Flow visualization of free convection in a vertical cylinder of water in the vicinity of the density maximum

Free convection in a vertical cylinder of water is studied in the vicinity of the density maximum at 4 °C. Results are presented both from cooling curve measurements (using a series of thermistors along the central axis of the cylinder) and from flow visualization experiments (using particle image velocimetry), and a comparison is made with simulations. The reversal of the overall flow direction as the temperature crosses the maximum density region gives rise to a plateau feature in the cooling curve, which may be explained by the formation of a rising toroidal structure along the column boundary. This feature effectively diverts flow away from the inner region of the cylinder, resulting in a temporary cessation of convective cooling within this region.