The scalar spectrum in the viscous-convective subrange

Abstract

A modified design of a fibre optic light probe was used for the measurement of turbulent concentration fluctuations and the spectra associated with these. The main object was to establish experimentally the nature of the scalar spectrum in the viscous-convective subrange. The existence of a spectral region with a (-1)-power law form supports the uniform straining model proposed by Batchelor (1959) for Schmidt numbers (v/D) much greater than unity. For this range, the data do not agree with the cascading process suggested by Pao (1965).An additional object was to study further the decay of the concentration fluctuations in terms of a measure of the turbulent mixing. More specifically, the limitations of applying the isotropic stationary mixing theory of Corrsin (1957, 1964a) to a shear pipe flow situation and the validity of the earlier data for this case (Lee & Brodkey 1964; Brodkey 1966a, b; and Gegner & Brodkey 1966) were investigated.