Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Experiments in the Swirling Wake of a Self-Propelled Axisymmetric Body
Experiments in the Swirling Wake of a Self-Propelled Axisymmetric Body
Pages 973-985
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From page 973... ...
The resulting flow is, of course, simpler in comparison to the propeller wake, but retains considerable complexity when compared to canonical turbulent shear flows such as boundary layers, jets and wakes. The flow in the near wake of the jetpropelled body, in reality, depicts the mixing between the body boundary layer and an axisymmetric swirling jet.
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From page 974... ...
This condition was achieved with a maximum axial velocity at the jet exit equal to twice the freestream velocity, i.e., Uj = 2Uo. The tangential momentum was adjusted such that it matched that of the propeller employed by Hyun and Patel (1~.
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From page 975... ...
where the velocity is maximum (Um) 2.0 .0 Figure 4a shows the mean velocity profiles at six representative streamwise stations, labeled A Trough F
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From page 976... ...
By x/D = 5, and well before station C, the velocity maximum occurs at the centerline, the through in the axial velocity profile disappears, and the profile is as sketched in Figure 3a. From Figure 4b it its seen that, within a distance of about AD = 4, the axial velocity along the wake centerline decreases rapidly from 2Uo at jet exit to very close to l.lUo where it becomes coincident with the maximum velocity, and the minimum velocity increases from zero at the jet exit to 0.9UO.
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From page 977... ...
Approach to Similarity of the Mean Flow The existence of different velocity and length scales, and their different behaviors in the near and intermediate regions, are of course the most obvious characteristics of interacting shear layers. On the other hand, classical similarity theory applied to free shear layers is based on the assumption that multiple scales, if present, are in some constant proportion, i.e., each flow is described by just one velocity scale and one length scale.
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From page 978... ...
Log Plot of Mean Flow Velocity and Length Scales that the axial velocity scale decays as x-l, the swirl velocity as x-3/4, and the length scale grows as x114. While a complete review of the theory is beyond the scope of this paper, it is of interest to plot the various velocity and length scales of Figures 4 and 5 to observe their approach to the predicted power laws.
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From page 979... ...
, which lie in the previously defined near field, x/D < 3, all turbulence profiles show the coexistence of two shear layers, one associated with the large gradient of the swirl velocity at the center, and the other with the mixing between the jet and the boundary layer. Both produce peaks in the turbulence kinetic energy and shear stresses.
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From page 980... ...
For all practical purposes, the shear stresses transporting axial and tangential momentum are now negligible, and the turbulent kinetic energy slowly decays. It was found by Ridjanovic (7)
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From page 981... ...
, length scales based on profiles of the turbulence quantities are better suited to define the overall length scale of the flow in the developed region because, as already implied, in that region the differences in mean velocity become small. Therefore, the scales presented in Figures 9b,c-1 lb,c are better suited for the study of the approach to similarity than those based on the mean velocity profiles (Figure 8~.
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From page 982... ...
These results should be interpreted with reference to Figure 3, which shows the velocity profiles in the near field. Immediately downstream of a propeller the maximum axial velocity occurs some distance away from the centerline (around r/Rp = 0.7 in Hyun and Patel (1)
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From page 983... ...
The magnitudes of the shear stresses are small in both cases, and the differences in their pro-files are consistent with the differences in the mean velocity shapes.
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From page 984... ...
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.
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From page 985... ...
and Jonsson, L., "Measurements of the Velocity Field Downstream an Impeller," to appear in Journal of Fluids Engineering 1996.
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