NEW MEASUREMENTS ON SMALL-SCALE INTERMITTENCY IN FULLY-DEVELOPED TURBULENCE

Small scale intermittency in a turbulent system is studied with low te mperature helium gas as the working fluid. The focus is on measurement s of the skewness and flatness factors of the velocity derivatives. By measuring these quantities in a large domain of microscale Reynolds n umbers R(lambda) (from 150 to 3200), we find a transitional behavior a round R(lambda) = 700. The flatness first increases below this thresho ld, then decreases and finally saturates or increases again. We study the intense events that give rise to the tails of the probability dens ity function of the velocity derivatives. Remarkable structures are ob served and compared with those previously found in numerical studies ( worms). We finally study the dynamical role of such structures.