Small-scale turbulent fluctuations beyond Taylor's frozen-flow hypothesis.

The space-time cross-correlation function C(T)(r,τ) of local temperature fluctuations in turbulent Rayleigh-Bénard convection is obtained from simultaneous two-point time series measurements. The obtained C(T)(r,τ) is found to have the scaling form C(T)(r(E),0) with r(E)=[(r-Uτ)(2)+V(2)τ(2)](1/2), where U and V are two characteristic velocities associated with the mean and rms velocities of the flow. The experiment verifies the theory and demonstrates its applications to a class of turbulent flows in which the requirement of Taylor's frozen flow hypothesis is not met.