Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Measurements of Hydrodynamic Damping of Bluff Bodies with Application to the Prediction of Viscous Damping of TLP Hulls
Measurements of Hydrodynamic Damping of Bluff Bodies with Application to the Prediction of Viscous Damping of TLP Hulls
Pages 622-634
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From page 622... ...
The hydrodynamic damping arises from a number of sources and includes wave drift damping of the hull which is related to wave radiation and diffraction and can be predicted using irrotational flow theory. Other forms of damping include mooring line damping, damping from the riser array, tether~damping in Me case of ILPs and additional damping from the hull arising as a consequence of the viscous nature of water.
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From page 623... ...
Apart from large amplitude slow drift oscillations, the flow around hulls is characterized by small KC numbers and is in regime where there is sparse information on CD values that can be applied with confidence to full scale structures. In the case of TLP hulls viscous damping arises from flow about the vertical columns and from the pontoon.
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From page 624... ...
For large values of KC and 0, drag coefficients can be estimated from existing data for circular cylinders and, because of the relatively smaller amount of data, perhaps less reliably for square sections.
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From page 625... ...
A number of bluff sections have been tested. These include circular cylinders with diameters of 150mm and 312mm and a square section with sides of 300mm.
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From page 626... ...
To ensure the models were almost neutrally buoyant lead weights were bolted to steel plates inside the models Various length models could be accommodated, by the use of different length rods, and various diameter models, by the addition of different fittings to the plate attached to the strut. Various length circular cross-section models with diameters of lSOmm and 312mm, along with a square cross-section model of 300mm across the flats, were constructed and tested.
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From page 627... ...
These theoretical results, which will be referred to here as the Wang results, apply to twodimensional, laminar attached flow about an oscillating circular cylinder. The Neon ves rise to the following expression for the drag coefficient: CD = 3 n3/2 KC (~/2 Alternatively, this can be written as: CD KC p/2 = 26.24.
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From page 628... ...
Circular cylinder models of diameter 312mm and lengths of 1214mm and 620mm, with 392mm end plates, were tested vertically using a single strut for a range of ~ values between 18,000 and 61,000. The drag coefficients were first obtained from these tests by subtracting the difference in the damping between the long and the short models.
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From page 629... ...
The results With the smaller end plates show that end conditions are important, even for a cylinder in oscillatory flow at low KC. It is apparent for all the circular cylinder data, when plotted in a log log form, that as KC reduces so the drag coefficient follows a line parallel to the theoretical values of Wang.
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From page 630... ...
have suggested that the drag of a bluff body in oscillatory flow can be considered as the sum of a boundary layer component and a vortex component. Further it is argued that at low KC the vortex drag coefficient for a circular cylinder should increase linearly with KC.
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From page 631... ...
At low enough KC the separated flow regions will be so localised near the edges that the drag contribution from vortices will be small and viscous damping will be due mainly to boundary layer drag arising from the regions of attached flow over the body. The TLP hull shown in figure 1 was constructed to a scale of 1:113.
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From page 632... ...
t ~ 1 ! 1 1 Figure 10 CD versus KC for a square section cylinder; o, ~ = 40,000; A, 39,000; x, 28,000; 0, 14,000; O
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From page 633... ...
8,000; -, prediction using equation (11) with ~ = 48,000; -- -- -, prediction using equation (11)
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From page 634... ...
10. Hill P., "On He Stability of Unsteady Boundary Layer on a Circular Cylinder Oscillating Transversely in a Viscous Fluid," J
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