Assessing the Local Isotropy of a Turbulent Plume in Linearly Stratified Saltwater using PIV Data

The local isotropy of turbulent plume flow in linearly stratified saltwater was examined by using the particle image velocimetry (PIV) technique. Linear stratification was produced in a 50×50×50 cm plexiglas tank by using the two-tank method. The plume was created by injecting dense fluid downward near the top of the stratified column. Images of seeding particles in the flow were recorded at 100 Hz. For statistically steady plume flow, instantaneous two-dimensional (2D) velocity vector fields were calculated by using the standard cross-correlation approach. The PIV flow data were then used to calculate local entrainment coefficient, turbulent energy spectra, and local isotropy. It is shown that the mean vertical velocity component fits the Gaussian distribution well. Local entrainment coefficient remains approximately constant (~0.12) until ~0.6Zmax but decreases above that height. The Kolmogorov -5/3 spectral slope is evident in the one-dimensional spatial spectra, indicating the existence of the inertial subrange. Values of urms/wrms in most chosen regions are larger than 0.7, suggesting good local isotropy in the plume turbulence. The present experiments show that buoyant plumes in a linearly stratified environment are isotropic and intensely turbulent even at low velocity. Themes: Experimental techniques; Oceanography; Turbulence