External Intermittency Simulation in Turbulent Round Jets

Abstract to study passive scalar mixing and intermittency in turbulent round jets. Both simulation techniques are applied to the case of a low Reynolds number jet with Re between time-averaged results for the scalar field of the low Re case demonstrate reasonable agreement between the DNS and LES, and with experimental data and the predictions of other authors. Scalar probability density functions (pdfs) for this jet derived from the simulations are also in reasonable accord, although the DNS results demonstrate the more rapid influence of scalar intermittency with radial distance in the jet. This is reflected in derived intermittency profiles, with LES generally giving profiles that are too broad compared to equivalent DNS results, with too low a rate of decay with radial distance. In contrast, good agreement is in general found between LES predictions and experimental data for the mixing field, scalar pdfs and external intermittency in the high Reynolds number jet. Overall, the work described indicates that improved sub-grid scale modelling for use with LES may be beneficialDirect numerical and large eddy simulation (DNS and LES) are applied= 2,400, whilst LES is also used to predict a high Re = 68,000 flow. Compariso

Effects of swirl on intermittency characteristics in turbulent non-premixed flames

Swirl effects on velocity, mixture fraction and temperature intermittency have been analysed for turbulent methane flames using LES. The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky and the steady laminar flamelet models respectively. Probability density function (pdf) distributions demonstrate a Gaussian shape closer to the centreline region of the flame and a delta function at the far radial position. However, non-Gaussian pdfs are observed for velocity and mixture fraction on the centreline in a region where centre jet precession occurs. Non-Gaussian behaviour is also observed for the temperature pdfs close to the centreline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to non turbulent flow is more rapid for the velocity than the mixture fraction and therefore indicates how rapidly turbulence affects the molecular transport in these regions of the flame