Mixing in isotropic turbulence with scalar injection and applications to subgrid modeling

A new technique for injecting scalar fluctuations in a DNS of isotropic tur bulence is presented. It is used to study statistically steady states assoc iated with different levels of mixing. The results are analysed in terms of spectra and PDF, and they are used as a data base to investigate the effec t of the filtering operation that is performed in LES. It is shown that the PDF of the scalar is substantially affected by the filtering operation. It is also shown that the Cook and Riley [1] subgrid model allows reconstruct ion of a PDF which is in fairly good agreement with the unfiltered DNS resu lts. The consequences of estimating the scalar subgrid variance by scale si milarity assumptions are investigated. It is found that the results are imp roved by a local determination of the model constant.