Estimation of the Turbulent Schmidt Number from Experimental Profiles of Axial Velocity and Concentration for High-Reynolds-Number Jet Flows

Computational Fluid Dynamics (CFD) is increasingly being applied to model turbulent jet flows in practical applications ranging from gas turbine combustion to noise control and thrust augmentation. The proper applications of the CFD tool, however, requires, among other things, using "universal" modelling constants derived from well-calibrated experiments for a specific type of flow. This paper looks at one of the most heavily studied flows, the axisymmetric turbulent free jet, and determines the turbulent Schmidt number based empirically on previous experimental data. The velocity field measurements of Hussein et al. (1994) carefully taken using LDV for a free round jet were used together with the passive scalar measurements of Becker et al. (1967). The results agreed with earlier measurements by other investigators and showed that the turbulent Schmidt number increases monotonously from a value of 0.62 on the jet axis to 0.82 in the region where the intermittency factor is still one. The effect of changing the turbulent Schmidt number was also studied using a commercial laser Doppler velocimetry software and comparisons were made with the experimental results. A constant average value of 0.7 is recommended for use in CFD applications involving axisymmetric free-jet flows.