopes present in the coolant are the 16N and 17N. Their prompt emission impacts the design, safety, and, ultimately, the economics of the plant as it can be the dominant source of radiation in regions far from the plasma. The flowing condition makes the calculation of radioisotope concentrations within an irradiated cooling circuit particularly challenging as it requires the coupling of the neutronics and fluid dynamics characteristics of the problem. Recent works showed that, except for simple and limited cases, the coupling requires the implementation of Computational Fluid Dynamics (CFD) methods. Nevertheless, the use of ad-hoc methodologies or the application of proprietary codes has hampered the participation and formation of common benchmarks among the neutronics community. In this work, we present the development of an open-source tool called FLUNED that studies the evolution of the concentration of radioactive species inside a fluid and that considers the fluid dynamics of the carrier. The tool is based on the open-source CFD code OpenFOAM and makes use of its modular nature to complement the fluid continuity equation with the radioisotope decay and production terms. In the second part of this work, we show the code validation using experimental data from the water activation experiment performed at FNG in Frascati in December 2019. The validation was performed by producing a computational chain that simulated the main components of the water circuit and estimated the counts in the two experimental detectors caused by the decay of the 16N and 17N isotopes. Finally, the positive comparison between the calculated and the experimental values allowed us to establish the validation of FLUNED for studies of fluid activation in water-cooled fusion experiments and reactors.

NNPred: Deploying neural networks in computational fluid dynamics codes to facilitate data-driven modeling studies

Data-driven modeling has contributed significantly to the field of computational fluid dynamics (CFD), but integrating machine-learning (ML) models into a CFD workflow still remains to be a challenging task. In this paper, we introduce an interface library for the deployment of ML models in CFD codes. The library supports multiple ML backends with binary interface compatibility and provides flexible data input/output (I/O) methods to match the type and layout of the data between ML models and CFD codes. For the convenience of CFD users, the library provides application programming interfaces for two widely used languages (i.e., C++ and Fortran), simplifying the deployment and prediction to only a few lines of code. Two data-driven modeling cases are demonstrated, along with the implementation details of the library in open-source CFD codes (i.e., OpenFOAM and CFL3D). The first case presents a simple heat-transfer problem with assumed experimental data of unknown emissivity, where the basic use of the library in OpenFOAM is demonstrated by solving a diffusive equation with the source term modeled by various ML algorithms. The second case discusses the modeling of turbulence in channel flow, and applies an ML-integrated closure model in both OpenFOAM and CFL3D. In both software, the ML-RANS model reproduces almost identical results to the reference data and favorable extrapolation performance is maintained. In addition, the parallel efficiency is compared with the traditional closure model under the same homogeneous parallel architecture, and only limited additional cost is imposed by running the ML prediction. This library could benefit CFD researchers in rapidly testing data-driven models in their CFD solvers. Program summaryProgram Title: NNPredCPC Library link to program files:https://doi.org/10.17632/v2jzcyjn8v.1Developer's repository link:https://github.com/Weishuo93/NN_PredLicensing provisions: MIT1, LGPL2, Apache-2.03Programming language: C++, FortranNature of problem: Coupling machine learning models with CFD programs with minimal modifications to facilitate the study of data-driven modeling methods (e.g., rapid modeling from experimental data, turbulence closure modeling, etc) with MPI supports.Solution method: First, an interfacing library is developed to bridge ML modeling and a CFD solver. The library supports different ML backends and provides flexible data I/O methods when interfacing with CFD codes. The API of this library can be accessed through C++ and Fortran, which are two most commonly used languages in the CFD community. Secondly, two data-driven modeling cases are demonstrated with the library's implementation details in OpenFOAM and CFL3D. The first case shows a semi-empirical modeling problem in heat transfer with assumed experimental data, whereas the second case reproduces the results of DNS with an ML-integrated turbulence model in two different CFD software (OpenFOAM and CFL3D). Finally, parallel computation is conducted using a homogeneous Message Passing Interface, which is adopted by most mainstream CFD programs. The extra costs are observed limited, and therefore users could validate their ideas on data-driven models with great convenience on parallel programs.Additional comments including restrictions and unusual features: Heterogeneous parallel strategy with GPU and associated applications need to be further developed.

