An experimental study of the far-field of incompressible swirling jets

The far field of an incompressible swirling jet has been studied using two-dimensional laser Doppler anemometry. Three pairs of symmetric injectors were used to produce weak-to-moderate swirling jets. Velocity profiles of the mean and fluctuating stream-wise and azimuthal velocity components were measured in jets with two levels of swirl (S = 0.15 and 0.25) at axial locations up to 50 jet exit diameters. The velocity and turbulence intensity profiles, centerline decay, and growth rates for the various swirling jets are compared to those obtained in the same facility without swirl (S = 0). Like the previous observations for the near jet,^1,2 there was no observable effect on the properly scaled far jet for the S = 0.15 case. The results were virtually identical to the non-swirling jet. For the S = 0.25 case, the only statistically significant effect was a shift in the virtual origin (from x/D* = 0.75 to -2.9). The recent predictions of equilibrium similarity theory³ were found to be in excellent agreement with the experimental results. In particular, the mean azimuthal component of velocity falls off as the inverse square of the downstream distance. By contrast, the mean stream-wise velocity and turbulence intensities fall off with the inverse of the downstream distance. As a consequence, the mean azimuthal equation uncouples from the rest, so the asymptotic swirling jet behaves like the non-swirling jet.