Turbulence and Combustion Group

Turbulent mixing models

In order to understand turbulent mixing, we require not only good physical models but also efficient numerical implementations of the same. In the current research, we are interested in developing a new near-neighbor implementation of the Interaction by exchange with the conditional mean (IECM) mixing model used in conjunction with PDF methods. This near-neighbor implementation is applicable to other local mixing models. In the particle method used to solve the modeled PDF equation, the near-neighbor implementation results in a particle's mixing with just one or two near neighbors (in the relevant space), and hence maximizes the localness of mixing.

Secondly, dispersion from line sources in decaying grid turbulence are interesting for they pose a significant challenge to statistical models, because the scalar length scale (of the initial plume) is much smaller than the turbulence integral scale. Consequently, this necessitates incorporating the effects of molecular diffusion in order to model the laboratory experiments. Past studies have shown that the mechanical to scalar timescale ratio can be taken to be a constant for most canonical flows. But, due to the wide separation of length scales in the initial stages of the plume development, the mechanical to scalar timescale ratio is dependent on time and a constant assumption leads to erroneous results. This work is partly also aimed at modeling the scalar timescale that is appropriate for all times in the development of the plume.

Figure 1 Centerline intensity of fluctuations i(0,t) against flight time from a single line source in grid turbulence. Symbols correspond to experimental data from Warhaft (1984) for various source conditions, black lines correspond to IECM model calculations of Sawford (2004) and colored lines correspond to present work. xo/M is the distance of the source from the turbulence generating grid and sigma_o is the size of the line source.

Selected publications
	S. Viswanathan and S.B. Pope (2008) ``Turbulent dispersion behind line sources in grid turbulenc'', Physics of Fluids , (submitted).