Chapter 5. SPATIALLY-COHERENT STRUCTURES
In the laboratory experiments of Van Heijst and Flor (1989) and
Voropaev et al. (1991), a dipole was generated by horizontal, turbulent
injection of a small amount of fluid in a tank filled with a continuously
stratified fluid (Figure 5-3). Although the generated motion was initially
three-dimensional, vertical motions of the injected turbulent cloud are
rapidly suppressed by the surrounding stratification. As a result, the
turbulence rather quickly assumes a two-dimensional character. Next, the
effect of the inverse energy cascade increases the vortex scales, for instance,
by merging vortices (as demonstrated in Figure 5-4), ultimately resulting in
the appearance of an organized dipolar structure.
The interaction between separate vortices is an important part of the
process of self-organization in two-dimensional flows. A basic interaction
occurs between two monopolar vortices, either of equal or different size,
strength and polarity. Figure 5-4 illustrates merging of two identical
monopolar vortices. At a specific distance from each other, the vortices
merge to form a new vortex. At first the vortex is elliptic, but later it
becomes axisymmetric. The ‘strain’ caused by the neighboring vortex causes
the development of two characteristic spiral arms in both initial vortices,
consisting of thin vorticity filaments. Formation of spiral arms is also
observed in a single vortex placed in a non-uniform background flow, which
may be caused by the presence of remote vortices.
The spiral eddies observed in the laboratory experiments can be seen in
images of the ocean surface from manned space flights. Dr. Robert
Stevenson was the first to discover such spiral eddies on the ocean surface.
The spirals appear to be globally distributed, 10-25 km in size and
overwhelmingly cyclonic in the northern hemisphere and anticyclonic in the
southern hemisphere. Figure 5-5 shows a visual image of spiral structures
taken during a Space Shuttle mission. During this mission, the crew had
excellent viewing conditions in the Southern Hemisphere:
“…During the first two days of the mission, the crew commander
reported that the southern oceans were ‘essentially featureless’. By the third
day, the ocean had changed. The commander reported to Mission Control
that the entire southern Indian Ocean, Tasman Sea, and southwestern Pacific
Ocean were covered with spiral eddies and remained so for the rest of the
mission. The crew took overlapping pairs of photographs showing the spiral
eddy field in great detail. Streamlines of the flow into and between eddies
formed slicks on the sea surface. At first the features were supposed to be
the result of local wind action over the sea, but other data indicated that
eddies extended to depths of several tens of meters and thus were an integral
part of the dynamics of the upper ocean. Other data showed that the
streamline slicks did indeed flow into eddies at speeds greater than the
surrounding water.“ (Stevenson, 1998; 1999.)
293
In the laboratory experiments of Van Heijst and Flor (1989) and
Voropaev et al. (1991), a dipole was generated by horizontal, turbulent
injection of a small amount of fluid in a tank filled with a continuously
stratified fluid (Figure 5-3). Although the generated motion was initially
three-dimensional, vertical motions of the injected turbulent cloud are
rapidly suppressed by the surrounding stratification. As a result, the
turbulence rather quickly assumes a two-dimensional character. Next, the
effect of the inverse energy cascade increases the vortex scales, for instance,
by merging vortices (as demonstrated in Figure 5-4), ultimately resulting in
the appearance of an organized dipolar structure.
The interaction between separate vortices is an important part of the
process of self-organization in two-dimensional flows. A basic interaction
occurs between two monopolar vortices, either of equal or different size,
strength and polarity. Figure 5-4 illustrates merging of two identical
monopolar vortices. At a specific distance from each other, the vortices
merge to form a new vortex. At first the vortex is elliptic, but later it
becomes axisymmetric. The ‘strain’ caused by the neighboring vortex causes
the development of two characteristic spiral arms in both initial vortices,
consisting of thin vorticity filaments. Formation of spiral arms is also
observed in a single vortex placed in a non-uniform background flow, which
may be caused by the presence of remote vortices.
The spiral eddies observed in the laboratory experiments can be seen in
images of the ocean surface from manned space flights. Dr. Robert
Stevenson was the first to discover such spiral eddies on the ocean surface.
The spirals appear to be globally distributed, 10-25 km in size and
overwhelmingly cyclonic in the northern hemisphere and anticyclonic in the
southern hemisphere. Figure 5-5 shows a visual image of spiral structures
taken during a Space Shuttle mission. During this mission, the crew had
excellent viewing conditions in the Southern Hemisphere:
“…During the first two days of the mission, the crew commander
reported that the southern oceans were ‘essentially featureless’. By the third
day, the ocean had changed. The commander reported to Mission Control
that the entire southern Indian Ocean, Tasman Sea, and southwestern Pacific
Ocean were covered with spiral eddies and remained so for the rest of the
mission. The crew took overlapping pairs of photographs showing the spiral
eddy field in great detail. Streamlines of the flow into and between eddies
formed slicks on the sea surface. At first the features were supposed to be
the result of local wind action over the sea, but other data indicated that
eddies extended to depths of several tens of meters and thus were an integral
part of the dynamics of the upper ocean. Other data showed that the
streamline slicks did indeed flow into eddies at speeds greater than the
surrounding water.“ (Stevenson, 1998; 1999.)
293