The One-Dimensional-Turbulence Model

Near-wall LES closure based on one-dimensional turbulence modeling

ABSTRACTThe incompressible temporally developing turbulent boundary layer (TBL) is analysed using the map-based stochastic one-dimensional turbulence (ODT) model. The TBL is a canonical flow problem, which is, in the present study, formed by a planar moving wall and a free stream at rest. An understanding of this idealised flow is of fundamental relevance for the numerical analysis of turbulent boundary-layer-type flows. In the present ODT simulations, the flow variables are resolved on all scales along a wall-normal, one-dimensional domain. These variables are evolved by a deterministic and a stochastic process. The latter models the effect of turbulent advection and pressure fluctuations, whereas the former represents molecular diffusion. The model is appropriate for high Reynolds numbers for which the turbulence field exhibits a broad range of scales and is notionally featureless. We show that ODT is able to capture salient features of the TBL by comparing the various statistics with available reference direct numerical simulation (DNS) results for different bulk Reynolds numbers in the range using fixed model parameters. The influence of the model parameters is analysed for and optimal parameter values are provided. The results discussed in this paper suggest that ODT is an economical and reasonably accurate approach for the simulation of transient turbulent boundary-layer-type flows.

A novel large-eddy simulation (LES) approach is developed for the study of turbulent non-premixed flames. The approach is based on the embedding of 1D flamelet solutions on the flame surface based the one-dimensional turbulence (ODT) model. LES is used to evolve mass conservation, momentum and a flame capturing scalar, the mixture fraction; while, ODT is used to solve for the momentum and thermo-chemical scalars (species and energy). The two solutions are implemented in parallel and coupled to account for the effects of turbulent on the flame and the effects of heat release on the flow. The ODT model resolution is designed to capture important fine-grained unresolved physics on the LES sub-filter scales, including finite-rate chemistry effects and the sub-filter scale flame wrinkling.

One-dimensional turbulence model simulations of autoignition of hydrogen/carbon monoxide fuel mixtures in a turbulent jet

Abstract. Atmospheric boundary layers (ABLs) exhibit transient processes on various time scales that range from a few days down to seconds, with a scale separation of the large-scale forcing and the small-scale turbulent response. One of the standing challenges in modeling and simulation of ABLs is a physically based representation of complex multiscale boundary layer dynamics. In this study, an idealized time-dependent ABL, the so-called Ekman–Stokes boundary layer (ESBL), is considered as a simple model for the near-surface flow in the mid latitudes and polar regions. The ESBL is driven by a prescribed temporal modulation of the bulk–surface velocity difference. A stochastic one-dimensional turbulence (ODT) model is applied to the ESBL as standalone tool that aims to resolve all relevant scales of the flow along a representative vertical coordinate. It is demonstrated by comparison with reference data that ODT is able to capture relevant features of the time-dependent boundary layer flow. The model predicts a parametric enhancement of the bulk–surface coupling in the event of a boundary layer resonance when the flow is forced with the local Coriolis frequency. The latter reproduces leading order effects of the critical latitudes. The model results suggest that the bulk flow decouples from the surface for high forcing frequencies due to a relative increase in detached residual turbulence.

One-dimensional turbulence (ODT) offers an alternative single-column solver for wall-bounded turbulence that sits between traditional 1D boundary-layer parametrisations and fully 3D direct numerical simulation: in fully resolved mode, molecular transport is explicitly resolved along a 1D vertical domain, while turbulent advection is represented by instantaneous spatial mappings ("eddy events"). For high-Reynolds-number, wall-bounded flows relevant to the atmospheric boundary layer (ABL), resolving Kolmogorov scales is prohibitively expensive. This motivates careful implementation of wall models and subgrid scale models in ODT similar to large eddy simulations (LES).Here, we develop an ODT formulation operated in an LES-like mode, in which unresolved eddy events are represented by a Smagorinsky–Lilly subgrid-scale (SGS) model, and surface coupling is provided through standard surface parametrisations for an extended range of resolved scales. We assess the formulation on two benchmark problems: (i) canonical smooth turbulent channel flow, using an algebraic wall model to supply surface stress consistent with resolved inertial-sublayer dynamics; and (ii) the GABLS1 intercomparison case (a weakly stable, shear-driven ABL with prescribed surface cooling rate), using Monin-Obukhov similarity theory to compute surface momentum and heat fluxes.Together, these two cases demonstrate the feasibility of combining ODT's eddy-event transport with LES-style SGS and surface models, thus providing a computationally efficient platform for future studies of ABL regimes, in which turbulence, surface fluxes, entrainment across sharp inversions, and multi-phase physics interact and remain challenging for coarse-resolution weather and climate models.

