CONDITIONAL STATISTICS OF VELOCITY FLUCTUATIONS IN TURBULENCE

Using experimental data recorded in a low temperature helium jet, we h ave studied the statistics of velocity increments: upsilon(r)(x) = ups ilon(x+r)-upsilon(x) conditioned on a ''rate of energy transfer'' anza tz, e(r):P(upsilon(r)/e(r)) For a fixed value of e(r), the histograms of upsilon(r) are found Gaussian at all scale, i.e. there is no interm ittency at fixed e(r). Intermittency is caused by the fluctuations of the latter quantity. If P(upsilon(r)/e(r)) is Gaussian, it is characte rized uniquely by its variance sigma(2) = [upsilon(r)(2)\e(r)] - [upsi lon(r)/e(r)](2) and mean upsilon(0) = [upsilon(r)\e(r)]. We show that sigma is related to e(r) by a power law, valid at any scale, and that upsilon(0) is close to logarithmic in e(r) in the inertial range. With these two relationships, the statistics of upsilon(r) at fixed e(r) a re completely determined by e(r). Therefore, the relevant quantity to describe intermittency is the transfer rate of energy, acting as a dri ving process for the velocity fluctuations.