Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Inception, Development, and Noise of a Tip Vortex Cavitation
Inception, Development, and Noise of a Tip Vortex Cavitation
Pages 851-864
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From page 851... ...
NOMENCLATURE a c Cl Cp D f H N p Pc Ps Pv V x r p 60 00 radius of the cavitating vortex core maximum chord length lift coefficient related to the surface of the foil pressure coefficient at the vortex axis cavitating core diameter frequency focal lenght hysteresis i angle of attack of the foil Kkx) experimental coefficient aperture pressure of He inlet flow at the middle of the test section critical pressure of nuclei susceptibility pressure of He fluid vapour pressure of water radius Reynolds number based on He maximum chord length time surface tension free stream velocity tangential velocity abscissa Euler constant circulation volumic mass of liquid cavitation number cavitation number at the beginning of the developed cavity far from He vortex axis, in the perpendicular plan of He centerline vortex PSD Power Special Density G.T.H.
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From page 852... ...
At the inception of the cavity, we observe bubbles growing in the center of the tip vortex for special water quality. These bubbles, captured by the vortex, could grow by diffusion of non condensible gas or could explose by vaporization of liquid.
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From page 853... ...
A map of the observed cavitation versus the angle of attack is drawn by looking at video records, for velocities between 5 m/s up to 15 m/s by step of 1 m/s. The water quality corresponds to a low oxygen content of 30 %, without nuclei injection.
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From page 854... ...
The coaxial lighting makes a shadow view of the vortex on the recorded image so that the edge of the tip vortex core is very contrasted. The vortex core has sufficient contrast such that its contour is directly obtained by converting the greyscale image into a binary image based on a threshold range.
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From page 855... ...
As presented in ~ 11 i, du to the unique Filly gas content device of the G.T.H., for water without nuclei injection, the modification of the oxygen content do not affect cavitation inception, provided saturation condition are not reached. For oversaturated flow without injection, ~ values at inception are similar to those obtained for low oxygen content and nuclei seeding.
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From page 856... ...
3.2 Correlation of the cavitation inception data The objective is to obtain a correlation regardless of the water quality and the flow velocity. Using the centerbody venturi VAG, the water quality is characterized by a single parameter, the susceptibility pressure Ps, for a specific nuclei concentration ~ 0.01 nuclei / cm3 )
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From page 857... ...
It increases with time in a way which is very dependent of the water quality. The diameter can be nearly constant for the water with 30°/0 oxygen content without nuclei injection, or increase by a factor 2 on 5 minutes for a water with 80% of oxygen content and no bubbles.
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From page 858... ...
9 8 7 6 5 4 3 2 o r ~ ~ r 0 2 4 6 8 10 12 14 80% oxygen without injection · inception of cavitation · hollow vortex core attached at the tip ~ desinenceofcavitation Fig. 15: incipient, attached and desinent cavitation parameter versus angle of attack, V = 10 m/s.
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From page 859... ...
16: PSD in subcavitating conditions. For a given water quality and a given inlet velocity, the acoustic level in subcavitating conditions is independent of the angle of attack and of the pressure level of the test section (Figure 161.
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From page 860... ...
1 X ~ 01 _ _ _ _ 1_ _ ~¢ _ _ _ _ ~ _ _ _ _ _ _ _ 1 OX x+ o <0 a_ _ i(°) l lo 5 10 30% oxygen with 'big' bubbles X 1 peak/e begin maximum peaks/e + 1 peak/e end ~ inception of cavitation o cavitating vortex core Fig.
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From page 861... ...
Similar results were observed for a variation of the inlet pressure at constant angle of attack: then the frequencies depend on the inlet pressure. At first order, it seems that frequencies are linked with the diameter of the cavitating vortex core.
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From page 862... ...
However, the infonnation of the diameter is not sufficient to deduce the frequencies emmitted by the cavity. 4.4 Morozov's model 3.0 2.5 2.0 I.5 I.0 In the following, we use measured frequencies and as associated measured diameter for several cavity within the tip vortex.
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From page 863... ...
The growth of the cavity within the tip vortex can be explained by diffusive effects linked to the water quality. This leads to understand the hysteris phenomenon at the desinence cavitation.
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From page 864... ...
2. It has been proposed, and used with good success, to obtain the "susceptibility" pressure directly from tip vortex tests (by plotting He difference between the free stream pressure for cavitation inception minus the vapor pressure as a function of the velocity to the power 2,4 for constant incidence angle)
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