Abstract
The instantaneous and time-averaged flow fields in the tip region of a ducted marine propulsor are examined. In this flow, a primary tip-leakage vortex interacts with a secondary, co-rotating trailing edge vortex and other co- and counter-rotating vorticity found in the blade wake. Planar particle imaging velocimetry (PIV) is used to examine the flow in a plane approximately perpendicular to the mean axis of the primary vortex. An identification procedure is used to characterize multiple regions of compact vorticity in the flow fields as series of Gaussian vortices. Significant differences are found between the vortex properties from the time-averaged flow fields and the average vortex properties identified in the instantaneous flow fields. Variability in the vortical flow field results from spatial wandering of the vortices, correlated fluctuations of the vortex strength and core size, and both correlated and uncorrelated fluctuations in the relative positions of the vortices. This variability leads to pseudo-turbulent velocity fluctuations. Corrections for some of this variability are performed on the instantaneous flow fields. The resulting processed flow fields reveal a significant increase in flow variability in a region relatively far downstream of the blade trailing edge, a phenomenon that is masked through the process of simple averaging. This increased flow variability is also accompanied by the inception of discrete vortex cavitation bubbles, which is an unexpected result, since the mean flow pressures in the region of inception are much higher than the vapor pressure of the liquid. This suggests that unresolved fine-scale vortex interactions and stretching may be occurring in the region of increased flow variability.
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Acknowledgements
The authors acknowledge the contributions of Dr. Stuart Jessup, Dr. Christopher Chesnakas, and Dr. David Fry during the design and implementation of the experiment and would like to thank them for their permission to use the images in Figs. 1 and 2. This work was funded by the Office of Naval Research under grant number N00014-99-1-0307, with Dr. Ki-Han Kim as the technical monitor.
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Oweis, G.F., Ceccio, S.L. Instantaneous and time-averaged flow fields of multiple vortices in the tip region of a ducted propulsor. Exp Fluids 38, 615–636 (2005). https://doi.org/10.1007/s00348-005-0938-z
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DOI: https://doi.org/10.1007/s00348-005-0938-z