Turbulence spectra and length scales measured in film coolant flows emerging from discrete holes

To date, very little attention has been devoted to the scales and turbulence energy spectra of coolant exiting from film cooling holes. Length scale documentation and spectral measurements have primarily been concerned with the freestream flow with which the coolant interacts. Documentation of scales and energy decomposition of the coolant flow leads to more complete understanding of this important flow and the mechanisms by which it disperses and mixes with the freestream. CFD modeling of the emerging flow can use these data as verification that flow computations are accurate. To address this need, spectral measurements were taken with single-sensor, hot-wire anemometry at the exit plane of film cooling holes. Energy spectral distributions and length scales calculated from these distributions are presented for film cooling holes of different lengths and for coolant supply plenums of different geometries. Measurements are presented on the hole streamwise centerline at the center of the hole, one-half diameter upstream of center, and one-half diameter downstream of center. The data highlight some fundamental differences in energy content, dominant frequencies, and scales with changes in the hole and plenum geometries. Coolant flowing through long holes exhibits smoothly-distributed spectra as might be anticipated in fully-developed tube flows. Spectra from short-hole flows, however, show dominant frequencies.