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The numerical computation of turbulent flows

The standard κ-ϵ equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the κ-ϵ equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.

Bubbles, Drops, and Particles

A new k-ϵ eddy viscosity model for high reynolds number turbulent flows

Presentation d'une methode numerique par volume fini pour la resolution des equations de Navier-Stokes bidimensionnelles, incompressibles, et stationnaires, en coordonnees generales curvilignes Application de la methode aux ecoulements turbulents sur des profils avec et sans separation au bord de sortie posterieur Comparaison des calculs avec des donnees experimentales

Numerical Study of the Turbulent Flow Past an Airfoil with Trailing Edge Separation

An industrial gas turbine combustion chamber operating at a pressure of 3 bar is simulated using the sgs-pdf evolution equation approach in conjunction with the Eulerian stochastic field solution method in the context of Large Eddy Simulation A dynamic version of the Smagorinsky model is adopted for the sub-grid stresses and eight stochastic fields were utilised to characterise the influence of the sub-grid fluctuations The chemistry was represented by an ARM reduced GRI 30 mechanism with 15 reaction steps and 19 species The results show good agreement with the experimental data in the flame region at different axial locations The measured NO emission levels are also reproduced to a good accuracy by the method The results serve to demonstrate that simulations of complex combustion problems using detailed, but reduced, chemistry in industrial geometries are achievable

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Large Eddy Simulation of an industrial gas-turbine combustion chamber using the sub-grid PDF method

A spark ignited turbulent jet flame is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function ( pdf ) equation approach, which is solved using the Eulerian stochastic field method The spark energy deposition is mimicked by a gaussian distributed source term in the enthalpy equation A dynamic model for the sub-grid stresses together with a simple gradient diffusion approximation for the scalar fluxes is adopted The chemistry is represented by a global 4-step chemistry involving seven species The different stages of the ignition sequence, namely the flame kernel growth, the triple flame propagation against the flow and the stabilisation were in good agreement with the experimental data

LES-pdf simulation of a spark ignited turbulent methane jet

The problem of modeling the velocity and acceleration of inertial particles in turbulent flows is discussed. Particular attention is focused on the modeling of the particle Lagrangian velocity increment, especially, but not exclusively, in the case in which only the low frequencies of the carrier turbulent flow field are available. The need for suitable models arises in the simulation of particle laden flows by the means of new computational techniques such as large-eddy simulation. For this, stochastic differential equations, sde, are often introduced, though there is a lack of clarity in how such models should deal with the experimental observed far from Gaussian statistics, intermittency, and heavy tailed probability density function for particle acceleration. It is well known that Langevin-type equations are not capable of reproducing such features. It is first shown how the stochastic model for the particle Lagrangian velocity increments is far from being a Langevin equation, and it is characterized ...

Particle acceleration in turbulent flows: A class of nonlinear stochastic models for intermittency

Ignition in an aircraft gas turbine combustion chamber is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The LES-pdf methodology is used for both dispersed (liquid) and gas phases. The liquid phase is described using a Lagrangian formulation whilst an Eulerian approach is employed for the gas phase. The spark energy deposition was mimicked by a distributed energy source term added to the enthalpy equation. Unsuccessful and successful ignition sequences have been simulated and the results suggest that spark ‘size’ is an important parameter in the ignition of kerosene fuelled combustion chambers.

W.P. Jones

Papers

Large Eddy Simulation of an industrial gas-turbine combustion chamber using the sub-grid PDF method

LES-pdf simulation of a spark ignited turbulent methane jet

Particle acceleration in turbulent flows: A class of nonlinear stochastic models for intermittency

Large Eddy Simulation of Spark Ignition in a Gas Turbine Combustor

Pdf modeling of finite-rate chemistry effects in turbulent nonpremixed jet flames