Subgrid-scale Turbulence Research Articles

On the influences of key modelling constants of large eddy simulations for large-scale compartment fires predictions

ABSTRACTAn in-house large eddy simulation (LES) based fire field model has been developed for large-scale compartment fire simulations. The model incorporates four major components, including subgrid-scale turbulence, combustion, soot and radiation models which are fully coupled. It is designed to simulate the temporal and fluid dynamical effects of turbulent reaction flow for non-premixed diffusion flame. Parametric studies were performed based on a large-scale fire experiment carried out in a 39-m long test hall facility. Several turbulent Prandtl and Schmidt numbers ranging from 0.2 to 0.5, and Smagorinsky constants ranging from 0.18 to 0.23 were investigated. It was found that the temperature and flow field predictions were most accurate with turbulent Prandtl and Schmidt numbers of 0.3, respectively, and a Smagorinsky constant of 0.2 applied. In addition, by utilising a set of numerically verified key modelling parameters, the smoke filling process was successfully captured by the present LES model.

A Physically Based Horizontal Subgrid-Scale Turbulent Mixing Parameterization for the Convective Boundary Layer

Abstract Compared to the representation of vertical turbulent mixing through various planetary boundary layer (PBL) schemes, the treatment of horizontal turbulent mixing in the boundary layer has received much less attention. In mesoscale and convective-scale models, subgrid-scale horizontal turbulent mixing has traditionally been associated with mesoscale circulations or eddies. Its parameterization most often adopts the gradient-diffusion model, where the horizontal mixing coefficients are usually set constant, or through the 2D Smagorinsky formulation, or in some cases based on the 1.5-order turbulence kinetic energy (TKE) closure. For horizontal turbulent mixing associated with boundary layer eddies, the traditional schemes are shown to perform poorly. This work investigates the characteristic turbulence velocity and length scales based on analysis of a well-resolved, wide-domain large-eddy simulation of a convective boundary layer (CBL). To improve the representation of horizontal turbulent mixing by CBL eddies, a class of schemes is proposed with different levels of sophistication. The first two schemes can be used together with first-order PBL schemes, while the third uses TKE to define its characteristic velocity scale and can be used together with TKE-based higher-order PBL schemes. The proposed parameterizations are tested a posteriori in idealized simulations of turbulent dispersion of a passive scalar. Comparisons show improved horizontal dispersion by the proposed schemes and further demonstrate the weakness of the existing schemes.

Large-Eddy Simulations of Atmospheric Flows Over Complex Terrain Using the Immersed-Boundary Method in the Weather Research and Forecasting Model

Atmospheric flow over complex terrain, particularly recirculation flows, greatly influences wind-turbine siting, forest-fire behaviour, and trace-gas and pollutant dispersion. However, there is a large uncertainty in the simulation of flow over complex topography, which is attributable to the type of turbulence model, the subgrid-scale (SGS) turbulence parametrization, terrain-following coordinates, and numerical errors in finite-difference methods. Here, we upgrade the large-eddy simulation module within the Weather Research and Forecasting model by incorporating the immersed-boundary method into the module to improve simulations of the flow and recirculation over complex terrain. Simulations over the Bolund Hill indicate improved mean absolute speed-up errors with respect to previous studies, as well an improved simulation of the recirculation zone behind the escarpment of the hill. With regard to the SGS parametrization, the Lagrangian-averaged scale-dependent Smagorinsky model performs better than the classic Smagorinsky model in reproducing both velocity and turbulent kinetic energy. A finer grid resolution also improves the strength of the recirculation in flow simulations, with a higher horizontal grid resolution improving simulations just behind the escarpment, and a higher vertical grid resolution improving results on the lee side of the hill. Our modelling approach has broad applications for the simulation of atmospheric flows over complex topography.

