Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Velocity and Turbulence in the Near-Field Region of Tip Vortices from Elliptical Wings: Its Impact on Cavitation
Velocity and Turbulence in the Near-Field Region of Tip Vortices from Elliptical Wings: Its Impact on Cavitation
Pages 865-881
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From page 865... ...
For a variety of flow conditions, foil planforms, cross sections, and locations of measurement along the vortex path, will found that the axial velocity in the vortex centre can be greater or smaller than the free stream velocity but this will not allow to conclude concerning the organization of the axial flow with respect to the rotating flow. Using the data collected within the context of the Action Concertee Cavitation research program on tip vortex cavitation, some of these results are presented in a way so that some trends are set out.
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From page 866... ...
UO* non dimensional mean axial velocity at the tip vortex centre (UO/UOO)
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From page 867... ...
Using such a procedure, the effects of the wing Reynolds number, the cross section, the tripping of the laminar to turbulent boundary layer transition, the water and polymer solution mass flow ejection, the background turbulence, etc., on tip vortex rollup in the very near region have been investigated for foils having an elliptical distribution of the chord along the span. It has been possible to show that at the tip of the wing the vortex intensity has already a finite value which, for the same flow conditions, increases when the foil tip moves from an upstream to a downstream position (Fruman et al.
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From page 868... ...
compares remarkable well with the desinent cavitation numbers for a variety of foil planform, cross section and Reynolds numbers. This satisfactory agreement seems to indicate that the peculiarities of the axial velocity profiles in the core region as well as the turbulent velocity fluctuations do not participate in the equilibrium conditions of the nuclei captured by the vortex and, therefore, in the onset of cavitation.
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From page 869... ...
the roll-up process is already initiated at the foil tip and the local vortex intensity is a fraction of the mid span bound circulation, d) the maximum tangential velocity reaches an absolute maximum along the vortex path at about one tenth of the maximum chord from the tip, corresponding to the position where the local vortex core radius is a minimum.
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From page 870... ...
Particular attention has been given to the very near region comprised within one chord from the tip. For an elliptical planform foil of cross section NACA 16020 at 10° incidence and equal lift coefficient, Figures 2 shows the non dimensional axial velocity at the vortex axis as a function of the distance to the tip.
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From page 871... ...
Figure 7 shows, for the flow conditions of St,8 Figure 6, the non dimensional axial velocity on the 0 E vortex axis as a function of the distance to the tip. LO STE The axial gradient, evaluated between the tip and the station where the maximum occurs, is at We most only one tenth of the value estimated above in the very vicinity of the tip (130 s-1~.
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From page 872... ...
The axial velocity profile modification can be correlated with observations 872 of ·':' , ~ C1=0,61 C1=0,35 Re x 10 0 2 4 6 Figure 8: Evolution of the pressure loss non dimensionalized by the dynamic pressure based on the upstream velocity as a function of the Reynolds number for the NACA 16020 E foil at 6 and 10 deg incidence. 6 _ ~ 4 5 _ ~ 1 E foil low Re 4.8 0020 2.9 0020 1.1 16020 _ 0.4 - 0.56 16020 bit ~ to on Cl l NACA 0 0.2 0.4 0.6 0.8 1.0 Figure 9: Evolution of the pressure loss non dimensionalized by the dynamic pressure based on the upstream velocity as a function of the lift coefficient.
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From page 873... ...
The absolute maximum tangential velocity fluctuation occurs very near the tip of the foil at x* <0.2, corresponding to the position of maximum axial velocity peak, it's also the position where the difference between the minimum and the maximum tangential velocity reaches an extremum and also, where the minimum pressure coefficient occurs, Fruman et al.
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From page 874... ...
Figure 14 shows the evolution of both the maximum tangential and axial velocity fluctuations with the vortex angular velocity measured at x* =0.2 for the elliptical foils having a NACA 0020 at 4, 6, 8, 10 deg for Re=4.8xlO6 and a NACA 16020 cross section at 6, 10 deg for Re=4.8xlO6 and 10 deg for Re=2.8xlO6.
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From page 875... ...
=0.2 from the tip of the NACA 0020 elliptical foil at 10 deg incidence and a Reynolds number of S.7x106. Associated to five positions of measurements within the vortex core, the non dimentionalized instantaneous velocity distributions for a sample of 600 measurements are shown.
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From page 876... ...
2.4 2.0 1.8 1.6 1.4 1.2 1.0 U* 0.8 1 1 1 1 ~.05 0 o ~ ~—1 1 1 0 0.05 o O -1 0 Figure 16 a: Radial distribution of the mean axial velocity measured at x*
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From page 877... ...
-0.05 o Figure 17: Radial distribution of the tangential velocity fluctuations corrected and non corrected, and the opposite value of the axial velocity fluctuations measured at x* =0.2 from the tip of the NACA 0020 elliptical foil at 10 deg incidence and a Reynolds number of 5.7x106.
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From page 878... ...
1 1 1 1 1 0.05 0 o N* ,% C ~ a, - CD o Is Figure 18 a: Radial distribution of the mean axial velocity measured at x*
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From page 879... ...
The pressure loss corresponding to the difference between the maximum axial velocity that should occur due to the minimum pressure'and the maximum axial velocity measured is for reasonable lift coefficients in order of magnitude, once the dynamic pressure. The pressure loss increases up to six times the dynamic pressure as the lift coefficient increases.
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From page 880... ...
~°° Mu* , 0 0,2 0,4 0,6 0,8 Figure 19: Radial distribution of the tangential velocity fluctuations corrected and non corrected as a function of the axial velocity fluctuations measured at x*
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From page 881... ...
"Recent results on the effect of cross section on hydrauŁoil tip vortex cavitation occurenceat highs Reynolds numbers"Cavitation and Multiphase Flow Forum, ASME-Vol.
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