Dynamic Interaction and Mixing of Two Turbulent Forced Plumes in Linearly Stratified Ambience

Abstract

The hydrodynamics of a two-vent turbulent forced plume propagating in a linear stratification has been studied by a series of laboratory experiments using the two-tank method and the particle image velocimetry (PIV) technique. The velocity fields and turbulence characteristics of a two-vent plume under different source buoyancy flux, ambient stratification, and nozzle distance are analyzed quantitatively. The increase of maximum penetration of a two-vent plume is mainly due to the attenuation of buoyancy gradient caused by less entrainment of ambient fluid in the region between the two vents. The energy spectra do not exhibit a sizable range of the Kolmogorov −5/3 slope, indicating that no substantial inertial subrange is present in this small-scale plume in a linear stratification generated in the laboratory. Both turbulent kinetic energy and dissipation increase with the decrease of distance between the two plumes, indicating that the interaction, entrainment, and mixing in a two-vent plume stem greatly enhance energy production and dissipation. The maximum turbulent viscosity in a two-vent plume, however, presents a three-stage variation. An increase of viscosity in the second stage appears in the mixing region of the neutral buoyancy layer between the two plumes when the normalized distance L/Zmax is in the range of 0.3–0.6; this increase is attributed to flow rebound after overshooting and a strong entrainment and mixing in this region due to lateral flow convection. A new semiempirical formula for the maximum penetration and new scaling relationships for the maximum turbulent viscosity of a two-vent plume are proposed and verified by the PIV experimental data.