Potential core lengths of round jets in stagnant and moving environments

Abstract

This paper investigates the change of velocity and concentration fields in the initial region of a round jet in stagnant fluid and in a moving environment of the co-flow, counter-flow or cross-flow situation. The aim was to discuss issues of flow establishment and in the initial region, to determine the potential core length, and to observe the effect of a moving ambient. Turbulent jet velocities and concentration were measured with the planar imaging techniques of particle-image velocimetry (PIV) and laser-induced fluorescence (LIF). The mean flow fields were obtained and analyzed to investigate the validity of 1/. x decay relationship for jet centerline velocity and concentration. Values for the decay constant and jet virtual origin were obtained from the data. While the results show that the decay constant is increased by a co-flow but reduced by a counter-flow or a cross-flow, the virtual origin was found to be affected as well. The overall effect is that any situation of the moving environment leads to a slight reduction of the physical length of the potential core. The paper also suggests an intermittency function for the analysis of fluctuating jet concentration field in the potential core. The mean intermittency function provides a direct and reliable estimate of the potential core length of a jet in a moving environment. © 2011 International Association of Hydro-environment Engineering and Research, Asia Pacific Division.