Relative velocity fluctuations in turbulent Rayleigh-Bénard convection

High-Rayleigh-number turbulent convection has been studied using the technique of photon-correlation homodyne spectroscopy to measure velocity differences at various length scales. The measured intensity correlation function G(q,t,L) is found to be of the scaling form G(qu(L)t), with u(L){~}L0.6 being the characteristic turbulent velocity at the legth scale L and q the scattering vector. It is also found that the lower cutoff length lc of the scaling range obeys a power law lc{~}Ra^-0.3. The measured power-law exponents are in excellent agreement with theoretical predictions.