Using photon correlation spectroscopy to study small-scale turbulence

The photon correlation spectroscopy technique was exploited to study turbulent pipe flow behind a grid. The correlation function of the scattered light intensity, g(t), was found to be a scaling function of qu(L)t(Lo/L)l when the Reynolds number Re > 460. Here q is the scattering vector, L,, the outer scale of the turbulent flow, and u(L) is the characteristic turbulent eddy velocity associated with eddies of size L. The exponent B is a function of Re. The measurements of the half-decay time of g(t) as a function of L and Re reveal an abrupt change in the character of the turbulent flow at Re = 460. A standard technique for studying laminar or turbulent flow is that of laser-Doppler velocimetry (LDV) [l, 21. The flowing fluid is seeded with small particles which scatter the light while following local flow. The Doppler shifted light is mixed with the incident beam in a square-law detector, usually a photomultiplier. The detected signal then contains a component of the Doppler shift, q * v(r, t), where v(r, t) is the