Download Time Domain VIV Models

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Orcina
The VT2 model uses a coalescing algorithm to control the number of
vortices being tracked.
The VT2 model has the following user-adjustable coefficients:
Vortex Smear Factor
Vortex Decay Constant and Thresholds (3 parameters)
All as for VT1 model – see previous section.
Creation clearance
Determines the distance from the cylinder surface at which new vortices are
created.
Coalesce same; Coalesce opposite
Determine the readiness with which vortices coalesce. Larger numbers mean
greater readiness to coalesce, hence fewer active vortices and shorter run
time.
Further details are available in the User Manual.
Experience suggests that small changes in Smear Factor and Creation Clearance
have opposite effects: provided the two values are kept the same there seems to be
little net change in results.
Recent work suggests that, in some cases, the Coalescence thresholds can be
increased considerably, with little effect on results and great benefit in reduced run
time. The run time for an VT2 model is dominated by the VIV calculation, and the time
for the VIV computation at each node increases as the square of the number of
vortices present. Increasing the Coalescence threshold values means that vortices are
more readily amalgamated which reduces the total number present and speeds the
computation. Recent experience with a 100 element model showed that setting the
threshold as high as 0.15 had little effect on the VIV response and reduced run time by
two orders of magnitude.
We recommend leaving the Smear Factor, Decay parameters and Creation Clearance
at their default values, and carrying out sensitivity analysis to determine the highest
Coalescence values which can be used without adversely affecting results. Separate
threshold values can be used for coalescence of same sign and opposite sign vortices.
Our experience suggests there is little to be gained by using different values.
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C:\Development\OrcaFlex\Help\Time Domain VIV Models.doc
Last saved: 22 August 2006