Robust Eddy Viscosity Turbulence Modeling with Elliptic Relaxation and Compound Wall Treatment

This paper presents a holistic Reynolds-averaged Navier–Stokes (RANS) turbulence modeling framework for the computational fluid dynamics (CFD) simulations of complex wall-bounded turbulent flows. Based on the elliptic relaxation idea, the deployed eddy viscosity turbulence model reconstructs the near-wall stress anisotropy and nonviscous effects. The appropriate selection of the turbulent quantities that are being solved for, together with the zero value wall boundary condition for the related turbulent quantities, renders the model less sensitive to the near-wall grid nonuniformities and resolution. The unified near-wall velocity profile, obtained based on the boundary layer theory, is used to devise the compound near-wall treatment that ensures the robustness of the numerical simulation. The proposed turbulence modeling framework is implemented into the general-purpose open-source CFD code and validated against the generic test cases with satisfactory agreement.

Numerical simulation of aerosol generation by cutting fuel debris and aerosol removal by spray droplets during Fukushima decommissioning

The proposed plan for decommissioning the damaged Fukushima Daiichi nuclear reactors involves cutting the melted and re-solidified fuel debris into small pieces for retrieval from the reactor buildings. However, the cutting process generates submicron aerosol particles that need to be removed from the containment vessel before they escape into the environment. The existing spray system inside the containment vessel can be used to remove these aerosol particles using various collection mechanisms. To simulate both the aerosol generation and removal processes, a new numerical model has been developed using the open-source Computational Fluid Dynamics code OpenFOAM. The simulation results indicate that the rate of aerosol generation does not affect the efficiency of aerosol removal, and larger particles can be removed faster due to stronger inertial impaction. The newly developed numerical model has also been used to simulate aerosol generation and removal in the real-size containment vessel of Fukushima Daiichi nuclear reactor unit 3. The simulation results provide information on the spatial distribution of aerosol particles under different multiple-nozzle layouts, which can be used to optimize the design of future multiple-nozzle spray systems for the real Fukushima decommissioning.

BROADCAST: A high-order compressible CFD toolbox for stability and sensitivity using Algorithmic Differentiation

The evolution of any complex dynamical system is described by its state derivative operators. However, the extraction of the exact N-order state derivative operators is often inaccurate and requires approximations. The open-source CFD (Computational Fluid Dynamics) code called BROADCAST discretises the compressible Navier-Stokes equations and then extracts the linearised N-derivative operators through Algorithmic Differentiation (AD) providing a toolbox for laminar flow dynamics. Furthermore, the gradients through adjoint derivation are extracted either by transposition of the linearised operator or through the backward mode of the AD tool. The software includes base-flow computation and linear global stability analysis via eigen-decomposition of the linearised operator or via singular value decomposition of the resolvent operator. Sensitivity tools as well as weakly nonlinear analysis complete the package. The numerical method for the spatial discretisation of the equations consists of a finite-difference high-order shock-capturing scheme applied within a finite volume framework on 2D curvilinear structured grids. The stability and sensitivity tools are demonstrated on two cases: a cylinder flow at low Mach number and a hypersonic boundary layer. Program summaryProgram Title: BROADCASTCPC Library link to program files:https://doi.org/10.17632/hcf893cr2n.1Developer's repository link:https://github.com/OneraHub/BroadcastLicensing provisions: Mozilla Public License 2.0Programming language: Python, FortranNature of problem: The extraction of the exact N-order state derivative operators useful to study complex dynamical systems is often inaccurate and requires approximations from the user.Solution method: This program discretises the compressible Navier-Stokes equations and then extracts the linearised state derivative operators through Algorithmic Differentiation providing a toolbox for laminar flow dynamics.

