Experimental investigation on the turbulent near-field of coaxial jets

Abstract

Abstract The flow field of a coaxial jet configuration having inner to outer diameter ratio Di/Do≅0.5 is studied for two values of the velocity ratio, Ui/Uo=0.30 and Ui/Uo=0.67, both with a 5 mm thick and with a sharp inner duct wall. LDA and hot-wire measurements are used to obtain the statistical moments up to the fourth order and the time histories of the axial and radial velocity components. As expected, the inner jet core length is seen to decrease with decreasing velocity ratio, and for Ui/Uo=0.30 it is comparable to that of the outer jet, a condition that gives rise to large fluctuations and to a high level of mixing between the two streams. The sharpening of the inner duct wall produces a reduction in the radial fluctuations and Reynolds stresses in the near field, particularly for the case Ui/Uo=0.67, at which a regular vortex shedding occurs with the 5 mm thick duct; however, the differences in the velocity moments between the two configurations rapidly decrease, and become negligible after the end of both potential cores. The prevailing frequencies of the fluctuating flow field, estimated from wavelet spectra of the time histories of both velocity components, decrease regularly moving downstream from the jet outlet, and seem to confirm the dominance of the stronger vortices of the outer shear layer. Finally, the trends of the higher order moments may be interpreted in terms of plausible physical schemes of the mixing processes of the developing shear layers.