PLIF and PIV measurements of the self-preserving structure of steady round buoyant turbulent plumes in crossflow

Abstract

Measurements of the mean concentration of source fluid and mean velocity fields were obtained for the first time in the self-preserving region of steady round buoyant turbulent plumes in uniform crossflows using Planar-Laser-Induced-Fluorescence (PLIF) and Particle-Image-Velocimetry (PIV), respectively. The experiments involved salt water sources injected into water/ethanol crossflows within a water channel. Matching the index of refraction of the source and ambient fluids was required in order to avoid image distortion and laser intensity nonuniformities. Further experimental methods and procedures are explained in detail. The self-preserving structure properties of the flow were correlated successfully based on the scaling analysis of [Fischer, H.B., List, E.J., Koh, R.C., Imberger, J., Brooks, N.H., 1979. Mixing in Inland and Coastal Waters, Academic Press, New York, pp. 315–389]. The resulting self-preserving structure consisted of two counter-rotating vortices having their axes nearly aligned with the crossflow direction that move away from the source in the streamwise (vertical) direction due to the action of buoyancy. This alignment, was a strong function of the source/crossflow velocity ratio, u 0/ v ∞. Finally, the counter-rotating vortex system was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to axisymmetric round buoyant turbulent plumes in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex system were 2–3 times larger than the transverse dimensions of self-preserving axisymmetric plumes at similar streamwise distances from the source.