Simulations of Hypersonic Boundary-Layer Transition over a Flat Plate with the Spalart-Allmaras One-Equation BCM Transitional Model

Transitional flow has a significant impact on vehicles operating at supersonic and hypersonic speeds. An economic way to simulate this problem is to use computational fluid dynamics (CFD) codes. However, not all CFD codes can solve transitional flows. This paper examines the ability of the Spalart–Allmaras one-equation BCM (SA-BCM) transitional model to solve hypersonic transitional flow, implemented in the open-source CFD code Eilmer. Its performance is validated via existing wind tunnel data. Eight different hypersonic flow conditions are applied. A flat plate model is built for the numerical tests. The results indicate that the existing SA-BCM model is sensitive to the freestream turbulence intensity and the grid size. It is not accurate in all the test cases, though the transitional length can be matched by tuning the freestream intensity. This is likely due to the intermittency term of the SA-BCM model not being appropriately calibrated for high-velocity flow, though if the model can be recalibrated it may be able to solve the general high-velocity flows. Although the current SA-BCM model is only accurate under certain flow conditions after one calibration process, it remains attractive to CFD applications. As a one-equation model, the SA-BCM model runs much faster than multiple-equation flow models.

A user-friendly lithium battery simulator based on open-source CFD

The growing use of lithium-ion batteries (LIBs) for automotive and stationary storage applications has put increasingly stringent requirements on battery thermal management and battery safety. An open-source platform that can bridge battery electrochemical models and computational fluid dynamics (CFD) can be of great benefit for designing advanced battery thermal management systems and safety countermeasures by allowing the simulation and prediction of battery responses to various thermofluidic environments and thermal boundaries. Here we develop a user-friendly battery simulator based on the open-source CFD code OpenFOAM. The simulator contains the in-house solvers for the two mostly used physics-based battery models, the single particle model, and the pseudo-two-dimensional model. GUIs are also developed based on Qt for simulation automation and ease of use. To demonstrate the functionality of the developed simulator, the electrochemical performance and internal states of half LIB cells and full LIB cells with different chemistries at different operating conditions are simulated. The obtained results agree well with other existing battery simulators. Due to its native integration with OpenFOAM, the new battery simulator is readily extendable to incorporate various CFD models and other physics to meet the simulation needs of thermal management and safety design for LIBs.

CFD analysis of water vapor condensation in large containments: Numerical model, verification and validation

In the event of a loss of coolant accident (LOCA) in a Pressurized Water Reactor (PWR), large quantities of water vapor and hydrogen might be released into the containment building. Increasing pressure and the risk of hydrogen deflagration can threaten the containment integrity. Water vapor condensation on cold containment structures modifies the pressure load on the containment structures. On the one hand, the total pressure reduces due to the decrease of the water vapor content of the containment atmosphere. On the other hand, the concentration of hydrogen increases, which can reach locally the threshold of deflagration or detonation. A Computational Fluid Dynamics (CFD) model is developed and tested in order to predict transients of the pressure and the concentrations of gaseous species in reactor containments after a LOCA. Since the free volume of the containment of typical French PWRs is between 50,000 m3 and 80,000 m3, the CFD model requires significant simplifications.The physical model and its simplifications as well the implementation in the CEA OpenSource CFD code TrioCFD are described. Model tests are presented in two steps. In a first step, verification tests are discussed to show that the condensation model is implemented correctly in the CFD code. In a second step, the model is validated against experimental data of the International Standard Problem ISP47. In this benchmark, steady state conditions between vapor injection and condensation were reached experimentally in the MISTRA test containment of the CEA. In phase A of the benchmark, an equilibrium was achieved between water vapor injection and water vapor condensation on temperature controlled cold walls. One incondensable gas, namely air, was present in the test vessel. In phase B, a second incondensable gas was added to the containment atmosphere, namely helium. Both steady state situations were analyzed with the CFD model. The calculations represent well the experiment when the predominant condensation paths are modelled. It is shown that the pressure in the containment vessel as well as the mean mass fractions of water vapor and air, as well as of helium in phase B, are calculated in accordance to the experiment. The temperature in the containment is overestimated. Measured vertical profiles of the species concentrations are reproduced correctly.

