Journal of Fluid Mechanics

Journal of Fluid Mechanics (1986), 170:139-168 Cambridge University Press
Copyright © 1986 Cambridge University Press

Experimental study of the two-dimensional inverse energy cascade in a square box

J.  Sommeria a1
a1 Gis Madylam, Institut de Mécanique de Grenoble, BP 68. No. 38402, Saint Martin D'Hères Cedex, France

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A quantitative experimental study of the two-dimensional inverse energy cascade is presented. The flow is electrically driven in a horizontal layer of mercury and three-dimensional perturbations are suppressed by means of a uniform magnetic field, so that the flow can be well approximated by a two-dimensional Navier–Stokes equation with a steady forcing term and a linear friction due to the Hartmann layer. Turbulence is produced by the instability of a periodic square network of 36 electrically driven alternating vortices. The inverse cascade is limited at large scales, either by the linear friction or by the finite size of the domain, depending on the experimental parameters. In the first case, $k^{-\frac{5}{3}$ spectra are measured and the corresponding two-dimensional Kolmogorov constant is in the range 3–7. In the second case, a condensation of the turbulent energy in the lowest mode, corresponding to a spontaneous mean global rotation, is observed. Such a condensation was predicted by Kraichnan (1967) from statistical thermodynamics arguments, but without the symmetry breaking. Random reversals of the rotation sense, owing to turbulent fluctuations, are more and more sparse as friction is decreased. The lowest mode fluctuations and the small scales are statistically independent.

(Published Online April 21 2006)
(Received April 3 1985)
(Revised February 14 1986)