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Show fluid out of the central region could account for the simultaneous broadening and weakening of the internal recirculation zone at increasing axial distance along the centerline (Fig. 3). The effect of this behavior on the mean locations of the vortex lines is sketched in Fig. 19 which shows both signs of vorticity being pulled out toward the walls as the fluid proceeds downstream. If this is indeed the case, we expect strong three-dimensional effects to be observed as the two signs of vorticity begin to intertwine. Possible evidence of this is seen in Fig. 13 which shows substantial incursions of unseeded fluid into the annular jet at somewhat regular intervals in the azimuthal direction. Once the vortex lines have been drawn out to furnace walls their fate is hard to predict. Two possibilities are worth considering. The frrst is what might result if the two signs of vorticity are highly intertwined by the time the fluid reaches the wall reattachment point. If this is the case, we expect that both signs of vorticity would be convected into the downstream flow and the external recirculation zone adjacent to the wall. Whether the vortex lines maintain a distinct character in these regions or are lost to diffusion is not known, but at minimum, some cancelation of the inductive power of the two signs of vorticity will certainly result. The other possibility is that the vortex lines are not so highly intertwined when the flow splits at the wall. In this case it is probable that more of the outer layer of fluid (negative helical vorticity) would be convected into the external recirculation zone while the inner layer (positive) is convected downstream. This configuration is consistent with the circulation in the external recirculation zone which reflects a positive azimuthal component of vorticity. CONCLUSIONS In this paper we have reported experimental data which illustrates the instantaneous spatial structure of an annular swirling flow after a sudden expansion. This data suggests a mental picture of the flow which is quite different from the time-averaged velocity-pressure (or momentum) viewpoint to which many of us have become accustomed. Borrowing from the rich base of information developed for flows with localized vorticity, we have attempted to construct a vorticity-based view of the flow. The mental model which has resulted from these efforts is based in large part on speculation, but is fully consistent with our experimental observations and provides a framework from which better-posed questions about the flow can result. The data, and resulting model, also call into question the wisdom of the common practice of applying Reynolds' equations to such a flow. While we are not in a position to propose a practical alternative at this time, we believe that the viewpoint constructed here may be helpful in analyzing the potential advantages and limitations of any mathematical modeling approach to this type of flow. - 8 - |