Session Ca - Turbulent Jets & Wakes.
ORAL session, Sunday, November 23
301, Moscone Center
We present an experimental study of the velocity fluctuations behind a comb held perpendicular to a falling soap film, a system which exhibits many aspects of two dimensional turbulence. The local velocity is measured at a single point behind the comb with a laser doppler velocimetry system. Using Taylor's hypothesis we obtain the velocity difference \delta v(r) on a scale r, and calculate the probability density function and its structure functions S_n(r) \equiv <(\delta v(r))^n>. A scaling region of the structure functions is observed for even n, consistent with an intermittent enstrophy cascade, whereas measurement of the third moment S_3(r) shows a completely different dependence. We discuss these measurements in terms of the exact calculation of the third moment obtained from the two dimensional Navier-Stokes equation.