  A starting plume or jet has a well-defined, evolving head that is driven
through the surrounding quiescent fluid by a localized flux of either buoyancy
or momentum, or both. We studied the scaling and morphology of starting plumes
produced by a constant flux of buoyant fluid from a small, submerged outlet.
The plumes were laminar and spanned a wide range of plume Richardson numbers
Ri. Ri is the dimensionless ratio of the buoyancy forces to inertial effects,
and is thus our measurements crossed over the transition between
buoyancy-driven plumes and momentum-driven jets. We found that the ascent
velocity of the plume, nondimensionalized by Ri, exhibits a power law
relationship with Re, the Reynolds number of the injected fluid in the outlet
pipe. We also found that as the threshold between buoyancy-driven and
momentum-driven flow was crossed, two distinct types of plume head mophologies
existed: confined heads, produced in the Ri > 1 regime, and dispersed heads,
which are found in the Ri < 1 regime. Head dispersal is caused by a breakdown
of overturning motion in the head, and a local Kelvin-Helmholtz instability on
the exterior of the plume.
