Drag Lift |
 
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Drag Lift |
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Flexcom provides an option to model drag lift forces. This is most typically used to compute lift forces on a pipeline from current and wave caused by the asymmetry of flow around the pipeline in an on-bottom or near-bottom configuration. The magnitude of the drag lift force is given by the following relation:
(1)
where 
is the density of the external fluid (typically seawater), 
is the drag lift coefficient, 
is the drag diameter, 
and 
are the normal components of fluid and structure velocity with respect to the orientation of the structure (the axis of the element). The lift coefficient 
may be specified as a constant value or expressed as a function of the ratio of the gap between structure and seabed to the contact diameter of the element.
The direction of the lift force is perpendicular to the element axis and to the water particle velocity. Specifically, the lift force acts parallel to the normal (
) of the plane defined by the vector of the local convected x axis of the element (
) and the vector of the water velocity (
). The normal (
) is he normalised cross-product 
.
The sense of direction for the lift force is defined as follows:
(i)If the angle between the normal (
) and the global X vector is strictly less than 90°, then the force has the same direction as the normal (
).
(ii)If the angle between the normal (
) and the global X vector is strictly more than 90°, then the force is in the opposite direction to the normal (
).
(iii)If the angle between the normal (
) and the global X vector is exactly 90°, then the force direction is given by the direction of (
).