Article: Rowinski DH, Pope S; (2013) An Investigation of Mixing in a Three-stream Turbulent Jet. Physics of Fluids, 25 (10)
Abstract: Mixing models are required to account for the effects of molecular diffusion in probability density function (PDF) methods for reacting flows, and these models entail one of the greatest sources of modeling uncertainty in PDF calculations. In this work, we perform PDF calculations of a turbulent coaxial jet using both Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) based methods. The configuration of the coaxial jet comes from a recently developed experiment, in which the conditional diffusion and scalar dissipation rate of two conserved scalars are measured. These measurements present the unique opportunity to compare directly the conditional diffusion implied by PDF mixing models to the measured conditional diffusion. In the RANS-PDF calculations, three classic mixing models are evaluated:
Interaction by Exchange with the Mean (IEM), Modified Curl (MC), and Euclidean Minimum Spanning Tree (EMST). Though calculations with all three mixing models yield time-averaged mean and RMS statistics of the two scalars in good agreement with the experimental measurements, the joint PDFs of the two scalars show a wide variability from the measurements. The EMST mixing model is unique in that it produces compositions which lie very close to the slow manifold identified in the experimental work. Next, LES calculations of the same flow are performed, and the dissipation rate and conditional diffusion are calculated. The resolved scalar dissipation rate is found to be in good agreement with the experimental data, but depends strongly on the resolution; the total dissipation rate from the RANS-PDF and LES calculations indicates significantly larger scalar dissipation rates than those measured experimentally. Finally, LES-PDF calculations of the same flow yield joint PDFs in very good agreement with the experimental data. The attenuation of variance production model is introduced and tested in this flow: this new model is found to yield calculations of improved accuracy in LES-PDF calculations on coarse grids. (C) 2013 AIP Publishing LLC.