The goal of this research is the development of Lagrangian stochastic models of acceleration in homogeneous turbulence (with and without mean deformations) that agree with the available data from DNS and experiments. We have identified a class of second-order Markovian stochastic models (conditional on a log-normal representation for the Lagrangian dissipation) that are exactly consistent with Gaussian (one-point, one-time) velocity statistics and conditionally Gaussian acceleration statistics (the Reynolds (2003) model belongs to this class). Preliminary calculations of two-time cross-correlations (below) using the Reynolds (2003) model and a cubic acceleration model (an easily identifiable nonlinear model in the class) show that the former has an unphysical cusp at the origin (as opposed to smooth behavior generated by the latter). DNS data are going to be used as a discriminator among different stochastic models in the class.
创建: Dec 15, 2017 | 07:27
A variety of modelling methods have been proposed to predict the complex, highly non-linear, interaction between turbulence and chemical reaction in both premixed and non-premixed flames. Among these, PDF methods have the advantages of representing convection and reaction exactly without modelling assumptions, and they have been demonstrated to be an effective approach for calculating turbulent reactive flows.
The PDF calculations performed on TNF workshop target flames (simple jet flames, piloted jet flames, bluff body flames) clearly demonstrate the ability of PDF method to account accurately for strong turbulent combustion interactions such as local extinction and re-ignition. In recent PDF calculations on turbulent lifted flames in a vitiated coflow, the numerical model is found to capture the most interesting feature of this particular lifted flame: the strong sensitivity of liftoff height to coflow temperature. More work is performed on piloted jet flames and bluff body flames to investigate the effect of detailed chemical mechanisms and mixing models.
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Recently, the Lagrangian investigations of local extinction, re-ignition and auto-ignition in piloted jet flame and lifted jet flames are performed. The processes of local extinction, re-ignition and auto-ignition are illustrated by the particle trajectories in composition space.
创建: Dec 15, 2017 | 07:26
