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6D buoys: Spar buoy and towed fish drag & slam |
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6D buoys: Spar buoy and towed fish drag & slam |
Slender bodies in near-axial flow experience a destabilising moment called the Munk moment. This effect can be modelled by specifying a non-zero Munk moment coefficient.
If this option is checked then the normal drag area for each cylinder is calculated directly from the cylinder geometry as the product of outer diameter and cylinder length.
If this option is not checked then the normal drag area for each cylinder must be defined explicitly.
Drag loads are the hydrodynamic and aerodynamic loads that are proportional to the square of fluid velocity relative to the cylinder. For details of the drag load formulae see the drag theory topic. For specific information on modelling a SPAR or CALM buoy see modelling a surface-piercing buoy.
| Note: | Aerodynamic drag is only included if the include wind loads on 6D buoys option is enabled in the environment data. |
The drag forces are calculated on each cylinder by the cross-flow principle. That is, the relative velocity of the fluid past the cylinder is split into its normal and axial components and these components, together with the drag areas and coefficients, determine the normal and axial components of the drag force.
The drag forces are specified by giving separate drag area and drag coefficient values for flow in the normal direction (local $x$ and $y$-directions) and the axial direction (local $z$-direction). The drag area is a reference area that multiplies the drag coefficient in the drag force formula. You can use any positive drag area that suits your needs, so long as the drag coefficient is consistent with that reference area.
The drag moments are specified and calculated in a similar way to the drag forces, except that the reference drag area is replaced by a reference area moment. This and the drag coefficient are multiplied together in the drag moment formula, so again you can use any positive area moment that suits your need, providing that the drag coefficient is consistent.
There are two different methods to choose from when specifying the drag data. The first is to set the OrcaFlex data to get the best possible match with real-world results for the buoy (e.g. from model tests or full scale measurements). This is the most accurate method, and we recommend it for CALM and discus buoys in particular. Because the drag area and drag coefficient are simply multiplied together, you can calibrate the model to the real results by fixing one of these values and adjusting the other. For example, you might set the axial drag coefficient to 1 and adjust the axial drag area until the heave response decay rate in your OrcaFlex model best matches your model test results.
The second method is to set the data using values derived from theory and/or given in the literature. For the drag forces, set the drag areas to the projected cylinder area exposed to drag in each direction, and base the drag force coefficients on literature values from Barltrop & Adams, 1991, Hoerner,1965 and DNV-RP-C205. Note that the drag area specified should be the total projected area exposed to drag when the buoy is fully submerged, since OrcaFlex calculates and allows for the proportion wet. For a simple cylinder of diameter $d$ and length $l$, the total projected drag area is $dl$ for the normal direction and $\pi d^2/4$ for the axial direction; if, however, the buoy has attachments that will experience drag then their areas must also be included.
For the moments, set the drag area moments to the third absolute moments of projected area exposed to drag in the direction concerned, and the drag moment coefficients based on values given in the literature.
The slam force as the buoy enters or exits the water can be modelled by defining non-zero slam data. Separate values are given for water entry and water exit, and each can be set either to a constant slam coefficient value or to be variable with submerged depth.
The constant coefficient case requires an associated waterplane slam area. For spar buoys and towed fish, this is calculated by OrcaFlex as the instantaneous water plane area. Lumped buoys, however, do not have a defined geometry so this cannot be calculated: you must specify this value yourself in this case.
If a slam coefficient is zero then no slam force is applied for motion in the corresponding direction. If both water entry and exit slam coefficients are zero then the slam force results will not be available.