Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Instability of Partial Cavitation: A Numerical/Experimental Approach
Instability of Partial Cavitation: A Numerical/Experimental Approach
Pages 599-615
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From page 599... ...
w re carried out m s I 9 cm square wster t m I md s geom h ically scaled up series of te ts w ~e carried out m th 30 cm sq me wster tmm I st fhe Versuchsmstalt fur Wasserbm (VAO) m Obemach, Germ my The te ts were designed to complement each other md to capitali e on the sp cisl fetmes of each facilit INTRODUCTION Marme propellers md hyd of oils must of operste m fhe cavitstmg regime Various typ s of cavi tstion c m be found in p~actice, mcludmg bubble es itation, shet ca vitation, cloud cavitstion, vortex tube cavitstion md vertex sh et cavitation, dep ndmg upon how the low pressure regions me genemted b pite of considemble ~esearch, fhe~e are still m my festmes of th problem thct have not ben properly e plored For example, mcepti m st dies me based m f lly wetted fl w properties, i e pressme dish ibuti m, t rbulff~ce level et in fhe absence of cavitstion On the other h md, chssical mod is of developed cavitation consid r mly cavitstion mmmber as fhe primary varisble Whst has not been given sd quste sttention is s class of partislly cavitating flows in which thee is on int roction betwen Jluid turbulence and cavitohon For enample, vort :x genemtion st the h sill g edge of shet cavitstion is s m mifestati m of fne cavitation itself Ve et sl, 1997)
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From page 600... ...
Unsteady cavity flows about hydrofoils are time wise and space wise highly variable two-phase flows, making it necessary to account for compressibility, viscosity as well as the gas dynamics. The experiments were carried at two different scales in two different water tunnels, at the Saint Anthony Falls Laboratory (SAFL)
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From page 601... ...
For a cylinder of diameter D in an infinite domain, the compressibility boundary layer thickness, , was determined, numerically as well as theoretically, to be U 5M `53 D where U is the ambient velocity. In other words, the weakly compressible flow equations contain the hydraulic transient flow solution, which continuously evolves into the steady incompressible flow.
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From page 602... ...
Recall that the outer solution of the weakly compressible flow equations is independent of M and represent the incompressible flow. Although much larger M can be used, the gain in computational speed diminishes as M is further increased.
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From page 603... ...
LIFT OSCILLATIONS Excellent agreement was obtained between the numerical simulations and the data obtained at SAFL and Obernach. As an example, the measured cavity length at various angles of attack is presented in Figure 3 in the form of 1/c vs /2 .
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From page 604... ...
At high values of /2, reentrant jet physics dominate, with sheet cavity oscillations at a frequency, based on cavity length, of fl/U 0.3. Atlow values of /2 ,bubbly flow shock wave phenomena dominate with a constant Strouhal number based on chord length of fc/U 0.2.
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From page 605... ...
Fig.ll shows vorticity fields at several instants within one cycle of vortex shedding or cloud cavity formation. This figure suggests that the cavity break off is due to the reentrant jet striking the cavity surface (reentrant jet is a thin positive vortex sheet)
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From page 606... ...
12. Sheet-Cloud Cavitation (full cavitations There is a significant change in the flow pattern when the maximum sheet cavity length exceeds the chord length.
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From page 607... ...
Numerical simulations of Type III oscillations due to bubble/patch cavitation. The lower curve displays lift fluctuations.
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From page 608... ...
In addition, Figure 13 shows that the lift oscillations also contain two peaks. In this case the primary peak is that of the sheet/ cloud cavity oscillation, which is dependent on the chord length rather than the cavity length.
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From page 609... ...
In the case of cavitating flow, it is important to note that the numerical simulations fit the data close to the trailing edge, but deviate from the measurements further downstream. It is conjectured that this is an effect of dissolved gas that has come out of solution.
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From page 610... ...
The observed self similarity of the mean velocity data for both cavitating and noncavitating conditions when compared with the numerical simulations suggest that dissolved incondensible gas plays an important role in the wake dynamics. In interpreting averaged velocity data from cavitating flows care must be taken because of the bias induced by slower moving bubble clouds that produce a higher than average data rate.
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From page 611... ...
Noting that the spectral characteristics of the lift oscillations vary considerably with /2, it is interesting to compare how the spectral characteristics of the noise vary over the same range. This is shown in Figure 21 where spectra at different values of /2 are compared over the same range shown in Figure 4 for lift oscillations.
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From page 612... ...
Spectral characteristics of surface pressure are compared at Vc = 0.74 for two values of /2 em. Reynolds number is kept constant at 105.
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From page 613... ...
The observed pressure characteristics are in good qualitative agreement with the numerical simulations and the measured lift oscillations. The array of techniques developed to study this complex flow show good promise for surface pressure/acoustic radiation and lift/acoustic radiation correlations.
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From page 614... ...
, Zhou, F and Wang, G (1997) , "Numerical Simulation of Cavitating and Non cavitating Flows over a Hydrofoil, " St A thony Falls Laborato y, Frolect Report No 402, University of Minnesota Tulin, M F (1958)
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From page 615... ...
AUTHOR'S REPLY Bubble cavity in this paper refers to c Urge detached cavity, which is generated near the nose of She foil Ed convected away m periodic maimer it usually occurs when the attack Ogle is small Ed the cavitation mmmber is only slightly below She critical value its occurrence is quite predictable Ed deterministic So the phenomenon described h re is quite different from the cavitation gas mmclei which occurs r mdomly Bec mse the size of She bubble cavity is quite large, it c mses She Ifft to oscillate It the Strouhal mmmber based on the cord length of cutout 0 I he computed lift oscillation agrees very well with the experimental data
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