A numerical study of intake valve jet flapping in a gasoline direct injection engine

This paper presents findings from a numerical study of intake valve jet flapping within a gasoline direct injection (GDI) engine, using a large eddy simulation (LES) turbulence modelling approach. The experimental test case and computational setup, including choice of sub-grid scale (SGS) turbulence model, are presented and discussed. An example cycle where intake valve jet flapping is seen to be prominent is discussed in detail. Intake valve jet flapping was found to be initiated as a consequence of turbulent fluctuations in the intake valve curtains. Cycle-by-cycle variations in valve curtain mass flux and the subsequent jet flapping events are investigated and significant cyclic variability is found. It was observed that when an ensemble-averaging procedure, typically used in LES simulations and experimental PIV data post-processing, is applied, due to the cyclic variability of the variations in valve curtain mass flux, most of the information related to this flow phenomenon is lost.

An improved anisotropy-resolving subgrid-scale model for flows in laminar–turbulent transition region

Some types of mixed subgrid-scale (SGS) models combining an isotropic eddy-viscosity model and a scale-similarity model can be used to effectively improve the accuracy of large eddy simulation (LES) in predicting wall turbulence. Abe (2013) has recently proposed a stabilized mixed model that maintains its computational stability through a unique procedure that prevents the energy transfer between the grid-scale (GS) and SGS components induced by the scale-similarity term. At the same time, since this model can successfully predict the anisotropy of the SGS stress, the predictive performance, particularly at coarse grid resolutions, is remarkably improved in comparison with other mixed models. However, since the stabilized anisotropy-resolving SGS model includes a transport equation of the SGS turbulence energy, kSGS, containing a production term proportional to the square root of kSGS, its applicability to flows with both laminar and turbulent regions is not so high. This is because such a production term causes kSGS to self-reproduce. Consequently, the laminar–turbulent transition region predicted by this model depends on the inflow or initial condition of kSGS. To resolve these issues, in the present study, the mixed-timescale (MTS) SGS model proposed by Inagaki et al. (2005) is introduced into the stabilized mixed model as the isotropic eddy-viscosity part and the production term in the kSGS transport equation. In the MTS model, the SGS turbulence energy, kes, estimated by filtering the instantaneous flow field is used. Since the kes approaches zero by itself in the laminar flow region, the self-reproduction property brought about by using the conventional kSGS transport equation model is eliminated in this modified model. Therefore, this modification is expected to enhance the applicability of the model to flows with both laminar and turbulent regions. The model performance is tested in plane channel flows with different Reynolds numbers and in a backward-facing step flow. The results demonstrate that the proposed model successfully predicts a parabolic velocity profile under laminar flow conditions and reduces the dependence on the grid resolution to the same degree as the unmodified model by Abe (2013) for turbulent flow conditions. Moreover, it is shown that the present model is effective at transitional Reynolds numbers. Furthermore, the present model successfully provides accurate results for the backward-facing step flow with various grid resolutions. Thus, the proposed model is considered to be a refined anisotropy-resolving SGS model applicable to laminar, transitional, and turbulent flows.

Turbulent indoor airflow simulation using hybrid LES/RANS model utilizing Lattice Boltzmann method

Abstract In this study the LES, RANS and hybrid RANS/LES turbulence models within the frame work of the Lattice Boltzmann method (LBM) were used to study turbulent indoor airflow. The turbulent airflow was simulated first using the LES where the sub-grid scale turbulence effects were accounted for using the Smagorinsky model. Then, the airflow was studied using the k − ɛ turbulence model in conjunction with the LBM by the addition of two population balance equations for k and ɛ. For the hybrid RANS/LES method, the near wall region was simulated using the RANS model in conjunction with the k − ɛ turbulence model, while the rest of the domain was analyzed with the LES model within the framework of the LBM. The present simulation results for the airflow showed good agreement with the experimental data and the earlier numerical results, particularly, for the LES and the hybrid RANS/LES models. The results also showed that the accuracy of the hybrid model is more than the RANS and less than the LES models. While the hybrid model predicts the large scale turbulence fluctuation velocities in the bulk of the flow, the needed computational time is less than that of the LES model and more than the RANS model.

