Experimental study of incompressible jets with different initial swirl distributions: Mean results

Incompressible swirling jets with Reynolds numbers of 1.0 x 10(5) have been studied using a miniature five-hole probe. The jet facility in which these measurements have been made has been specifically designed to produce a highly conditioned, swirling jet flow. To provide a comprehensive investigation of the effect of low-to-moderate swirl (well below vortex breakdown) on jet growth rate, two tangential velocity profiles, a solid-body type and a q-vortex type with swirl numbers of both 0.10 and 0.23, have been investigated and compared with a nonswirling jet. The different tangential velocity distributions appear to have a minimal effect on the growth-rate enhancement observed. In contrast, the swirl number does affect growth rate enhancement (up to similar to 40% for the cases studied), but only when it exceeds a certain level. This result contradicts some earlier studies that assumed a linear increase in growth rate with swirl, but supports the results from newer studies. These newer results suggest three regimes for swirl-enhanced growth rates: 1) low swirl levels where no enhancement occurs, 2) moderate swirl levels where the enhancement scales with the swirl number, and 3) high swirl levels where vortex breakdown dominates the process.