Convection in a long vertical tube due to unstable stratification - a new type of turbulent flow?

Abstract

We present experimental results of free convection in a vertical tube due to an unstable density difference imposed between the two (open) ends of the tube. Two tanks of fluids connect the two ends of the tube with the top-tank fluid heavier than the bottom-tank fluid. We use salt mixed with water to create the density difference. The convection in the tube is in the form of relatively heavier fluid going down and lighter fluid going up simultaneously; the mean flow at any cross section of the tube is zero. Depending on the Rayleigh number we observe different types of flow, with turbulent flow being observed at the higher Rayleigh numbers. We believe this is a new type of turbulent flow - a nearly homogeneous, buoyancy-driven flow with zero mean shear.