OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
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Clearly DecayConstant must be in the range 0 to 1, and DecayThreshold1 must be less than DecayThreshold2. Note
that DecayThreshold1 and DecayThreshold2 can be set to Infinity. If either of them is Infinity then λ = 1-
DecayConstant always, so the vortices always lose DecayConstant of their strength per variable time step.
The same decay model is used in model 2, except that the factor λ is adjusted to allow for the fact that model 2 uses
the outer time step instead of the variable time step used in model 1. The adjustment results in the same rate of
decay per unit time.
Drag Coefficients
The vortex tracking model includes the drag effects in both the transverse and inline directions, but not in the axial
direction. When the vortex tracking model is used, OrcaFlex therefore suppresses the components of the usual
Morison drag force in the transverse and inline directions, but includes the component in the axial direction. The
drag coefficients for the normal directions are therefore not used, but the axial drag coefficient is used.
Results
The Vortex Force is available as line force results variables. This reports the total lift and drag force. Note that this is
the sum of the force generated by the vortex tracking model, which is in the inline and transverse directions and
already includes the drag force in those directions, plus the standard Morison drag force in the axial direction.
The stagnation and separation points are available as line angle results variables.
Transverse VIV Offset is also available as a line position results variable.
9.2.3 VIV Drawing
With the time-domain VIV models you can control how various aspects of VIV are drawn on the 3D view, by setting
data on the VIV Drawing page on the line data form.
Arc Length Intervals
You can control which nodes have VIV detail drawn, by specifying one or more Arc Length Intervals. An arc length
interval specifies a contiguous length of line From one specified arc length To another. The VIV details are drawn
for all nodes whose arc lengths fall in any one of the specified intervals. For convenience '~' in the From column
means End A of the line, and '~' in the To column means End B.
For example, to view the detail for one node only, specify 1 arc length interval and set both its From and To values
equal to the arc length of that node. Whereas to view the detail for the whole line specify 1 arc length interval and
set both its From and To values equal to '~'.
What is Drawn
The following VIV details are drawn for all nodes whose arc lengths fall in any one of the specified arc length
intervals.
For all the time domain VIV models, the vortex force is drawn as a line radiating from the disc centre in the direction
of the vortex force. The line length is scaled so that a vortex force equal to the standard Morison drag force with
Cd=1 in a relative velocity of 1m/s, is represented by a line 1 disc radius long (i.e. just reaching the edge of the disc).
For the vortex tracking models only, and only if the maximum number of vortices logged is set greater than zero,
then the following extra details are drawn.
� The node is drawn as a disc (even if you have not specified nodes drawn as discs) and the stagnation and
separation points are drawn on the edge of the disc.
� The positive and negative vortices are drawn as circles whose areas are proportional to the vortex strengths.
The constant of proportionality can be controlled by setting Area per Unit Strength.
� The centre-lines of the positive and negative vortex sheets, and the wake line are drawn.
Note: The vortices and sheet centre-lines drawn are limited by the specified maximum number of vortices
logged.
For all these items you can control the pen used for drawing. With the vortex tracking models, for example, this
allows you to suppress (by setting the pen style to null) or downplay (by choosing a suitable colour) individual
aspects of the detail.