STRUCTURE OF A SWIRLING TURBULENT MIXING LAYER

A single-stream swirling turbulent mixing layer was produced in which the angular momentum instability acts over most of the layer for most of its development length. The instability is associated with a large increase in the level of the conventional Reynolds stresses over those in an unswirled mixing layer. This paper presents measurements of the three-dimensionality of the mean flow at one axial location well downstream of the origin. The output of a fixed hot-wire probe was ensemble-averaged to produce an approximation to the mean flow field that would be seen by a rotating observer. The main goals of this study were to establish the presence or absence of Taylor-Gortler-type vortices and to document the effects of phase dependence on conventional turbulence measurements. The mean velocities show some three-dimensionality that does not contribute significantly to the conventional, fixed-point Reynolds stresses and is not associated with the presence of longitudinal vortices such as the Taylor-Gortler vortices found in unstable laminar boundary layers. The variations in the mean velocities and Reynolds stresses are generally consistent with a bodily " wrinkling" of the mixing layer, without a significant variation in the layer thickness. The wrinkling is most probably a result of circumferential variations in the initial boundary layer, which in turn originate at the swirl generator.