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Twenty-First Symposium on NAVAL HYDRODYNAMICS
This paper reports detailed mean-flow and turbulence measurements in the momentumless wake of an axisymmetric body propelled by a swirling jet. The data elucidate the process of mixing between the boundary layer of the body and the jet and the evolution of the momentumless wake. It is found that the wake evolves in at least three stages. The first of these is the near-wake, extending to about 3 jet diameters, where the flow from the near-wall region of the boundary layer mixes with the jet periphery to produce an intense shear layer, distinct from the swirl-induced shear layer that is present at the jet center. There is rapid decay of the mean shear and turbulence in this region. In the second, intermediate region, extending to about 13 jet diameters, the mixing penetrates to the wake centerline, the individual shear layers are assimilated, the pressure field induced by the stern geometry and the swirling jet decays, and the mean shear and the Reynolds shear stresses become negligible by the end of this region. In the third region, called the developed-wake, the flow acquires the characteristics of a single shear layer, with very low levels of mean shear and shear stresses, implying negligible production of new turbulence, and decay of the normal stresses produced upstream.
Analysis of the data in the format of classical similarity theory reveals that the axial and swirling flows develop at quite different rates, as do the corresponding turbulence characteristics. Not all properties of the flow conform with the power-laws predicted by similarity theory. The decay of the swirl initially follows the trends predicted for high swirl and gradually moves towards those expected for weak swirl. However, not all flow properties show asymptotic behaviors, and therefore, it is concluded that a considerably larger streamwise distance is needed for the wake to achieve complete similarity. Further analysis of the data is needed to establish this limit.
The present flow was compared with that of Hyun and Patel (1) to reveal similarities and differences between momentumless wakes of jet- and propeller-driven bodies. Although the near-fields of the two flows are grossly different, as expected, there is strong similarity between the two after a distance of about 7 jet and propeller diameters. Hyun and Patel had shown that the periodicity of the flow associated with the wakes of the individual propeller blades died out beyond a distance of about 2 propeller diameters. The present data reveal that the identity of the jet and the body boundary layer is preserved up to at least 3 jet diameters. Further mixing is needed in both cases for the flow to acquire the characteristics of a single free shear layer. Similarity theory indicates that the two flows must evolve into a single unique state. Neither the experiments of Hyun and Patel nor the present extend into this range, but the comparisons presented here suggest that the two flows acquire considerable resemblance, justifying the intent of the present study to reproduce some elements of propeller wakes in a simpler environment.
Finally, it is important to point out that the present data, along with the data from complementary experiments in wakes and jets without swirl (see Sirviente (2)), comprise a rather comprehensive and unique set documenting the mixing of shear layers with diverse velocity and length scales, and their evolution toward a single shear layer. Consequently, these data are likely to prove of great value in the development and validation of models for nonequilibrium turbulent flows.
This research was partially supported by the Office of Naval Research, Grant N00014–91-J-1204, monitored by Dr. L.P.Purtell.
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