Vertical Density Currents-ii

Examples of vertical density currents wholly within the domain of laminar flow, one in a water solution, the other in air, have come to my attention. Both examples illustrate new ways of introducing and dispersing microscopic particles into static fluids and both demonstrate that a stable, clearly defined layer of dispersed particles forms first and that the vertical density currents originate and flow from the lower part of this layer. The new information comes from wholly unrelated lines of research, one in virology, and the other in mycology. Neither investigation was aimed at hydrodynamics yet both provide good experimental support for vertical density currents. In an earlier paper ( Bradley 1965)) I showed by experiments that, given a supply of particles dispersed in the upper part of a body of fluid, a vertical density current will form and flow downward if the effective diameters of the particles are appreciably less than 0.14 mm and when the space between the particles reaches some critical minimum. Such vertical density currents, in quiet fluid, flow in stream tubes that have a circular cross section and a diameter d, that varies inversely as the difference in specific gravity of the density current Ss, and the surrounding fluid F, of density p.