A stochastic modeling approach, the so‐called One‐Dimensional Turbulence (ODT), is used to study passive scalar transport in incompressible, fully‐developed, turbulent channel flows up to very high Schmidt numbers Sc = ν/Γ ∼ 104 (kinematic viscosity ν, scalar diffusivity Γ). Good agreement is obtained between ODT and Direct Numerical Simulation (DNS) results with respect to the mean, the fluctuation statistics, and the dimensionless mass transfer coefficient K+. ODT yields the mass transfer coefficient to become independent of the Reynolds number at high Schmidt numbers, which is similar to reference laboratory measurements and DNS. The present ODT results exhibit the power law K+ ∝ Sc−0.65 with a scaling exponent that is only slightly larger than in the reference laboratory measurements and DNS. The results obtained suggests that ODT can be a versatile tool for turbulent transport modeling in a wide range of physical parameters. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved understanding of fire suppression mechanism with an idealized extinguishing agent

A framework for data-based turbulent combustion closure: A priori validation

Local reignition scenarios in H2/N2 turbulent diffusion flames are investigated using a one-dimensional turbulence (ODT) model and chemical explosive mode analysis (CEMA). Through analogy with the flame fronts in homogeneous ignition and laminar premixed flames, four reignition scenarios are distinguished and analyzed by CEMA. Results show that the reignition scenario via premixed flame propagation corresponds to a high-temperature explosion index and the reignition process is segmentally dominated by the reactions related to the consumption and production of the hydrogen radical. While reignition mode through an independent flamelet corresponds to a high radical explosion index, the whole reignition process is dominated by the production reaction of the hydrogen radical. When these two reignition processes are terminated by turbulent eddies, a hybrid reignition process or reignition scenario through turbulent flame folding may occur, and the turbulent folding mode corresponds to a high dissipation rate and obvious temperature jump near the end of the reignition process. In addition, statistical analysis shows that the premixed flame propagation mode is more effective to ignite the fluid parcel with a low temperature and the reignition scenario via an independent flamelet is a quicker process.

One-dimensional turbulence simulations and chemical explosive mode analysis for flame suppression mechanism of hydrogen/air flames

In this project, a new methodology was proposed to bridge the gap between Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). This novel methodology, titled Lattice-Based Multiscale Simulation (LBMS), creates a lattice structure of One-Dimensional Turbulence (ODT) models. This model has been shown to capture turbulent combustion with high fidelity by fully resolving interactions between turbulence and diffusion. By creating a lattice of ODT models, which are then coupled, LBMS overcomes the shortcomings of ODT, which are its inability to capture large scale three dimensional flow structures. However, by spacing these lattices significantly apart, LBMS can avoid the curse of dimensionality that creates untenable computational costs associated with DNS. This project has shown that LBMS is capable of reproducing statistics of isotropic turbulent flows while coarsening the spacing between lines significantly. It also investigates and resolves issues that arise when coupling ODT lines, such as flux reconstruction perpendicular to a given ODT line, preservation of conserved quantities when eddies cross a course cell volume and boundary condition application. Robust parallelization is also investigated.

In this work we study energy transport in a gliding arc discharge with two diverging flat electrodes in argon gas at atmospheric pressure. The discharge is ignited at the shortest electrode gap and it is pushed downstream by a forced gas flow. The current values considered are relatively low and therefore a non-equilibrium plasma is produced. We consider two cases, i.e. with high and low discharge current (28 mA and 2.8 mA), and a constant gas flow of 10 l min−1, with a significant turbulent component to the velocity. The study presents an analysis of the various energy transport mechanisms responsible for the redistribution of Joule heating to the plasma species and the moving background gas. The objective of this work is to provide a general understanding of the role of the different energy transport mechanisms in arc formation and sustainment, which can be used to improve existing or new discharge designs. The work is based on a three-dimensional numerical model, combining a fluid plasma model, the shear stress transport Reynolds averaged Navier–Stokes turbulent gas flow model, and a model for gas thermal balance. The obtained results show that at higher current the discharge is constricted within a thin plasma column several hundred kelvin above room temperature, while in the low-current discharge the combination of intense convective cooling and low Joule heating prevents discharge contraction and the plasma column evolves to a static non-moving diffusive plasma, continuously cooled by the flowing gas. As a result, the energy transport in the two cases is determined by different mechanisms. At higher current and a constricted plasma column, the plasma column is cooled mainly by turbulent transport, while at low current and an unconstricted plasma, the major cooling mechanism is energy transport due to non-turbulent gas convection. In general, the study also demonstrates the importance of turbulent energy transport in redistributing the Joule heating in the arc and its significant role in arc cooling and the formation of the gas temperature profile. In general, the turbulent energy transport lowers the average gas temperature in the arc, thus allowing additional control of thermal non-equilibrium in the discharge.

This work is a continuation of the Montréal stellar evolutionary model development (Richard et al., 2001 and references therein) where the effects of atomic diffusion are taken into account for 28 elements (see Turcotte, these proceedings), along with an approximate treatment of turbulent transport (Richer et al., 2000).We used the CEFF equation of state and the Bahcall nuclear energy generation routine. The Rosseland opacity is computed at each time step, from the OPAL monochromatic opacities for 21 elements (to which we added locally computed spectrum tables for Li, Be, and B) for each mesh point in the model for the current local chemical composition. Convection and semi-convection are taken into account in every species’ diffusion velocity and in the determination of the thermal gradient (Richard et al., 2001). The models also take into account gravitational settling, thermal diffusion, radiative accelerations and turbulent transport. The radiative accelerations are from Richer et al. (1998). The turbulent transport is modeled as a diffusion process (Richer et al., 2000) with diffusion coefficient: DT = ωD(He)0 (ρ/ρ0)n where D(He)0 is the Ηe atomic diffusion coefficient at density ρ0.

Off-lattice reconstruction of porous media: critical evaluation, geometrical confinement and molecular transport