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Nonuniversal velocity probability densities in two-dimensional turbulence: The effect of large-scale dissipation
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Image of FIG. 1.
FIG. 1.

Snapshots of the vorticity from simulations using different models of . Upper panel is hypo-drag with ; middle panel is linear drag with ; and lower panel is quadratic drag with . Strong vortices are indicated by the larger scale used on the -axis in the top panel.

Image of FIG. 2.
FIG. 2.

Lagrangian trajectories of selected particles from the simulation shown in the middle panel of Fig. 1 (linear drag ). Three of the particles were released close to a vorticity extremum and exhibit “looping” trajectories, characteristic of trapping within a vortex. The other two particles were released in the filamentary sea between the vortices.

Image of FIG. 3.
FIG. 3.

Energy spectra for the three simulations shown in Fig. 1.

Image of FIG. 4.
FIG. 4.

Top panel: vorticity PDFs corresponding to the three simulations in Fig. 1. The abscissa is , and the dotted parabola is a standard Gaussian. Lower panel: velocity PDFs and the standard Gaussian (the dotted parabola). The abscissa is normalized with and . PDFs of and almost coincide and are close to the standard Gaussian for linear drag and quadratic drag.

Image of FIG. 5.
FIG. 5.

Vorticity (upper panel) and velocity (lower panel) PDFs from a sequence of simulations using linear drag, with decreasing from 0.1 to 0.002. The abscissa uses standardized variables etc. The velocity PDF remains close to the standard Gaussian (the smooth parabola) at all values of , while the vorticity PDF becomes increasingly non-Gaussian as is decreased.

Image of FIG. 6.
FIG. 6.

Snapshots of the velocity for hypo-drag (upper) and linear drag (lower), regions in which are inside the black solid bubbles.

Image of FIG. 7.
FIG. 7.

Distribution of vorticity extremum for hypo-drag , linear drag , and quadratic drag simulations. The solid straight line is a least-square-fit to the exponential.

Image of FIG. 8.
FIG. 8.

Insensitivity of velocity PDF obtained by the Gaussian vortex reconstruction Eq. (9) to the threshold for hypo-drag . Results are similar for linear and quadratic drag.

Image of FIG. 9.
FIG. 9.

Comparison of velocity PDF from simulations (circles) to that obtained by the Gaussian vortex reconstruction Eq. (9) (solid) for hypo-drag (upper) and linear drag (lower). Velocity PDFs obtained by GVR but with randomized vortex positions (dashed) and the exponential distribution in vorticity extremum replaced by constant (dot-dashed) are also plotted in the hypo-drag case.

Image of FIG. 10.
FIG. 10.

Probability distribution of vortex pair separation for hypo-drag and linear drag . Opposite-signed and like-signed vortex pairs are considered separately. The main figure focuses on small , and the inset shows for the full range of .

Image of FIG. 11.
FIG. 11.

The evolution of standardized vorticity PDF , standardized velocity PDF , energy spectrum , and velocity kurtosis when the value of in hypo-drag simulations is varied. corresponds to the simulation with linear drag . The dotted curve in the lower left panel is the standard Gaussian. In the lower right panel, the dashed line indicates the value of for linear drag ; the solid curve is the fit in Eq. (10).


Generic image for table
Table I.

A summary of the statistical properties of nine runs. All quantities are nondimensionalized using to scale length and to scale time. In the third column, is the rate of energy dissipation by large-scale drag (as opposed to hyperviscosity), and in the fourth column, .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Nonuniversal velocity probability densities in two-dimensional turbulence: The effect of large-scale dissipation