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Effect of density stratification on vortex merger
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View: Figures


Image of FIG. 1.
FIG. 1.

Schematic of a two-dimensional initial vortex configuration, which will lead to merger. The arrows indicate the sense of vorticity, such that the two vortices initially describe an anti-clockwise revolution of period , where the subscript 0 stands for initial conditions, about the vorticity centroid “O.” The 2πΓ is the circulation of each vortex.

Image of FIG. 2.
FIG. 2.

Time evolution of vorticity (upper panel) and density (lower panel) contours for merger at higher levels of stratification than in Figure 4 with = 5000, = 1, and = 1. Significant generation of small scales can be observed. This figure is not to the same scale as the previous figure.

Image of FIG. 3.
FIG. 3.

Evolution of kinetic (.. - solid), potential (.. - dashed), and total ( - dashed-dotted lines) for = 5000 and = 1 for four different Froude numbers indicated with different colors. The kinetic and potential energies are always in anti-phase with each other.

Image of FIG. 4.
FIG. 4.

Time evolution of vorticity (color) and density (gray scale) contours for merger in the presence of density stratification with = 5000, = 2, and = 10.

Image of FIG. 5.
FIG. 5.

Effect of Prandtl number, shown in the legend, on the separation distance for a fixed = 1000 and different Froude numbers, (a) = 2 and (b) = 1.

Image of FIG. 6.
FIG. 6.

Variation of separation distance with time for = 5000 with a mean non-dimensional density gradient of = 0.0954. For the purely Boussinesq flow, gravity is chosen to give = 3. In the purely inertial case, gravity is neglected and baroclinic torque is generated only due to the nonlinear terms in the governing equation.

Image of FIG. 7.
FIG. 7.

Variation of separation distance with time for = 3000 by varying the Atwood number from 0.0106 to 0.0954 in nine equal steps of 0.0106. All the nine curves are clearly identical to each other.

Image of FIG. 8.
FIG. 8.

Non-Boussinesq effects on the time evolution of vorticity contours for merger in the presence of density stratification with = 5000, = 0.0954, and = 1. The black line shows the drift of the vorticity centroid.

Image of FIG. 9.
FIG. 9.

Trajectory of a single vortex with = 10 000 (a) no stratification, (b) with Boussinesq approximation at = 3, (c) with only inertial effects of stratification at = 0.0954. For visual clarity, the curves are shifted vertically from each other.

Image of FIG. 10.
FIG. 10.

Same as Figure 9 , but showing the -coordinate of both the vortices as a function of time. (a) Unstratified fluid, (b) Boussinesq fluid, (c) non-Boussinesq fluid.

Image of FIG. 11.
FIG. 11.

Drift of the vortex centroid as a function of Atwood number for four different Reynolds numbers with = 1.

Image of FIG. 12.
FIG. 12.

Variation of drift velocity as a function of Atwood number for four different Reynolds numbers with = 1.

Image of FIG. 13.
FIG. 13.

Baroclinic torque produced due to centrifugal effects and gravity. Shown here is the locus at a given time of an initially horizontal interface separating light and heavy fluid. The solid arrow represents the normal to this line at one point. The effect of gravity (centrifugal acceleration) produces the same (opposite) sign of vorticity along diametrically opposite points as shown in the figure. The sign of the vorticity produced depends on the orientation of the normal vector to the interface with respect to gravity and to the radial vector. The first in the pair of signs shown indicates the sense of torque produced by gravity effects, while the second shows the sign of torque production due to centrifugal effects. The small dashed circle shows the locus of the two point vortices.

Image of FIG. 14.
FIG. 14.

A schematic of the dominant vorticity due to gravity alone, based on Figure 13 . The two primary vortices are shown in solid circles, and marked “P,” and the “clubbed” baroclinic vortices are marked “B” and shown by open circles (not to any scale). In (a), the primary vortices are at such a phase with respect to the baroclinic vortices causing them to move away from each other. In (b), the net effect on the primary vortices is to push them towards each other leading to accelerated merger.

Image of FIG. 15.
FIG. 15.

Vorticity (lines) and density (gray scale) contours for = 5000, = 1 at time * = 0.5 for various : (a) = ∞, (b) = 3, (c) = 2, (d) = 1. Solid and dashed lines represent positive and negative vorticity levels. Note that in (a), the density field is a passive scalar.

Image of FIG. 16.
FIG. 16.

Same as Figure 14 with “B” now representing baroclinic vorticity generated from inertial effects of stratification. The baroclinic vortices act like a dipole imparting a push on the primary vortices. In both (a) and (b), the net effect on the primary vortices is to push them leftward.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Effect of density stratification on vortex merger