Resolving vorticity and dissipation in a turbulent boundary layer by tomographic PTV and VIC+

The existing time-resolved tomographic particle image velocimetry (PIV) measurements by Jodai and Elsinga (J Fluid Mech 795:611–633; Jodai, Elsinga, J Fluid Mech 795:611–633, 2016) in a turbulent boundary layer (Reθ = 2038) are reprocessed using tomographic particle tracking velocimetry (PTV) and vortex-in-cell-plus (VIC+). The resulting small-scale flow properties, i.e. vorticity and turbulence dissipation, are compared. The VIC+ technique was recently proposed and uses the concept of pouring time into space to increase reconstruction quality of instantaneous velocity. The tomographic PTV particle track measurements are interpolated using VIC+ to a dense grid, making use of both particle velocity and Lagrangian acceleration. Comparison of the vortical structures by visualization of isosurfaces of vorticity magnitude shows that the two methods return similar coherent vortical structures, but their strength in terms of vorticity magnitude is increased when using VIC+, which suggests an improvement in spatial resolution. Further statistical evaluation shows that the root mean square (rms) of vorticity fluctuations from tomographic PIV is approximately 40% lower in comparison to a reference profile available from a DNS simulation, while the VIC+ technique returns rms vorticity fluctuations to within 10% of the reference. The dissipation rate is heavily underestimated by tomographic PIV with approximately 50% damping, whereas the VIC+ analysis yields a dissipation rate to within approximately 5% for y+ > 25. The fact that dissipation can be directly measured by a volumetric experiment is novel. It differs from existing approaches that involve 2d measurements combined with isotropic turbulence assumptions or apply corrections based on sub-grid scale turbulence modelling. Finally, the study quantifies the spatial response of VIC+ with a sine-wave lattice analysis. The results indicate a twofold increase of spatial resolution with respect to cross-correlation interrogation.

Large eddy simulation of upward co-current annular boiling flow using an interface tracking method

Towards the full understanding of the mechanism of boiling in annular flow regime, a computational fluid dynamics method with interface tracking has been developed. Smagorinsky model is used for the sub-grid scale turbulence, the constant turbulent Prandtl number being used to model the turbulent thermal conductivity. A sharp-interface phase change model is used to simulate heat and mass transfer phenomenon at the liquid-vapor interface. The developed numerical method is applied to a simulation of vertical upward co-current boiling flow in the annular flow regime. The experiment of Barbosa et al. is selected for validation: the mass flow rate is 30kg/m2·s with the heat flux 159kW/m2 under the system pressure of 1.9bar. The computed results clearly show how the disturbance waves are generated: the waves are caused from inhomogeneous shear stress acting on the liquid-vapor interface, which is amplified by flow separation in the vapor phase. Subsequently, the interaction between the disturbance waves, the mass transfer and the temperature distribution over the heat-transfer surface is investigated. The temperature gradient in the normal direction to the heat-transfer surface is larger in the thin liquid film than that in the disturbance wave, and consequently, mass transfer rate is also higher there. The higher temperature region is observed underneath the disturbance waves resulting from lower temperature gradient (i.e. heat flux). The computed results imply that the bubble nucleation observed in the experiment may be caused by the higher temperature underneath the disturbance wave on the heat-transfer surface.

VoIgt profile Parameter Estimation Routine (viper): H i photoionization rate at z < 0.5

We have developed a parallel code called the VoIgt profile Parameter Estimation Routine (viper) for automatically fitting the H i Lyα forest seen in the spectra of quasi-stellar objects (QSOs). We obtained the H i column density distribution function (CDDF) and linewidth (b) parameter distribution for z < 0.45 using spectra of 82 QSOs obtained using the Cosmic Origins Spectrograph and viper. The consistency of these with existing measurements in the literature validates our code. By comparing this CDDF with those obtained from hydrodynamical simulations, we constrain the H i photoionization rate (⁠|$\Gamma _{\rm H\,\small {I}}$|⁠) at z < 0.45 in four redshift bins. viper, together with the Code for Ionization and Temperature Evolution (cite), which we developed for gadget-2, allows us to explore parameter space and perform χ2 minimization to obtain |$\Gamma _{\rm H\,\small {I}}$|⁠. We notice that the b parameters from the simulations are smaller than those derived from observations. We show that the observed b parameter distribution and b versus |$\log {N_{\rm H\,\small {I}}}$| scatter can be reproduced in simulations by introducing subgrid-scale turbulence. However, it has very little influence on the derived |$\Gamma _{\rm H\,\small {I}}$|⁠. The |$\Gamma _{\rm H\,\small {I}}(z)$| obtained here, (3.9 ± 0.1) × 10−14 (1 + z)4.98 ± 0.11 s−1, is in good agreement with the values derived by us using flux-based statistics in a previous article. These values are consistent with the hydrogen ionizing ultraviolet (UV) background being dominated mainly by QSOs, without needing any contribution from non-standard sources of UV photons.