Study on the air resistance of pigs in groups based on open-source CFD code: Influence of stocking density and live weight

Animal stocking density (STKD) and live weight (LIVW) are the main factors determining the air resistance in ventilation system. To study the impact of LIVW (30, 45, 60, 75, 90, 105, 120 and 135 kg) and STKD (0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 2.0, and 2.6 pig m −2 ) on air resistance of pig groups, an open-source computational fluid dynamics (CFD) code, OpenFOAM, was adopted. Firstly, pressure drop of tube bundles which included 15 staggered tubes with a diameter of 0.40 m, equivalent to pig groups with 50-kg and 0.9 pig m −2 , were simulated for the accuracy of CFD models which were then used for calculating pig groups' air resistance for horizontal (HD) and vertical (VD) airflow directions. The validation of CFD models was based on semi-empirical equations for the tube bundle. The results indicated that: 1) The CFD model (realisable k-ε model, 10 boundary layers with 0.04 m total thickness) was reliable for pressure drop of complex bluff bodies with an average relative error of 6.80%. 2) The pig groups' friction factor for HD or VD almost increased with the rising STKD or LIVW, and regression equations were established to estimate friction factors according to LIVW and STKD of pig groups. 3) Regarding to pig groups’ air resistance, STKD is much more important than LIVW for VD, but LIVW and STKD have almost the same importance for HD. The interaction of LIVW and STKD for HD and VD has little effect. • OpenFOAM was reliable to calculate the pressure drop of complex bluff bodies. • Varied mass and stocking densities affect the air resistance of pig groups. • Pig groups' friction factors can be predicted by exponential regression equations.

A comparison of major steady RANS approaches to engineering ABL simulations

Engineering atmospheric boundary layer (EABL) simulations performed using Computational Fluid Dynamics (CFD) can give key insights in successfully addressing a variety of topics in environmental and structural aerodynamics. To improve understanding of this complex topic, a computational evaluation of the shear stress-driven (SSD), pressure-driven (PD) and body-force-driven (BFD) EABL flows was performed using the open-source CFD code OpenFOAM®. The EABL simulations were computationally evaluated by using the successor domain technique (SDT) and precursor domain technique (PDT). The boundary conditions applied in the CFD algorithm using OpenFOAM® to computationally model the SSD, PD, and BFD EABL flows were reported and discussed. A developed CFD approach may also be satisfactorily used in other relevant CFD codes, including but not limited to Ansys CFX®, Ansys Fluent®, STAR−CCM+®. The impact of the EABL models on surface pressure distribution on a low-rise cubic building, the total building drag force and the wind velocities in pedestrian-level areas was computationally evaluated by using major Reynolds-averaged Navier-Stokes (RANS) turbulence models, i.e., the standard k−ε, re-normalization group (RNG) k−ε, realizable k−ε, Wilcox's k−ω, and Menter's k−ω shear stress transport (SST) turbulence models. The obtained computational results indicate that the SSD and PD EABL flows may be successfully modeled in cases where additional modifications of the officially released CFD code are not required in regard to the PDT. The PD EABL flow may be successfully modeled by using all studied RANS turbulence models in combination with the PDT. The EABL flow throughout an empty computational domain is homogeneous even without using a body force because the employed CFD code is capable of creating the required pressure gradient that drives the flow along the computational domain. While the surface pressure estimation on the windward body surface and in the recirculation zones was generally proven to be an issue when using steady RANS two-equation turbulence models, surface pressures in the stagnation zone on the windward cubic building surface and pressures on the top and the side building surfaces in the present study agree well with the experiments when using the RNG k−ε and Menter's k−ω SST turbulence models. The pressure distribution on the building surfaces, the total building drag force and wind characteristics in the vicinity of a lift-up building are not considerably affected by the choice of the EABL model, while they are substantially affected by the type of RANS turbulence model. The best agreement with the experiments was achieved when using the RNG k−ε and Menter's k−ω SST turbulence models, so these models may be recommended for future applications.

