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

We present experimental results of free convection in a vertical tube due t o 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-ta nk fluid heavier than the bottom-tank fluid. We use salt mixed with water t o create the density difference. The convection in the tube is in the form of relatively heavier fluid going down and lighter fluid going up simultane ously; 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 flo w being observed at the higher Rayleigh numbers. We believe this is a new t ype of turbulent flow-a nearly homogeneous, buoyancy-driven flow with zero mean shear.