EXPERIMENTAL INVESTIGATION ON THE TURBULENT NEAR-FIELD OF COAXIAL JETS

The flow field of a coaxial jet configuration having inner to outer di ameter ratio D-i/D-o congruent to 0.5 is studied for two values of the velocity ratio, U-i/U-o = 0.30 and U-i/U-o = 0.67, both with a 5 mm t hick and with a sharp inner duct wall. LDA and hot-wire measurements a re used to obtain the statistical moments up to the fourth order and t he time histories of the axial and radial velocity components. As expe cted, the inner jet core length is seen to decrease with decreasing ve locity ratio, and for U-i/U-o = 0.30 it is comparable to that of the o uter jet, a condition that gives rise to large fluctuations and to a h igh level of mixing between the two streams. The sharpening of the inn er duct wall produces a reduction in the radial fluctuations and Reyno lds stresses in the near field, particularly for the case U-i/U-o = 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 con figurations rapidly decrease, and become negligible after the end of b oth potential cores. The prevailing frequencies of the fluctuating flo w 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 moment s may be interpreted in terms of plausible physical schemes of the mix ing processes of the developing shear layers. (C) 1998 Elsevier Scienc e Inc. All rights reserved.