Rate of Entropy Extraction in Compressible Turbulence.*

The rate of change of entropy is measured for a system of particles floating on the surface of a fluid maintained in a turbulent steady state. This rate of entropy ${\dot S}$ equals the time integral of the two point temporal velocity divergence correlation function with a negative prefactor. The measurements satisfactorily agree with the sum of Lyapunov exponents (Kolmogorov-Sinai entropy rate) measured from previous simulations, as expected of dynamical systems that are very chaotic (Sinai-Ruelle-Bowen statistics).