An Idealized Numerical Simulation Investigation of the Effects of Surface Drag on the Development of Near-Surface Vertical Vorticity in Supercell Thunderstorms

AbstractIdealized simulations are used to investigate the contributions of frictionally generated horizontal vorticity to the development of near-surface vertical vorticity in supercell storms. Of interest is the relative importance of barotropic vorticity (vorticity present in the prestorm environment), baroclinic vorticity (vorticity that is principally generated by horizontal buoyancy gradients), and viscous vorticity (vorticity that originates from the subgrid-scale turbulence parameterization, wherein the effects of surface drag reside), all of which can be advected, tilted, and stretched. Equations for the three partial vorticities are integrated in parallel with the model. The partial vorticity calculations are complemented by analyses of circulation following material circuits, which are often able to be carried out further in time because they are less susceptible to explosive error growth.Near-surface mesocyclones that develop prior to cold-pool formation (this only happens when the environmental vorticity is crosswise near the surface) are dominated by only barotropic vertical vorticity when the lower boundary is free slip, but both barotropic and viscous vertical vorticity when surface drag is included. Baroclinic vertical vorticity grows large once a cold pool is established, regardless of the lower boundary condition and, in fact, dominates at the time the vortices are most intense in all but one simulation (a simulation dominated early by a barotropic mode of vortex genesis that may not be relevant to real convective storms).

Enhanced method for multiscale wind simulations over complex terrain for wind resource assessment

Due to the natural variability of the wind, it is necessary to conduct thorough wind resource assessments to determine how much energy can be extracted at a given site. Lately, important advancements have been achieved in numerical methods of multiscale models used for high resolution wind simulations over steep topography. As a contribution to this effort, an enhanced numerical method was devised in the mesoscale compressible community (MC2) model of the Meteorological Service of Canada, adapting a new semi-implicit scheme with its imbedded large-eddy simulation (LES) capability for mountainous terrain. This implementation has been verified by simulating the neutrally stratified atmospheric boundary layer (ABL) over flat terrain and a Gaussian ridge. These preliminary results indicate that the enhanced MC2-LES model reproduces efficiently the results reported by other researchers who use similar models with more sophisticated sub-grid scale turbulence schemes. The proposed multiscale method also provides a new wind initialization scheme and additional utilities to improve numerical accuracy and stability. The resulting model can be used to assess the wind resource at meso- and micro-scales, reducing significantly the wind speed overestimation in mountainous areas.

Characterizing plasmoid reconnection by turbulence dynamics

In weakly dissipative plasmas, the plasmoid instability may lead, in principle, to fast magnetic reconnection through long current sheets. On the other hand, it is well known that weakly dissipative large-Reynolds-number plasmas easily become turbulent. We address the question of whether turbulence can enhance the reconnection rate of plasmoid-unstable current sheets by carrying out high resolution MHD simulations. Instead of resolving all scales down to dissipation, we utilize a turbulence model to investigate the influence of turbulence on the plasmoid instability. For this sake, we extend a Reynolds-averaged turbulence model expressing the energy, cross-helicity, and helicity due to the turbulence to a subgrid-scale (SGS) model of turbulence by means of a Gaussian filter. We then use the SGS turbulence model to investigate the contributions of the turbulent energy and cross-helicity to the plasmoid reconnection rate. In particular, we address the consequences of a finite guide magnetic field parallel to the reconnection electric field on the reconnection rate in terms of the residual turbulent helicity. To validate the turbulence model, we compare the SGS electromotive force with that obtained statistically from the high resolution simulations. This way, we characterize the influence of turbulence on the reconnection rate of plasmoid-unstable current sheets and attribute the plasmoid reconnection rate at large-magnetic-Reynolds-numbers to turbulence.

