Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Hull Vibration Excitation by Propeller Sources: A Link Between Hydrodynamics and Marine Acoustics
Hull Vibration Excitation by Propeller Sources: A Link Between Hydrodynamics and Marine Acoustics
Pages 760-773
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From page 760... ...
Rath Spivack (DAMTP, University of Cambridge, Cambridge, U.K.) Abstract Acoustic boundary element models are used to solve the Helmholtz equation, in order to explore the nature of fluctuating hull pressures due to propeller sources, when the wavelength of underwater sound is comparable to hull dimensions.
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From page 761... ...
Thirdly, the quenching effect of the free surface changes with frequency, which has a particularly marked influence on hull forces due to sources that are near the stern waterline of a surface ship. None of these features are reproduced correctly in a numerical analysis that solves Laplace's, rather than the Helmholtz equation.
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From page 762... ...
Finally, we show how results for a floating ellipsoid are related to those for a real cruise liner hull shape, using results in Kinns & Bloor (2002a,b)
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From page 763... ...
(17) and for the Solid Boundary Factor (SBF)
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From page 764... ...
. In addition to the total pressure on the hull surface, it is interesting to calculate the Solid Boundary Factor (SBF)
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From page 765... ...
In the case of an axial dipole the distribution of the fields (and consequently of the Solid Boundary Factor) on the hull is axisymetric, as was the case for a monopole source.
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From page 766... ...
We present here results for a half-submerged ellip soid ensonified by a monopole, and show the effect of changing frequency and source depth. The monopole source is placed below the ellipsoid at typical propeller position, at d/10 and d/5 forward of stern waterline.
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From page 767... ...
Cumulative hull forces can be calculated simply by summation of forces on hull elements up to a given station forward of the stern, taking account of the surface orientation and dealing with the in-phase and out-of-phase components separately. The cumulative force magnitude can be derived from the two components at the specified summation limit.
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From page 768... ...
Results for a cruise liner hull A numerical description of a real cruise liner hull was used in Kinns & Bloor (2002a,b) to illustrate applica tion of the modelling techniques to a real hull shape.
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From page 769... ...
The force due to the vertical dipole reaches a maximum at about 15 metres forward of the stern, while the maximum is only reached between about 60 Figure 13 and 80 metres forward of the stern for the transverse Figure 13 shows the cumulative vertical force on the and axial dipoles. hull at frequencies of 12, 24 and 48 Hz for a monopole source having a maximum rate of change of mass flux equal to 1 n4 kg/sec2 The effects of the ch~n~?
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From page 770... ...
Acoustic boundary element models have been used to explore the nature of fluctuating hull pressures due to propeller sources. The Helmholtz equation has been solved, so that the effects of a finite speed of sound are included.
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From page 771... ...
For the surface ship, this causes departures from SBF distributions calculated using Laplace's equation at frequencies that are well below typical propeller blade passing frequency. Finally, we used an independent boundary element model, with explicit representation of the sea surface, to show how hull forces depend on the nature and frequency of submerged sources for a cruise liner hull with twin screws.
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From page 772... ...
However, in order to evaluate compressibility effects on the vibration response of the ship, analyses such as those presented by the authors must be viewed in the light of vibration analyses of the ship's structure, which are usually expressed in terms of its structural vibration eigenmodes. It is the interaction of the distribution of the magnitude of the local pressure fluctuations with the eigenmode shapes, rather than the cumulative hull forces that determines the vibration response of the ship.
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From page 773... ...
In a related paper (Kinns and Bloor, 2002b) , hull forces for the cruise liner have been presented in the non-dimensional form: FIMob as a function of xib, where Fis the cumulative hull force, b is the beam, Mo is the monopole source strength and x is the distance forward from the stern waterline.
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