도로협곡내 보행공간에서 식생에 의한 교통 대기오염 저감효과에 대한 CFD 시뮬레이션 연구

The changes of traffic-induced sidewalk air pollution are investigated after trees are planted in a street canyon, using FDS(Fire Dynamics Simulator), an open source CFD(Computational Fluid Dynamics) code. The roles of tree permeability, its planting location, and its shapes are examined. Extensive amount of simulations are carried out to produce 399 sets of valid data out of 400 scenario cases, which include 1) the shrub layer only cases, 2) the small-tree layer over shrub layer cases, and 3) the tall-tree layer over shrub layer cases. The steady state pollution concentrations averaged over the sidewalk planes are reduced more than 10% after the trees are planted, regardless of its planting type. The largest reduction of about 45% is achieved for a shrub layer only case. The vortical flows which transport the pollution materials directly from vehicle lanes to the sidewalk are distorted favorably by the shrub layer, which provide the underlying physical explanations for these reductions.

Numerical study of the impact of glottis properties on the airflow field in the human trachea using V-LES

The influences of the profiles and cross-sectional areas of glottal aperture on the upper respiratory airway are investigated using an idealized cast-based mouth-throat model and three dimensional computational fluid dynamics (CFD). The open source CFD code OpenFOAM is employed. The transient flows are modeled using the very-large eddy simulation with the Smagorinsky sub-grid scale (SGS) model. Five different shapes of glottis are considered, including circular glottis with 100 %, 75 % and 50 % cross-sectional area and elliptic glottis with 75 % and 50 % cross-sectional area. Both instantaneous and averaged flow fields are analyzed. It is found that the variations of glottis have great impacts on the properties of downstream flow fields such as the secondary flow, laryngeal jet, recirculation zone, turbulent kinetic energy, and vortex. Evident impacts are observed in the region within 6 tracheal diameters downstream of the glottis. The profile of the glottis has more impacts on the laryngeal shape, while the cross-sectional area has more impacts on velocity of the laryngeal jet and turbulent intensity. It is concluded that both the glottal areas and profiles are critical for an idealized geometrical mouth-throat model.

Assessment of the Manoeuvrability Characteristics of a Twin Shaft Naval Vessel Using an Open-Source CFD Code

The purpose of this study is to assess the quality of the manoeuvre prediction of a twin-shaft naval vessel by means of a time-domain simulator based on Computational Fluid Dynamics (CFD) hydrodynamic coefficients. The simulator uses a modular approach in which the hull, rudders, appendices and propellers are based on different mathematical models. The hydrodynamic coefficients of the hull in the bare and appended configurations are computed using virtual captive tests performed with an open-source CFD code: OpenFoam. This paper demonstrates that the application of the CFD hydrodynamic coefficients led to a good estimate of the macroscopic characteristics of the main IMO manoeuvres with respect to the experimental measures. The adopted test case is the DTMB 5415M frigate both with and without appendages. This test case has been investigated in several research studies and international benchmark workshops, such as SIMMAN 2008, SIMMAN 2014 and many CFD workgroups.

Computational simulation of a test facility in reduced scale for analysis of boron dispersion in a pressurizer of an integral compact and modular reactor

