The standard κ-ϵ equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the κ-ϵ equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.

Bubbles, Drops, and Particles

Combustion in swirling flows: A review

The 'plug' flow emerging from a long rotating tube into a large stationary reservoir has been used in an experimental investigation of centrifugally unstable swirling jets. A moderate Reynolds number, Re = 1000, was studied extensively, and swirl numbers, S, the ratio of nozzle exit rotating speed to the mean mass axial velocity, were in the range 0-1.1. Four regimes were covered: non-swirling jets with S=0, weakly swirling jets in the range 0 S c2 , where S c1 =0.6 and S c2 = 0.88. Particular attention was paid to the dominant role of the underlying vortical flow structures and their dynamic evolution. Kelvin-Helmholtz (K-H) instability in the axial shear layer, generated by the axial velocity, leading to vortex ring formation, dominated non-swirling and weakly swirling jets

An experimental investigation of swirling jets

Measured values of mean velocity, Reynolds stresses and probability density distributions of fluctuating velocity are reported for the turbulent coaxial jets, with and without swirl, emerging into stagnant surroundings from a long pipe and an annulus concentric with the pipe. They were obtained using hot-wire anemometry and on-line data processing with the aid of a mini-computer. The results show that non-swirling coaxial flow configurations approach a self-similar state in a much smaller distance than that of the round jet, for velocity ratios ranging between 0·65 and 1·5; this is due to the mixing layer and vortex shedding that occur in the region downstream of the separation wall between the two streams. In the presence of swirl, the coaxial jet was found to develop at a faster rate. An assessment of turbulence models, based on Reynolds stress closures, suggests that previous assumptions for turbulent diffusion of turbulent kinetic energy are in error.

Coaxial jets with and without swirl

An existing cold-jet facility at NASA Lewis Research Center was modified to produce swirling flows with controllable initial tangential velocity distribution. Distinctly different swirl velocity profiles were produced, and their effects on jet mixing characteristics were measured downstream of an 11.43 cm (4.5 in.) diameter convergent nozzle. It was experimentally shown that in the near field (i.e., x/D Q.6. This remarkable result leads to the conclusion that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field. The relative size (i.e., diameter) of the vortex core emerging from the nozzle and the corresponding tangential velocity distribution are the controlling parameters influencing the swirling turbulent free-jet evolution.

N. A. Chigier

Papers

Experimental Investigation of Swirling Vortex Motion in Jets