Turbulent motion and mixing in a pipe

Turbulent mixing of a dye solution injected at the center of a pipe was studied by means of a new light probe developed for the measurement of concentration fluctuations were made both along the axial distance and across the pipe. The concentration fluctuations persisted longer at the center-line region than anywhere else; this suggested the use of the decay of intensity at the center as an upper limit. Approximately 99% of the initial intensity decayed within 7.5 ft. (30 L/d). The concentration spectra along the axial distance indicated that the eddies initially undergo a scalar energy cascade. Because of the huge difference in the kinematic viscosity and molecular diffusivities the concentration spectra extend much farther toward high wave numbers than the velocity spectrum. Apparently a state of imbalance resulted between the supply of small eddies and their dissipation; this was marked by the appearance of a small hump in the spectra. The phenomenological approximations of Corrsin and of Beek and Miller could be used for the practical problem of predicting the decay of the intensity of the concentration fluctuations or mixing in the pipe geometry studied.