Natural versus forced convection in laminar starting plumes
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 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 flows was crossed, two distinct types of plume head morphologies exist: 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.
- Publication:
-
Physics of Fluids
- Pub Date:
- August 2009
- DOI:
- 10.1063/1.3207837
- arXiv:
- arXiv:0905.0494
- Bibcode:
- 2009PhFl...21h3601R
- Keywords:
-
- 47.55.P-;
- 47.15.Uv;
- 47.15.Cb;
- 47.15.Fe;
- Buoyancy-driven flows;
- convection;
- Laminar jets;
- Laminar boundary layers;
- Stability of laminar flows;
- Physics - Fluid Dynamics;
- Physics - Space Physics
- E-Print:
- 8 pages, 8 figures, accepted for publication in Physics of Fluids (final version with corrections)