J. Janicka

A digital filter based generation of inflow data for spatially developing direct numerical or large eddy simulations

In contrast to time-evolving turbulence, direct numerical or large eddy simulations of spatially inhomogeneous flows require turbulent inflow boundary conditions, that make the results strongly influenced by the velocity profiles to be prescribed. This paper aims to present a new approach for generating artificial velocity data which reproduces first and second order one point statistics as well as a locally given autocorrelation function. The method appears to be simple, flexible and more accurate than most of the existing methods. This is demonstrated in two cases. First, direct numerical simulations of planar turbulent jets in the Reynolds number range from 1000 to 6000 are performed. Because of the importance of the primary breakup mechanism of a liquid jet in which inflow influences are evident, the new procedure is secondly used, to study atomization in dependence of the flow inside the nozzle by means of a Volume of Fluid scheme.

Synthetic turbulence inflow conditions for large-eddy simulation

Direct numerical or large-eddy simulations of the majority of spatially inhomogeneous turbulent flows require turbulent inflow boundary conditions. A potential implication is that any results computed may be strongly influenced by the prescribed instantaneous inlet velocity profiles. Such profiles are practically never available, and a usual practice is to generate synthetic inflow data satisfying certain statistical properties, which may, for example, be known from experimental data or empirical correlations. The present paper describes a new method for generating turbulent inflow data based on digital filters that is capable of reproducing specified statistical data. Two variants of the approach are presented: a simple method in which the Reynolds stresses and a single length scale are prescribed, and a more detailed approach that is able to reproduce the complete Reynolds-stress tensor as well as any given, locally defined, spatial and temporal correlation functions. The application of the methods to a...

Investigation of the influence of the Reynolds number on a plane jet using direct numerical simulation

Abstract Free jets represent a benchmark for research into the physics of turbulent fluid flow and are furthermore of great interest for many engineering applications. In the present work we investigate the influence of the Reynolds number on the evolution of a plane jet. The effect on the global jet characteristics, as well as on some spectral properties, is particularly addressed. A strong influence on the jet evolution is found for Re⩽6000, but also that the jet is close to a converged state for higher Reynolds numbers. Although it is believed that the jet reaches a universal self-similar state, there was early evidence that the inflow conditions can have a downstream effect on the development of the turbulent flow field. Therefore some results, concerning the influence of inflow boundary conditions on the simulations, are also reported.

Prediction of turbulent jet diffusion flame lift-off using a PDF transport equation

A recently developed theory for the stability of turbulent diffusion flames is evaluated topredict lift-off of jet flames. A fundamental quantity of the theory is the scalar dissipation rate, conditioned at stoichiometric mixture. This quantity appears in the transport equation for the probability density function (pdf) of the conserved scalar. It can be evaluated on the basis of an a-priori assumption about the pdf. For two assumptions about the pdf the conditioned scalar dissipation rate in a turbulent round jet is calculated and compared to the one obtained under the assumption of statistical independence between the scalar and the scalar dissipation rate. Comparison with measured lift-off data for methane flames show a good agreement for small but discrepancies for large lift-off heights. The effects of turbulence inhomogenities on the results are discussed.

The probability density function of a passive scalar in turbulent shear flows

The transport equation for the probability density function of a scalar in turbulent shear flow is analyzed and the closure based on the gradient flux model for the turbulent flux and an integral model for the scalar dissipation term is put forward. The probability density function equation is complemented by a two‐equation turbulence model. Application to several shear flows proves the capability of the closure model to determine the probability density function of passive scalars.

The Calculation of Mean Radical Concentrations in Turbulent Diffusion Flames

Abstract A prediction model for turbulent diffusion flames is presented and applied to H 2 air-diffusion flames and in particular to the calculation of mean radical concentrations. The fast shuffle reactions in the H 2 air reaction system are assumed to be in partial equilibrium, whereas the three-body recombination reactions are treated kinetically. These assumptions lead to a two variable formalism which allows the calculation of nonequilibrium values of radicals. It is shown that these values differ considerably from calculations obtained with equilibrium assumptions. The results of the calculations compare favourably with measurements of mean OH concentrations by El-Dib [9].

