Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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A BEM Technique for the Modeling of Supercavitating and Surface-Piercing Propeller Flows
A BEM Technique for the Modeling of Supercavitating and Surface-Piercing Propeller Flows
Pages 819-838
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From page 819... ...
They tend to have smaller volume change and produce bubbles which collapse downstream of the blade trailing edge, which results in reduced noise and blade surface erosion. However, they are also difficult to model due to the unknown size of and the pressure in the separated region behind thick blade trailing edges, which are characteristic of supercavitating propellers.
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From page 820... ...
However, the length of the separated zone did have an effect on the pressure and cavity length near the blade trailing edge under fully wetted and partially cavitating conditions. Finally, (Kinnas et al 1999)
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From page 821... ...
The boundary element method inherently includes the effect of non-linear thickness-loading coupling by discretizing the blade surface instead of the mean camber surface. Thus, it requires more CPU time and memory than the lifting surface method.
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From page 822... ...
, and the portion of the wake surface which is overlapped by the cavity, sow. Boundary Conditions · Kinematic Boundary Condition on Wetted Blade and Hub Surfaces The kinematic boundary condition requires the flow to be tangent to the wetted blade and hub surface.
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From page 823... ...
13 is defined normal to the wake surface. In addition, the quantity hw at the blade trailing edge is determined by interpolating the upper and/or lower cavity surface over the blade and computing its normal offset from the wake sheet.
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From page 824... ...
. SUPERCAVITATING PROPELLERS Experimental evidence shows that the separated zone behind the thick blade trailing edge forms a closed cavity that separates from the practically ideal irrotational flow around a supercavitating blade section (Russel 1958~.
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From page 825... ...
3, the present method is applicable to fully wetted, partially cavitating, and supercavitating conditions in steady and unsteady flows. Cavitation patterns on supercavitating propellers that can be predicted by the present method are shown in Fig.
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From page 826... ...
10. Boundary Conditions · Dynamic Boundary Condition on the Free Surface and Ventilated Cavity Surfaces The dynamic boundary condition requires that the pressure everywhere on the free surface and on the ventilated cavity surface to be constant and equal to the atmospheric pressure, Palm.
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From page 827... ...
In addition, the 8oh term is difficult to evaluate due to the interruption of the ventilated cavity by the free surface. · linearized Free Surface Boundary Condition on the Free Surface As a first step to model partially submerged propellers in 3-D, the linearized free surface boundary condition is applied: 6~J~°((x, y, z, t)
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From page 828... ...
Thus, the cavity detachment locations on the suction side of the blade are searched for iteratively at each time step until the smooth detachment condition is satisfied. In addition, due to the interruption of the free surface, the following detachment conditions must also be satisfied for partially submerged propellers: · The ventilated cavities must detach at or prior to the blade trailing edge; and · During the exit phase (i.e.
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From page 829... ...
15 to 18, the predicted ventilated cavity patterns and blade forces agree well with experiments for JS = 1.2. In addition, the method also converged quickly with time step size and grid size, as shown in Figs.
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From page 830... ...
2-D STUDY OF FREE SURFACE EFFECTS In order to quantify the added hydrodynamic forces associated with jet sprays generated at the AD = 9O AD = 6° AD = 3° o 0.005` ) 90 180 to blade angle (degrees)
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From page 831... ...
. · Kinematic Boundary Condition on SF: The kinematic free surface condition requires fluid particles on the free surface and ventilated surface to remain
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From page 832... ...
23. · Dynamic Boundary Condition on SF: On the exact free surface and ventilated surface, where the pressure should be constant and equal to the atmospheric pressure: bt + 2 [( 0~ )
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From page 833... ...
23. · Kinematic Boundary Condition on SWB: The kinematic boundary condition requires the following condition to be satisfied on the wetted body surface: ( |
From page 834... ...
CONCLUSIONS A 3-D boundary element method has been extended to predict the performance of supercavitating and surface-piercing propellers. The current method is able to predict complex types of cavity patterns on
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From page 835... ...
The method also appeared to converge quickly with grid size and time step size. A 2-D study using the exact free surface boundary conditions has been initiated to quantify the added hydrodynamic forces associated with jet sprays during the entry phase.
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From page 836... ...
AND KINNAS, S 1995 Application of vortex/source lattice method on supercavitating propellers.
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From page 837... ...
SCHERER, J 1977 Partially submerged and supercavitating propellers.
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From page 838... ...
AND FALTINSEN, O 1998 Water entry of axisymmetric bodies with and without flow separation.
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