VELOCITY-FIELD OF AN AXISYMMETRICAL PULSED, SUBSONIC AIR-JET

Laser Doppler anemometer measurements in a fully pulsed, subsonic air jet with a significant no-flow period between pulses have been conducted and show much higher entrainment than steady or partially pulsed jets of the same mass flow. The mean centerline velocity decay is linearly related to the inverse of the effective distance from exit for some 50 diameters, but centerline velocity decay is much slower than for steady jets due to domination by the periodic component and its associated pressure field, which affects jet momentum. For larger distances, the decay changes to the steady jet rate. Reynolds stresses are considerably larger than for a steady jet and are considered to be responsible for the increased entrainment. Results are, except for a small increase in the constant of proportionality, consistent with Taylor's entrainment hypothesis. Phase averaged results through a cycle show the ratio of shear stress to turbulent kinetic energy to be in the range of 0.2-0.3 for the bulk of the flow.