Diffusion of heat as a passive contaminant in a slightly pulsating jet

The diffusion of a passive contaminant (heat) by a round jet subjected to small-amplitude periodic forcing at the frequency of the preferred mode is investigated experimentally using cold-wire anemometry. Within a distance of three nozzle diameters downstream, the low-amplitude pulsation forced on the jet produces temperature perturbations which grow in the shear layer, while the potential core remains perfectly isothermal. The instantaneous isotherm pattern shows the growth of coherent structures in the mixing layer which begin to lose their coherence beyond three nozzle diameters and are partially blurred by the developing turbulence. Nevertheless, the structures convected downstream have a strong influence on the temperature on the jet axis. The isotherms are slowly stretched and then rapidly compressed during a cycle. Hot fluid is ejected outwards from the axis on the leading edge of the structures, and cold fluid is trapped inwards on the trailing edge.