Measurements of Velocity and Turbulence in Vertical Axisymmetric Isothermal and Buoyant Jets

Abstract

The current study examines the transition region of axisymmetric isothermal and buoyant jets of low Reynolds number, directed vertically upward into a stagnant, unstratified ambient. The region in which measurements were obtained allows examination of two types of transition occurring in the jet: from nozzle exit dominated to fully developed, and from momentum to buoyancy-dominated flow. Isothermal velocity data were acquired using a two-channel laser-Doppler anemometer for Reynolds numbers ranging from 850 to 7405. The buoyant cases studied had Froude numbers ranging from 12 to 6425 and Reynolds numbers from 525 to 6500. In each case data were taken from 5 to 44 nozzle diameters downstream. Curve fit approximations of the data were developed by assuming polynomial similarity profiles for the measured quantities. Each profile was individually curve fit because in the transition region under consideration the flow field is not necessarily similar. Profile constants were then curve fit to determine profile variation as a function of nozzle exit parameters and downstream location. These allow prediction of the downstream velocity flow field and turbulent flow field as a function of the Reynolds number, Froude number, and density ratio at the nozzle exit. Profile width and entrainment increased at low Reynolds number. Axial and radial velocity fluctuations were found to increase at low Reynolds number. The buoyant cases studied were found to have lower velocity fluctuations and significantly lower Reynolds stresses than isothermal cases of similar Reynolds number.