Assessment of LES Subgrid-scale Models and Investigation of Hydrodynamic Behaviour for an Axisymmetrical Bluff Body Flow

This work is concerned with the investigation of fluid-mechanical behaviour and the performance of different subgrid-scale models for LES in the numerical prediction of a confined axisymmetrical bluff-body flow. Four subgrid-scale turbulence models comprising the Smagorinsky model, Dynamic Smagorinsky model, WALE model and subgrid turbulent kinetic energy model, are validated and compared directly against the experimental data. Two different mesh counts are used for the LES studies, one with a higher mesh resolution in the shear layer than the other. It is found that increasing the mesh resolution improves the time-averaged fluctuating velocity profiles, but has less effect on the time-averaged filtered velocity profiles. A comparison against experiment shows that the recirculation zone length is well predicted using LES. The accuracy of the four different subgrid scale models is then assessed by comparing the LES results using the dense mesh with the experiment. Comparisons with the time-averaged axial and radial velocity profiles demonstrate that LES displays good agreement with the experimental data, with the essential flow features captured both qualitative and quantitatively. The subgrid velocity also matches well with the experimental results, but a slight underprediction of the inner shear layer is observed for all subgrid models. In general, it is found that the Smagorinsky and WALE models are more dissipative than the Dynamic Smagorinsky model and subgrid TKE model. Comparison of the spectra against the experiment shows that LES can capture dominant features of the turbulent flow with reasonable accuracy, and weak spectral peaks related to the Kevin-Helmholtz instability and helical vortex shedding are present.

Unstructured LES-CMC modelling of turbulent premixed bluff body flames close to blow-off

A finite volume Large Eddy Simulation-Conditional Moment Closure (LES-CMC) formulation is applied to turbulent premixed bluff body methane-air flames at conditions far from (A1) and close to (A4) blow-off. The unstructured topology of the CMC grid allows refinement in regions where turbulence inhomogeneity is expected, providing an improved description of the turbulence-chemistry interaction phenomenon, non-negligible at conditions investigated in this study. Subgrid scale (SGS) progress variable variance and scalar dissipation rate are closed with models associated with the SGS combustion, turbulence and molecular diffusion processes of premixed flames using detailed kinetics (GRI-3). The simulations effectively reproduce the general trends, especially considering the challenging conditions at very lean mixtures (down to φ=0.64): the characteristic ‘M’-shaped morphology of flame A4 and the significant increase in flame brush thickness compared to flame A1 is accurately replicated, although overall flame heights are under-predicted. Formaldehyde and OH distributions are compared with PLIF measurements and excellent agreement is reported supporting previous experimental observations: for flame A1, OH is seen throughout the recirculation zone while CH2O is present only close to the shear layer along which heat release occurs in a thin, unbroken region of CH2O/OH overlap, disturbed only occasionally by vortex-like structures. For flame A4 close to blow-off, the calculation generally shows appreciable quantities of CH2O throughout the recirculation zone with isolated pockets of OH, surrounded by high heat release, in agreement with experimental findings. The simulation further evidences that in regions experimentally void of both CH2O and OH, large quantities of partially reacted fluid are present which entered the recirculation zone from the top. Quantitative comparisons of flame surface density and local flame curvature statistics are also calculated which agree well with experimental data, suggesting a comprehensive description of the physico-chemical processes of the proposed modelling strategy.

Large Eddy Simulation of gas–particle kinematic decoupling and turbulent entrainment in volcanic plumes