In an iPWR (integral pressurized water reactor), the pressurizer is located at the top of the reactor vessel; this configuration requires the investigation of the mechanisms adopted for the homogenization of boron. In previous work, three representative experiments were conducted in an experimental installation made of stainless steel with a volumetric scale equal to 1:200, representing one-fourth of the pressurizer of an iPWR. The test facility layout was mounted so that the test section was fed with water or saline solutions with different concentrations, representing scenarios of boration or deboration. To determine the concentration at the inlet and outlet of the test section, samples have been collected every 10 min during the experiments representing some scenarios. The main goal in this work was to investigate how well mixing during boron dilution transients in the pressurizer of a small modular reactor can be modelled accurately by CFD (Computational Fluid Dynamics) codes. Two CFD codes were used, the commercial ANSYS CFX and the open source CFD code OpenFOAM. The use of open source software such as OpenFOAM offers a way to CFD gain acceptance in the licensing. The results of the comparison of simulations with an experiment at the test section were presented and showed a very good agreement. It was verified that deviations are less than 4%, both codes can be used to accurately represent this phenomenon. In order to improve future experiments, the dispersion of the solution inside the test section is studied with the aim to propose positions for sensors. A first simple configuration is proposed, but some further simulations will be done to find an optimum configuration.

A new Simulation Model for Grate Firing Systems in OpenFOAM

A grate firing system can be seen as a multiphase flow problem. High volume fractions of solid fuel particles travel over the grate of a power plant, while heterogeneous reactions occure at the surface of the particles in combination with homogeneous combustion reactions inside the flue gas. Up to now, grate firing systems are still difficult to simulate with standard computational fluid dynamics (CFD). The movement of the solid particles inside the fuel bed and their heterogeneous reactions are difficult to combine with the homogeneous reactions of the gas phase species and the heat transfer. The emitted radiation from the gas has to be coupled with the heat balance inside the fuel bed.To solve this problem, we integrated a new library into a open source CFD code for coupling radiation models with the solids phase. We describe how to handle the workflow of the new library and present simulation results for two different furnace designs. An automatic mesh generator was developed. The work closes with simulation results for two different grate firing systems. Heterogeneous reactions and homogeneous reactions, as well as temperature fields inside the phase are presented. A validation of the simulations against measurement data is given.

Comparative Study of CFD and LedaFlow Models for Riser-Induced Slug Flow

The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas industry to consider making the switch from LedaFlow to CFD. Presently, the industry use relatively simple 1D-models, such as LedaFlow, to predict flow patterns in pipelines. The reduction in cost of computational power in recent years have made it relevant to compare the performance of these codes with high fidelity CFD simulations. A laboratory test facility was used to obtain data for pressure and mass flow rates for the two-phase flow of air and water. A benchmark case of slug flow served for evaluation of the numerical models. A 3D unsteady CFD simulation was performed based on Reynolds-Averaged Navier-Stokes (RANS) formulation and the Volume of Fluid (VOF) model using the open-source CFD code OpenFOAM. Unsteady simulations using the commercial 1D LedaFlow solver were performed using the same boundary conditions and fluid properties as the CFD simulation. Both the CFD and LedaFlow model underpredicted the experimentally determined slug frequency by 22% and 16% respectively. Both models predicted a classical blowout, in which the riser is completely evacuated of water, while only a partial evacuation of the riser was observed experimentally. The CFD model had a runtime of 57 h while the LedaFlow model had a runtime of 13 min. It can be concluded that the prediction capabilities of the CFD and LedaFlow models are similar for riser-induced slug flow while the CFD model is much more computational intensive.

The burning characteristics and flame evolution of hydrocarbon and hydrogen flash fires

A flash fire is a sudden, intense fire caused by ignition of a mixture of air and a dispersed flammable substance such as a solid (including dust), flammable or combustible liquid (such as an aerosol or fine mist), or a flammable gas. The present study aims to gain insight about the combustion processes and flame structure and dynamics associated with flash fires through computational fluid dynamics (CFD) based numerical studies using FireFOAM, the large eddy simulation based fire solver with the frame of open source CFD code OpenFOAM. It will focus on the initial transient development to gain insight about flash fire growth and the underlying combustion process. The scope of the study is, however, limited to flash fires formed following rapid release of relatively large quantities of flammable gas. The predicted flash fire diameter and the lifting height were found to be in reasonably good agreement with published experimental data. To gain further insight of the flash fire transient behaviour, the flame structures, temperature profiles and pressure fields have also been analysed. The predicted incident radiation at different locations is discussed in relation to the resulting thermal radiation hazards.