The Proceedings Fifth International Conference on Numerical Ship Hydrodynamics (1990) / Chapter Skim
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Finite Difference Analysis of Unsteady Cavitation on a Two-Dimensional Hydrofoil
Finite Difference Analysis of Unsteady Cavitation on a Two-Dimensional Hydrofoil
Pages 667-684
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model, to explain the interaction between viscous fluid and bubble dynamics. This BTF cavity model treats the inside and outside of a cavity as one continuum by regarding the cavity as a compressible viscous fluid whose density varies widely.
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In a microscopic view, this model treats cavitation structurally as bubble clusters. By coupling these two views, the BTF cavity model can clarify the nonlinear interaction between macroscopic vortex motion and microscopic bubble dynamics.
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This is because it regards the cavity flow field as a compressible viscous fluid whose density varies greatly. According to this phenomenological modeling, contour lines of void fraction (volume fraction of cavities)
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2.2 Microsconin Mod ~ 1 i n ~ - Local Homogeneous Model To calculate the macroscopic flow field, it is necessary to know the local void fraction function fg(t,x,y~z)
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The purpose of the SACT series is to study theoretically the unsteady structure of cavitation using the BTF model. Over several years, we developed a program SACT-II (SACT, version II; two
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. The present SACT-III employs the finite difference method in the body fitted coordinates to solve the governing partial differential equations given in the preceding section.
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Equation(13) , however, has no spatial diffusive term for bubble radius R and its time-derivative.
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The foil shape was modified by adding the computed displacement thickness of the foil surface boundary layer [33]
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0 x 10 s. Figure 11 shows the chordwise distribution of boundary layer displacement thickness on the foil surface.
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. Before analyzing the cavitating flow field, we should check the effect of the SGS bubble interaction model.
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. It is therefore concluded that the BTF cavity model can express the features of sheet-type cavitation beyond its microscopic model, which is essentially suitable to the bubble cluster flow.
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These facts also suggest that the present BTF model can be applied to sheet type cavitation beyond its micro structure limitation. Vertical Exaggeration of 4:1 Contour Interval of 0.1 ~\ _ _ _ o °L i gu re 17 T i me-averaged pres sure coef f i c i ent contours and velocity vectors around NI\CAOOlS hydrofoi l, a =1.
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The highly distorted attached cavity sheds cavitation clouds cyclically (T=5.9 and 7.1) , which soon collapse.
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Assuming bubble density and a typical bubble radius, a local void fraction function is given. The BTF cavity model is significant in the following points: (l)
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For instance, the pressure gradient of the vortex cavitation cloud attracts bubbles towards its center. We must therefore take account of the convection of bubbles and the slip between bubbles and liquid to predict the behavior of cavitation clouds more accurately.
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Odeg. ;Re=3.0X lOs, the bold broken lines are void fraction contours of 0.1.
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The computed result showed a close relationship between the behavior of the separated shear layer and the cavitation cloud. This is because the cavitation cloud shedding frequency also depends on the Reynolds number.
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