Jets and fountains with reversing buoyancy occur in numerous natural and man-made situations (1) including, for example, oxygen jets directed into molten metal furnaces (2), the overshoot stage of a smokestack discharging into a stratified atmosphere (3), and underwater volcanic eruptions (4). For simplicity, we assume an upward directed jet or fountain with downward buoyancy as shown in Fig. 1, although the results apply equally to downward directed jets and fountains with upward buoyancy. The flow structure consists of a rising central core, surrounded by an annular downward flow (5,6), and under appropriate conditions includes mixing (7,8) and phase mingling (9,10). The primary parameter governing this flow structure is the Richardson number (11), which is the ratio of negative buoyancy to inertial forces
$Ri=Dg...$