Shear layers driven by turbulent plumes

A turbulent plume from a continuous source of buoyancy in a long tank is sh own to generate a series of quasi-steady counterflowing horizontal shear la yers throughout the tank. Both the horizontal flow velocity and the depth o f the shear layers are observed to decrease with distance above/below the p lume outflow. The shear layers are supported by the stable density stratifi cation produced by the plume and are superimposed on the vertical advection and entrainment inflow that make up the so-called 'filling box' circulatio n. Thus, at some depths, the surrounding water flows away from the plume in stead of being entrained, although we see no evidence of 'detrainment' of d ense plume water. Given the stratification produced by the plume at large t imes, the timescale for the velocity structure to adjust to changes in forc ing is proportional to the time for long internal gravity waves to travel t he length of the tank. The shear layers are interpreted in terms of interna l normal modes that are excited by, and which in turn determine, the horizo ntal plume outflow. The sixth and seventh baroclinic modes typically domina te because at the level of the plume outflow their phase speed is approxima tely equal and opposite to the vertical advection in the 'filling box'. Als o, the approximate balance between phase speed and advection is found to ho ld throughout the tank, resulting in the observed quasi-steady flow structu re. Viscosity causes the horizontal velocity in the shear layers to decreas e with distance above/below the plume outflow, and is thought to be respons ible for a low-frequency oscillation in the flow structure that is observed during experiments.