Using photon correlation spectroscopy to study small-scale turbulence

The small-scale grid-generated turbulence at moderate Reynolds number in pipe flow through a grid is studied here using the rarely exploited technique of photon correlation spectroscopy. Measurements of this type give access to the probability density P(V/R/) that a pair of particles in the fluid, having separation R, differ in velocity by V(R,t). The intensity correlation function exhibits a certain behavior only if Re is larger than 460. All the measurements suggest that the flow changes its character at this point. (C.D.)