Turbulent Flow Structure in the Similarity Region of Swirling Jet

Paper in proceeding, 2007

Several previous investigations of the near field of swirling jets have shown that these jets grow at a faster rate than non-swirling jets and experience significant changes in the turbulence quantities (c.f.\ Gilchrist, Naughton). A recent study by Shiri et al., however, showed that the growth rate enhancement does not persist in the far-field of a swirling jet flow with moderate swirl numbers (0.15 and 0.25). The results were shown to be consistent with the equilibrium similarity theory of Ewing in which the mean swirl velocity was argued to decrease downstream as $1/(x-x_o)^2$, while the mean stream-wise velocity decreased as $1/(x-x_o)$. In fact the only statistically significant effect of the swirl on the mean velocity for even the highest swirl number was a shift in the virtual origin (to $x/D_* = 0.75$ from $-2.9$). The present investigation extends the previous study to include all three velocity components of the turbulence quantities at a swirl number of $S=0.25$. Since only a two-component LDA was available, this was accomplished by making traverses in both the vertical and horizontal directions. All moments to third order were obtained, excepting those involving both the azimuthal and radial components simultaneously. Some of the results show the earlier measurements of Hussein et al. in the same jet (but without swirl). The second and third-order moments are quite close to the earlier non-swirling results. But unlike the mean velocity and spreading rate (which were nearly identical), the differences may be significant and a consequence of the swirl. As noted by George, if there were an effect of the source conditions on the similarity profiles, it is in the second and higher moment profiles where it would be expected to appear. On the other hand, the differences in second-order moments near the centerline could also be accounted for by a slight angular misalignment in either experiment ($<0.5\deg$).