Experimental observation of the two-dimensional inverse energy cascade

We report the first clear experimental evidence of a stationary two-dimensional inverse energy cascade. The experiments are performed in electromagnetically driven flows, using thin, stably stratified layers; measurements of the instantaneous velocity fields show that the fluctuations are homogeneous and isotropic in the inertial range. The energy spectrum displays the expected k(-5/3) law with a Kolmogorov constant lying in the range 5.5-7, which is consistent with the current numerical estimates.