RANS Context Research Articles

Partially Averaged Navier–Stokes Formulation of a Two-Layer Turbulence Model

Partially averaged Navier–Stokes (PANS) is a scale-resolving simulation (SRS) approach wherein a two-equation closure model is suitably adapted for computing unresolved stresses. The objective of the present study is to develop a two-layer PANS closure for improved near-wall simulation capability. In the RANS context, the two-layer turbulence model displays superior near-wall behavior compared with other closures of similar order. Our goal is to adapt these near-wall closure advantages to PANS-SRS model. In the outer region of the PANS two-layer closure, a PANS turbulence model is employed. In the inner layer, an unresolved kinetic energy equation is solved. The eddy viscosity and dissipation are specified using algebraic equations based on known scaling relationships. The proposed model is evaluated in a turbulent channel flow of friction Reynolds numbers in the range of . Comparison with DNS data demonstrates that two-layer PANS is potentially an accurate and robust SRS approach.

Numerical analyses of a high pressure sooting flame with multiphysics approach

The development of a new standard for soot emissions proposed by ICAO-CAEP to reduce the environmental impact of civil aviation is moving increasingly research effort on the investigation of sooting flames. Formation and oxidation of the particulate matter are strongly affected by gas temperature, requiring an accurate prediction of the flow field from a numerical point of view. On the other hand, the temperature distribution within the combustor is modified by radiation, which depends on the soot concentration, leading to a very challenging coupled problem.In this work, a series of sensitivity analyses in RANS context are performed on soot, radiation and heat transfer modelling to assess their impact on the prediction of soot emission, gas temperature as well as wall heat fluxes distribution in the context of a high pressure sooting flame which is representative of a RQL combustor. These results are employed to set up a CHT (Conjugate Heat Transfer) simulation, using the multiphysics THERM3D procedure in a loosely-coupled manner where reactive CFD, radiation and heat conduction calculations are computed sequentially with a separate solver in a dedicated framework. These sensitivities can provide useful information for the numerical setup in high-fidelity simulations, as Scale Resolving Simulations.

A multiple mapping conditioning mixing model with a mixture-fraction like reference variable. Part 1: Model derivation and ideal flow test cases

Probability density function (PDF) methods have significant advantages in modeling turbulent combustion, in particular because the highly non-linear chemical source terms appear in closed form. The micromixing term in the PDF transport equations, representing diffusion in composition space, is however unclosed. Physically, micromixing occurs between regions of fluid having similar compositions. It is expected and generally shown that models that account for this localness of mixing perform better. In this work, a novel variation of the multiple mapping conditioning (MMC) mixing model is proposed. MMC makes use of reference variables to localise the mixing and was originally formulated for stochastic reference variables with standard Gaussian distributions. Here we follow the original interpretation but modify it by using a reference variable that has an evolving distribution, according to an Ornstein–Uhlenbeck process, with statistics that are similar to those of a physical scalar. In the present application of the model to nonpremixed combustion, mixture fraction is considered as the scalar; extensions of the model may be envisaged for other combustion modes by considering a different scalar variable. While the original version of MMC is mathematically elegant, it is conceptually complex and evaluation of model coefficients is difficult. The new model is both physically intuitive and its coefficients are easy to determine according to desired principles in canonical mixing situations. In Part I of this two-paper set, the model is studied analytically for homogeneous and mean scalar gradient flows without chemical reaction leading to an approach to set the model parameters that controls the unconditional scalar dissipation rate and delivers tunable localness. The behaviour of the model is also examined numerically in simple homogeneous mixing situations with and without chemical reaction. In Part II, the new MMC mixing model is implemented in the RANS context and validated against experimental data for the Sandia D–F flame series.

Direct Numerical Simulation of Complex Fuel Combustion with Detailed Chemistry: Physical Insight and Mean Reaction Rate Modeling

Direct numerical simulation (DNS) of freely-propagating premixed flames of a multi-component fuel is performed using a skeletal chemical mechanism with 49 reactions and 15 species. The fuel consists of CO, H2, H2O, CH4, and CO2 in proportions akin to blast furnace gas or a low calorific value syngas. The simulations include low and high turbulence levels to elucidate the effect of turbulence on realistic chemistry flames. The multi-component fuel flame is found to have a more complex structure than most common flames, with individual species’ reaction zones not necessarily overlapping with each other and with a wide heat releasing zone. The species mass fractions and heat release rate show significant scatter, with their conditional average however remaining close to the laminar flame result. Probability density functions of displacement speed, stretch rate, and curvature are near-Gaussian. Five different mean reaction rate closures are evaluated in the RANS context using these DNS data, presenting perhaps the most stringent test to date of the combustion models. Significant quantitative differences are observed in the performance of the models tested, especially for the higher turbulence level case.

Formulation Of A Two-phase Filtered DensityFunction Approach For Large Eddy Simulation

Two-phase velocity-scalar filtered density function (FDF) methods have recently been extended for large eddy simulation (LES) of separated two-phase flows [1]. Several terms in the two-phase FDF transport equation are unclosed and must be modeled for practical application of the two-phase FDF theory. Some terms, such as conditional dissipation in momentum, scalar, and momentum-scalar phase spaces are analogous to those in the single-phase velocity-scalar FDF formulation for which closures are readily available. Phase-coupling and interphase conversion terms, however, which are unique to the two-phase FDF formulation are unclosed and must b e modeled. There are, then, th ree d istin ct types o f unclo sed terms in th e two-phase FDF formulation: conditional dissipation terms, phase-coupling terms and interphase conversion terms. In this study, the mathematical formulation of this approach is presented and conditional dissipation, phase-coupling and interphase conversion terms are identified and their connection to standard LES closures identified . 1 Two-phase FDF transport equation Two-phase probability density function (PDF) methods have recently been formulated and successfully applied for dispersed/continuum phase flows [2, 3, 4]. Probability density function methods are based on defining a suitable set of flow realizations over which the PDF may be defined in the ensemble statistics context and therefore in practice are devoid of any spatial information about the two-phase flow. That is to say, turbulent flow field information in the PDF approach must be given in the time-averaged or RANS context, and consequently coupling to a RANS calculation for the carrier gas field is common [4, 2]. Filtered density func© 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 50, www.witpress.com, ISSN 1743-3533 (on-line) Computational Methods in Multiphase Flow III 275