Fourth-order moments of longitudinal- and transverse-velocity structure functions

The relationship between fourth-order moments of δu1 and δu2, the longitudinal- and transverse-velocity increments, respectively, is examined in some detail in the context of local isotropy and internal intermittency. Measurements in a plane jet at a sufficiently large value of the turbulence Reynolds number indicate an inertial-range behaviour of the flatness factors of δu1 and δu2 that is in reasonable agreement with the lognormal hypothesis of Kolmogorov and the log-Poisson model. In the inertial range, the fourth-order structure function exhibits a moderately larger departure from local isotropy than the second-order structure function.