In:
Journal of Fluid Mechanics, Cambridge University Press (CUP), Vol. 388 ( 1999-06-10), p. 289-313
Abstract:
We present the results of an experimental investigation of the generation of coherent
vortical structures by buoyant line plumes in rotating fluids. Both uniform and stratified ambients are considered. By combining the scalings describing turbulent
plumes and geostrophically balanced vortices, we develop a simple model which predicts the scale of the coherent vortical structures in excellent accord with laboratory
experiments. We examine the motion induced by a constant buoyancy flux per unit length B ,
released for a finite time t s , from a source
of length L into a fluid rotating with angular
speed Ω = f /2. When the plume discharges into a uniformly stratified environment
characterized by a constant Brunt–Väisälä frequency, N 〉 f , the fluid rises to its level
of neutral buoyancy unaffected by the system rotation before intruding as a gravity current. Rotation has a strong impact on the subsequent dynamics: shear develops
across the spreading neutral cloud which eventually goes unstable, breaking into a chain of anticyclonic lenticular vortices. The number of vortices n emerging from the
instability of the neutral cloud, n = (0.65±0.1) Lf 1/2 /
( t 1/2 s B 1/3 ),
is independent of the ambient stratification, which serves only to prescribe the intrusion height and aspect
ratio of the resulting vortex structures. The experiments indicate that the Prandtl ratio characterizing the geostrophic vortices is given by P = Nh /( fR ) = 0.47±0.12; where h and R are, respectively, the half-height and radius of the vortices.
The lenticular vortices may merge soon after formation, but are generally stable and persist until they are spun-down by viscous effects. When the fluid is homogeneous, the plume fluid rises until it impinges on a free
surface. The nature of the flow depends critically on the relative magnitudes of the layer depth H and the rotational lengthscale L f = B 1/3 / f .
For H 〉 10 L f , the ascent
phase of the plume is influenced by the system rotation and the line plume breaks into a series of unstable anticylonic columns of characteristic radius
(5.3±1.0) B 1/3 / f which typically interact and lose their coherence before surfacing. When H 〈 10 L f , the
system rotation does not influence the plume ascent, but does control the spreading of the gravity current at the free surface. In a manner analogous to that observed in the
stratified ambient, shear develops across the surface current, which eventually becomes unstable and generates a series of anticyclonic surface eddies with characteristic radius
(1.6±0.2) B 1/3 t 1/3 s / f 2/3 . These surface eddies are significantly more stable than their
columnar counterparts, but less so than the lenticular eddies arising in the uniformly stratified ambient. The relevance of the study to the formation of coherent vortical structures by leads
in the polar ocean and hydrothermal venting is discussed.
Type of Medium:
Online Resource
ISSN:
0022-1120
,
1469-7645
DOI:
10.1017/S0022112099004759
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
1999
detail.hit.zdb_id:
1472346-3
detail.hit.zdb_id:
218334-1
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