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Attachments: Clumps |
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Attachments: Clumps |
A clump is a concentrated attachment that is connected to a node on a line. It can be buoyant or heavy and represents a small body that experiences forces (weight, buoyancy, drag etc.) in the same way as a 3D buoy. But instead of being free to move it is constrained to move with the node to which it is attached, and the forces acting on it are transferred to that node. A clump therefore adds to the mass, buoyancy and hydrodynamic force of the line through its connecting node.
Clumps only have the three translational degrees of freedom, $X$,$Y$, and $Z$, which are determined by the position of the node to which they are attached. You may choose whether a clump's axes are aligned with global axes directions or with the connecting node.
Each clump is assigned a height and an offset from its connecting node, which are used to determine the $Z$ position of the clump for the purposes of evaluating buoyancy and hydrodynamic forces: no moment is applied to the connecting node by the clump. Where the clump pierces the water surface, buoyancy and hydrodynamic forces are applied in proportion to the immersed length of the clump.
Each clump is of a named clump type, from which it inherits all its properties. The clump types are defined on the attachment types form and have the following data.
Used to refer to the clump type.
The mass of the clump.
Used to calculate buoyancy and added mass for each clump of this type on a line. Clumps may be either net buoyant or heavy as desired.
Used in drawing the clump and also to determine how much of the clump is below the water surface.
If the clump is aligned with global axes then it is centred at the offset position above the connecting node, and extends for half its height above and below this point.
If the clump is aligned with line axes then it is centred at the connecting node, and extends for half its height either side of this point in the node's axial direction.
A clump may be offset vertically from the line, for example to represent a line supported below the surface by floats. The connection is not modelled fully: the clump is always treated as being at the specified offset vertically above (positive offset) or below (negative offset) its connecting node.
If the clump is aligned with line axes then the clump offset is zero and not editable.
Determines whether the clump's axes are aligned with global axes or line axes.
This setting determines the clump's local axes directions. If it is aligned with global axes then the clump's local axes are in the same directions as the global axis system. If it is aligned with line axes then its local axis directions are the same as those of its connecting node.
Drag forces are calculated in clump axis directions for each clump on a line. The drag force $\vec{f}_\mathrm{D}$ on a clump is, in component form \begin{equation} \begin{aligned} f_\mathrm{Dx} &= -\PW\ \tfrac12\ \rho\ \C{Dx} A_\mathrm{x} v_\mathrm{x} \lvert \vec{v} \rvert \\ f_\mathrm{Dy} &= -\PW\ \tfrac12\ \rho\ \C{Dy} A_\mathrm{y} v_\mathrm{y} \lvert \vec{v} \rvert \\ f_\mathrm{Dz} &= -\PW\ \tfrac12\ \rho\ \C{Dz} A_\mathrm{z} v_\mathrm{z} \lvert \vec{v} \rvert \\ \end{aligned} \end{equation} where
$\PW=$ the clump's proportion wet, based on its immersed length
$\rho=$ water density
$\CD=$ the given drag coefficient for each component
$A=$ the given drag area for each component
$\vec{v}=$ clump velocity relative to fluid velocity.
For isotropic clumps types, the $y$ values for drag coefficient and area may be set to '~', to mean 'same as the $x$ value'.
The added mass contribution to clump inertia, $\mathrm{AM}$, in each clump axis direction is calculated as \begin{equation} \mathrm{AM} = \PW\ \rho\ \Ca V \end{equation} where
$\Ca=$ the corresponding component of the given added mass coefficient
$V=$ the given volume.
Defines the pen used for drawing clumps of this type.