Swirl-induced intermittency: A novel effect modifying the turbulence structure of swirling free jets

Swirling jets are used in many combustion devices, and so their investigation, is of particular interest both for fundamentally oriented research for design purposes. In the first part of the paper we present a detailed data base for isothermal swirling free jets in stagnant surroundings with vortex breakdown. The measurements were conducted with the quintuple-hotwire technique and comprise three mean velocities and the complete Reynolds stress tensor. The data obtained are best suited for validation purposes of mathematical models and, particularly, turbulence closures. Beyond that, they contribute to a better understanding of the nature of free, turbulent swirling flows.

The second part introduces a novel mechanism we term swirl-induced intermittency. We feel that it is decisive in describing the considerable turbulent radial transport of momentum and scalar quantities observed in swirling free jets. Swirl-induced intermittency is elucidated by physical reasoning and by means of measured joint probability functions (JPDFs) of the Reynolds shear stresses. In addition, conditioned JPDFs measured by a new six-wire technique in the slightly heated jet allow the quantitative analysis of the contribution of swirl-induced intermittency to the overall momentum exchange. This analysis reveals that swirl-induced intermittency is mainly responsible for the large discrepancies between meansurement and prediction of turbulent, swirling free jets in the literature. Since swirl-induced intermittency also strongly enhances entrainment rates and modifies mixing characteristics of swirling jets, it can be assumed that many characteristics of nonpremixed combustion are significantly influenced.