Turbulent Prandtl Number in Neutrally Buoyant Turbulent Round Jet

A method which combines two nonintrusive imaging techniques, particle tracking velocimetry and laser induced fluorescence, was used to make simultaneous measurements of velocity and concentration in a neutrally buoyant turbulent round jet. The measurements were made at two different Reynolds numbers (R), 360 and 4,210, at a Schmidt number of 1,930. The mean velocity 〈u〉, mean concentration 〈c〉, Reynolds stress $-\langle u^{\prime }{v}^{\prime }\rangle \text{,}$ and turbulent scalar flux $\langle v^{\prime }{c}^{\prime }\rangle$ were obtained and the eddy viscosity, eddy diffusivity, and turbulent Prandtl number $\left({\mathrm{Pr}}_t\right)$ calculated from these measurements. Both the low and high Reynolds number results show self-similar characteristics that are dependent on R with ${\mathrm{Pr}}_t$ a function of radial position. For the R=4,210 case, it was found that $0.7<{\mathrm{Pr}}_t<0.9$ for $0<y/x<0.15$ (y is the radial coordinate and x the streamwise coordinate) with ${\mathrm{Pr}}_t$ decreasing monotonically for $y/x>0.12\text{.}$ For the R=360 case, it was found that ${\mathrm{Pr}}_t\approx 0.4$ for $0.06<y/x<0.15$ but significantly smaller at the center (approximately half of the peak value) and outer regions of the jet profile. Comparatively, for the R=4,210 case, ${\mathrm{Pr}}_t$ at $y/x=0$ is only slightly smaller than the peak value (0.7 versus 0.9). In both R cases, the maximum of ${\mathrm{Pr}}_t$ occurred at approximately $y/x=0.1\text{.}$