Velocity fluctuations in a turbulent soap film: The third moment in two dimensions

Quasi-two-dimensional decaying turbulence is studied in a flowing soap film by measuring the moments of the probability density function P(delta upsil on(r)) for the longitudinal velocity differences delta upsilon(r) on a scal e r. As in three-dimensional (3-D) turbulence. P becomes non-Gaussian with decreasing r. The third moment S-3(r)=[(delta upsilon(r))(3)] is small and negative at small scales, but becomes positive at larger scales. The exact calculation of S-3(r) for 2-D homogeneous isotropic turbulence relates this change in sign to the development of the velocity correlation function as the turbulence decays. (C) 1999 American Institute of Physics. [S1070-6631( 99)01905-4].