  The effects of passive scalar anisotropy on subgrid-scale (SGS) physics and
modeling for Large-Eddy Simulations are studied experimentally. Measurements
are performed across a moderate Reynolds number wake flow generated by a heated
cylinder, using an array of four X-wire and four cold-wire probes. By varying
the separation distance among probes in the array, we obtain filtered and
subgrid quantities at three different filter sizes. We compute several terms
that comprise the subgrid dissipation tensor of kinetic energy and
scalar-variance and test for isotropic behavior, as a function of filter scale.
We find that whereas the kinetic energy dissipation tensor tends towards
isotropy at small scales, the SGS scalar-variance dissipation remains
anisotropic independent of filter scale. The eddy-diffusion model predicts
isotropic behavior, whereas the nonlinear (or tensor eddy diffusivity) model
reproduces the correct trends, but overestimates the level of scalar
dissipation anisotropy. These results provide some support for so-called mixed
models but raise new questions about the causes of the observed anisotropy.
