Lagrangian description of nonpremixed passive scalars in differential diffusion: direct numerical simulation.
Abstract
We study differential diffusion in isotropic turbulence in a Lagrangian frame, for two scalars initially segregated in space and evolving with different molecular diffusivities. This mimics, for example, the mixing of multiple chemical species which may be the rate-controlling factor in nonpremixed combustion problems. Direct numerical simulations are conducted for scalars of Schmidt numbers 1/8 and 1 at different Reynolds numbers. As mixing proceeds the scalar fluctuations decrease in magnitude while their probability density function (PDF) evolves from a bimodal (``double-delta'') form towards a quasi-Gaussian state. Although this process is non-stationary, it is found that the scalar fluctuations are quasi-stationary if normalized by an instantaneous r.m.s value. Lagrangian statistics computed for normalized scalar and scalar dissipation fluctuations show many qualitative similarities to those of stationary scalar fields with a uniform mean gradient (Yeung 2000, J. Fluid Mech., in press). Quantitative differences include longer Lagrangian time scales for the scalars and their differential diffusion variable, and a lesser degree of intermittency for the normalized scalar dissipation. These results are useful for tests of Lagrangian stochastic modeling.
- Publication:
-
APS Division of Fluid Dynamics Meeting Abstracts
- Pub Date:
- November 2000
- Bibcode:
- 2000APS..DFD.DB004Y