In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) intercomparison study on volcanic plume models, we present three-dimensional (3D) numerical simulations carried out with the ASHEE (ASH Equilibrium Eulerian) model. The ASHEE model solves the compressible balance equations of mass, momentum, and enthalpy of a gas-particle mixture and is able to describe the kinematic decoupling for particles characterized by Stokes number (i.e., the ratio between the particle equilibrium time and the flow characteristic time) lower than 0.2 (or particles smaller than about 1mm). The computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a Large Eddy Simulation approach, and is thus suited to analyze the role of particle non-equilibrium in the dynamics of turbulent volcanic plumes. The two reference scenarios analyzed correspond to a weak (mass eruption rate=1.5*106 kg/s) and a strong volcanic plume (mass eruption rate=1.5*109 kg/s) in absence of wind. For each scenario, we compare the 3D results, averaged in space and time, with theoretical results obtained from integral plume models. Such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid-scale turbulence model, and the influence of gas-particle non-equilibrium processes on the large-scale plume dynamics. We thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant (of the order of 20%), but still lower than that typically associated with input data and integral model approximations. In the Weak Plume case, 3D results are consistent with the predictions of integral models in the jet and plume regions, with an entrainment coefficient around 0.10 in the plume region. In the Strong Plume case, the self-similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable, with an average value of 0.24. For both cases, integral model predictions diverge from the 3D plume behavior in the umbrella region. The presented analysis of 3D numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies. In addition, high-resolution 3D runs allow reproduction of observable quantities (such as infrasound signals) which can be useful for constraining eruption dynamics during real events.

Study of three LES subgrid-scale turbulence models for predictions of heat and mass transfer in large-scale compartment fires

ABSTRACTNumerical assessment was performed to investigate the wall-adaptive features offered by two subgrid-scale (SGS) turbulence models: Wall-Adapting Local Eddy Viscosity (WALE) and Vreman against the Smagorinsky model. The gas temperature and velocity field predictions were enhanced using WALE over Smagorinsky, especially at the flaming and near-wall regions since WALE considers both strain and rotation rates of the turbulent structure and the turbulent viscosity approaches zero at the wall. Conversely, the simulation results by Vreman were under-predicted against the experimental data. The WALE model could notably enhance the simulation accuracy for large-scale compartment fires due to significant improvements of the flow diffusivity modeling.

Large Eddy Simulation of an Enclosed Turbulent Reacting Methane Jet With the Tabulated Premixed Conditional Moment Closure Method

The tabulated premixed conditional moment closure (T-PCMC) method has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes in Reynolds-averaged Navier–Stokes (RANS) environment by Martin et al. (2013, “Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method,” ASME Paper No. GT2013-95092). Here, the premixed conditional moment closure (PCMC) method is extended to large eddy simulation (LES). The new model is validated with the turbulent, enclosed reacting methane backward facing step data from El Banhawy et al. (1983, “Premixed, Turbulent Combustion of a Sudden-Expansion Flow,” Combust. Flame, 50, pp. 153–165). The experimental data have a rectangular test section at atmospheric pressure and temperature with an inlet velocity of 10.5 m/s and an equivalence ratio of 0.9 for two different step heights. Contours of major species, velocity, and temperature are provided. The T-PCMC model falls into the class of table lookup turbulent combustion models in which the combustion model is solved offline over a range of conditions and stored in a table that is accessed by the computational fluid dynamic (CFD) code using three controlling variables: the reaction progress variable (RPV), variance, and local scalar dissipation rate. The local scalar dissipation rate is used to account for the affects of the small-scale mixing on the reaction rates. A presumed shape beta function probability density function (PDF) is used to account for the effects of subgrid scale (SGS) turbulence on the reactions. SGS models are incorporated for the scalar dissipation and variance. The open source CFD code OpenFOAM is used with the compressible Smagorinsky LES model. Velocity, temperature, and major species are compared to the experimental data. Once validated, this low “runtime” CFD turbulent combustion model will have great utility for designing the next generation of lean premixed (LPM) gas turbine combustors.

