Spatial characteristics and modelling of mixture fraction variance and scalar dissipation rate in steady turbulent round jets

Abstract

The spatial characteristics of scalar dissipation rate (SDR) are investigated in a steady-state turbulent round jet using Direct Numerical Simulation (DNS). Radial distribution of mixture fraction is found to agree well with the experimental data. Radial distributions of the turbulent diffusivities and the turbulent Schmidt numbers show different behaviours, based on how these quantities are defined. Point-wise statistics confirm that the log-normal distribution fits the probability distribution functions (PDFs) of the SDR well, except for a small positive skewness attributed to flow anisotropy. Joint PDFs of mixture fraction and SDR are symmetric with respect to mixture fraction. Self-similarity of the mean SDR, as well as its axial, radial and azimuthal components appears after a certain downstream distance. In the self-similar region, centreline SDR scales with downstream distance, x-4. Normalised SDR radial profiles show an increase near the centreline, followed by a steep decrease, after an off-centreline peak. Magnitudes of the axial, radial and azimuthal components of the mean SDR are approximately the same in the self-similar region. The budget analysis of the mean SDR transport equation reveals that the production term by scalar field stretch and the destruction term due to local curvature effects are the two dominant terms. A modification of an existing model for the self-similar mean SDR radial profiles is proposed. This is found to improve the prediction of mean SDR distributions.