Natural versus forced convection in laminar starting plumes

TL;DR

This study investigates the scaling, velocity, and morphology of laminar starting plumes driven by buoyancy, revealing distinct head types and a power law relationship between ascent velocity and flow parameters across buoyancy and momentum regimes.

Contribution

It provides new insights into the transition between buoyancy-driven and momentum-driven laminar plumes, including head morphology and velocity scaling laws.

Findings

Ascent velocity follows a power law with Reynolds number scaled by Richardson number.

Two plume head types exist: confined (Ri > 1) and dispersed (Ri < 1).

Dispersed heads result from instabilities and overturning breakdown.

Abstract

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.