Experimental Assessment of ‘subgrid’ scale Probability Density Function Models for Large Eddy Simulation

Filtered density functions (FDFs) of mixture fraction are quantified by analyzing
experimental data obtained from two-dimensional planar laser-induced fluorescence scalar
measurements in the isothermal swirling flow of a combustor operating at a Reynolds number of
28,662 for three different swirl numbers (0.3, 0.58 and 1.07). Two-dimensional filtering using a
box filter was performed on the measured scalar to obtain the filtered variables used for
presumed FDF for Large Eddy Simulations (LES). A dependant variable
from the measured scalar, which was a pre-computed temperature, was integrated over the
experimentally obtained FDF as well as over the presumed beta or top-hat FDFs and a relative
error in temperature prediction was calculated. The experimentally measured FDFs depended on
swirl numbers and axial and radial positions in the flow. The FDFs were unimodal in the regions
of low variance and bimodal in the regions of high variance. The influence of the filter spatial dimension on the measured FDF was evaluated and consequences for subgrid modeling for LES discussed.