Sub-grid-scale description of turbulent magnetic reconnection in magnetohydrodynamics

Magnetic reconnection requires, at least locally, a non-ideal plasma response. In collisionless space and astrophysical plasmas, turbulence could transport energy from large to small scales where binary particle collisions are rare. We have investigated the influence of small scale magnetohydrodynamics (MHD) turbulence on the reconnection rate in the framework of a compressible MHD approach including sub-grid-scale (SGS) turbulence. For this sake, we considered Harris-type and force-free current sheets with finite guide magnetic fields directed out of the reconnection plane. The goal is to find out whether unresolved by conventional simulations MHD turbulence can enhance the reconnection process in high-Reynolds-number astrophysical plasmas. Together with the MHD equations, we solve evolution equations for the SGS energy and cross-helicity due to turbulence according to a Reynolds-averaged turbulence model. The SGS turbulence is self-generated and -sustained through the inhomogeneities of the mean fields. By this way, the feedback of the unresolved turbulence into the MHD reconnection process is taken into account. It is shown that the turbulence controls the regimes of reconnection by its characteristic timescale τt. The dependence on resistivity was investigated for large-Reynolds-number plasmas for Harris-type as well as force-free current sheets with guide field. We found that magnetic reconnection depends on the relation between the molecular and apparent effective turbulent resistivity. We found that the turbulence timescale τt decides whether fast reconnection takes place or whether the stored energy is just diffused away to small scale turbulence. If the amount of energy transferred from large to small scales is enhanced, fast reconnection can take place. Energy spectra allowed us to characterize the different regimes of reconnection. It was found that reconnection is even faster for larger Reynolds numbers controlled by the molecular resistivity η, as long as the initial level of turbulence is not too large. This implies that turbulence plays an important role to reach the limit of fast reconnection in large Reynolds number plasmas even for smaller amounts of turbulence.

Simulations of indoor airflow and particle dispersion and deposition by the lattice Boltzmann method using LES and RANS approaches

In this paper the Lattice Boltzmann method (LBM) in conjunction with the large eddy simulations (LES) and Reynolds Averaged Navier–Stokes (RANS) models was used to study the airflow and particle dispersion and deposition in indoor environments. The turbulent airflow was simulated first using the LES in framework of LBM, in which the sub-grid scale turbulence effects were simulated through a shear-improved Smagorinsky model (SISM). The airflow was also simulated using the k−ε turbulence model within the framework of LBM by the addition of two populations for the turbulence kinetic energy, k, and the dissipation rate, ε. The present simulation results for the airflow showed good agreement with the experimental data and the earlier numerical results.To simulate the particle dispersion and deposition in the room, particles with diameters of 10 nm–10 μm were investigated. For generating the instantaneous turbulence fluctuations for thek−ε model, the discrete random walk (DRW) method was used. Effects of SGS turbulence fluctuation on deposition rate of different size particles were also studied. It was shown that inclusion of the SGS turbulence fluctuations improves the model predictions. The simulated results for particle dispersion and deposition showed that the predictions of the present method are quite similar to the earlier finite volume method and are in good agreement with the experimental data.

Importance of detailed chemical kinetics on combustion and soot modelling of ventilated and under-ventilated fires in compartment

A novel in-house computation code based on large eddy simulations (LES) incorporating fully coupled subgrid-scale (SGS) turbulence, combustion, soot and radiation models for turbulent reacting flows in compartment fires has been developed. It uniquely embraces the detailed reaction mechanisms for the chemical processes involved during combustion, which provides a comprehensive description of the fuel oxidation processes. Furthermore, it gives a more complete representation of the generation of intermediate chemical by-products including toxic gases such as carbon monoxide and irritant gases such as soot. Since the modelling of hydrocarbons by-products are enabled when considering the full chemical profile, the formation of soot particles is correlated to the concentration of main soot incipient acetylene, which provides an appropriate representation of nucleation, surface growth processes. The LES code has been validated with experimental fire tests results for both ventilated and under-ventilated fires in compartment for confirmation of its robustness. The importance of incorporating the detailed reaction mechanisms in compartment fire simulations has been confirmed by comparing with experiments. For under-ventilated fires in compartment, the chemical kinetics become increasingly important since the combustion efficiency drops significantly involving generation of intermediate chemical species. It is discovered that species concentrations especially CO2 and CO are more accurately predicted by the detailed scheme comparing to the multi-step scheme, since the formation of hydrocarbons and nitrogen oxides are considered. In general, the simulation incorporating detailed kinetics showed an averaged improvement of 9.2% and 81.7% in the prediction of CO2/CO ratio and volume fraction of soot respectively. This also improves the replication of the flame structure as the fire is chemically-driven within the combustion zone.