TURBULENCE DECAY IN LIQUID JETS

A study has been made of the flow patterns and turbulence characteristics in free liquid jets in order to determine the rate of decay of turbulence properties along the jet. Mean streamwise velocities and streamwise and cross-streamwise turbulence intensities were measured using laser Doppler velocimetry. The jet Reynolds number was varied between 1000 and 30000, with the diameter of the liquid jet D = 3.051 mm. Using a power law model for the time decay of turbulent kinetic energy, it was found that turbulence decays, on average with an exponent N=1, independent of the Reynolds number. A constant power for the decay implies Reynolds number similarity throughout this range. Substantial reductions in the degree of anisotropy occur in the downstream direction from the injector exit as the jet relaxes from fully developed turbulent pipe flow profile to a flat profile. For the intermediate range of Reynolds numbers (10000-20000), the relaxation distance was 20D, almost independent of the Reynolds number. At high values of Reynolds number (20000-30000), the relaxation process was very fast, generally within 3 diameters from the injector exit.