Power fluctuations in a closed turbulent shear flow

We study experimentally the power consumption P of a confined turbulent flo w at constant Reynolds number Re. We analyze in details its temporal dynami cs and statistical properties, in a setup that covers two decades in Reynol ds numbers. We show that nontrivial power fluctuations occur over a wide ra nge of amplitudes and that they involve coherent fluid motions over the ent ire system size. As a result, the power fluctuations do not result from ave raging of independent subsystems and its probability density function Pi(P) is strongly non-Gaussian. The shape of Pi(P) is Reynolds number independen t and we show that the relative intensity of fluctuations decreases very sl owly as Re increases. These results are discussed in terms of an analaogy w ith critical phenomena. [S1063-651X(99)50709-5].