A two-variables formalism for the treatment of chemical reactions in turbulent H2—Air diffusion flames

A prediction model for turbulent H2-air diffusion flames is developed by allowing three-body reactions in non-equilibrium. This leads to the definition of reaction variable and mixture fraction as variables describing the reacting mixtures instantaneously. A form for the two-dimensional pdf of these variables as a function of first nd second order moments is suggested. The model is complemented by the k−e model. It is applied to the calculation of mean field quantities including mitric oxide formation of the flames investigated by Kent and Bilger18 and Lavoire and Schlader.20

Gas compositional and pressure effects on thermographic phosphor thermometry

In the present study, the influence of gas compositional and pressure conditions on thermographic phosphor thermometry was investigated. A heatable pressurized and optical accessible calibration chamber was built to measure the phosphorescence decay time at different temperatures as well as at different partial and absolute pressures. At room temperature, the absolute pressure could be increased to 30 bar. To vary the gas composition, nitrogen, oxygen, carbon dioxide, methane, helium as well as water vapour were used. Three different phosphors were investigated: Mg4FGeO6:Mn, La2O2S:Eu and Y2O3:Eu. Phosphorescence was excited by the third and the fourth harmonics of a pulsed Nd:YAG-laser (355 nm and 266 nm, respectively) and recorded temporally resolved by a photomultiplier. Mg4FGeO6:Mn as well as La2O2S:Eu were not influenced significantly by varying partial and absolute pressures. In contrast, Y2O3:Eu showed a strong sensitivity on the oxygen concentration of the surrounding gas phase as well as irreversible changes in the phosphorescence decay time after increasing the absolute pressure.

A detailed investigation of the stabilization point of lifted turbulent diffusion flames

The stabilization point of lifted turbulent hydrogen diffusion flames is investigated by Raman/Rayleigh/laser-induced fluorescence (LIF) spectroscopy. The stabilization point is determined from simultaneously taken planar laser-induced fluorescence (PLIF) images. It is shown from averaged statistics that lift-off height has negligible influence on the flame length and the far region of the jet. Reactants, premixed downstream of the stabilization point, are rapidly consumed over a very short distance. A new method to generate stabilization point conditioned species and temperature data is proposed and applied to the data. With this method it is possible to describe the surrounding of an observer located at the instantaneous stabilization point. The data are presented by constant contour plots of mixture fraction, species, and temperature in a stabilization point fixed coordinate system. The data obtained by this method are used to assess previously proposed theories on the behavior of lifted turbulent diffusion flames. Experimental findings presented are inconsistent with predictions by the concept of premixed flame stabilization as well as with the flamelet concept. The insensitivity of the spatial location of the stabilization point to the variation of the stoichiometric mixture fraction of the fuels investigated suggests a stabilization mechanism through large-scale turbulent structures. Large-scale structures also explain the existence of products upstream of the stabilization point. The conclusion of this analysis is that large-scale turbulent structures play a dominant role in the stabilization mechanisms of the lifted turbulent diffusion flames, subject to this study.

LARGE EDDY SIMULATION OF A TURBULENT HYDROGEN DIFFUSION FLAME

In this work a large-eddy simulation (LES) of a turbulent hydrogen jet diffusion flame is presented. The numerical method handles fluctuations of density in space and in time, but assumes density to be independent of pressure (incompressibility). The chemical composition of the fluid is described by solving the filtered transport equation for mixture fraction f. Density, viscosity and temperature are evaluated assuming chemical equilibrium. To account for sub-grid fluctuations of f, its sub-grid distribution is presumed to have the shape of a β-function.The results of the simulation are discussed extensively. The influence of inlet boundary conditions is addressed and radial profiles at different axial positions are shown for a complete set of one-point statistical data. Agreement of numerical results and experimental data is very good. Furthermore, a comparison of Reynolds- and Favre-averages is done and energy spectra at different locations in the flame are discussed.