Session Ca - Turbulent Jets & Wakes.
ORAL session, Sunday, November 23
301, Moscone Center
We use a two probe laser doppler system to simultaneously measure the local velocity fluctuations at spatially separated points in a falling soap film. Turbulence is produced by a comb passing through the film. The strong version of Taylor's hypothesis, that the fluctuating component of the velocity u' is unchanged as it is advected downstream by the mean velocity U_0, is checked by measuring the cross correlation between the probes as a function of the time delay \tau for several downstream separations y. Here u'/U_0 \simeq 0.14. We find that the maximum correlation occurs in accordance with Taylor's hypothesis at \tau = y/U_0, and decreases as e^-y. We calculate the probability density function and structure functions of the velocity difference \delta v(r) on a scale r from single point measurements using this hypothesis, comparing to direct spatial measurements as a test of the weak version, that statistical quantities are invariant under the substitution t = r/U_0.