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VIVA data |
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VIVA data |
Some data needed by VIVA cannot be derived automatically by OrcaFlex from the ordinary OrcaFlex data. These are described below and appear usually on the VIVA data form, or (exceptionally) on the VIV page of the line data form.
By default the VIVA data are not included in text data files saved by OrcaFlex. This option allows you to override that default.
If you are using the coupled VIVA interface, then the VIVA data are always included in text data files irrespective of this setting.
In order for OrcaFlex to run VIVA, the location of the VIVA executable file viva.exe must be defined. This file location may be given either as a relative or full path.
The tolerance to use in the coupled statics iteration.
VIVA predicts which modes of oscillation might be excited and it gives separate VIV results for each possible excited mode. In addition it gives VIV results for multi-mode response, i.e. response that is a mixture of the possible modes.
You select which set of VIVA $\CD$ results to use, based on the type of response you expect, by setting the target mode. This can be a mode number, to use the single mode results for that mode number, or it can be set to ~ to use the multi-mode results.
If you set the target mode to a mode number that VIVA does not predict will be excited, then OrcaFlex will report a warning and use the OrcaFlex line type $\CD$ values.
VIVA requires an equally-spaced discretisation of the line. The data supplied to VIVA will contain NP equally-spaced points covering the line. OrcaFlex may need to use interpolation to produce the VIVA input data, for instance if the OrcaFlex line is not uniformly segmented, or if the number of nodes in the line differs from NP.
For some input files, VIVA detects that the discretisation may be too coarse. IF so, OrcaFlex will report a warning which can usually be resolved by increasing the value of NP.
The VIVA damping type data, taking a value of either modal or constant. For details see the VIVA documentation, where it is referred to as G_M.
The structural damping ratio that VIVA should use. It is a single value that applies to the whole line. For details see the VIVA documentation, where it is referred to as ZETA.
VIVA needs to know the flow velocity at each point along the line. You can choose to use either the relative velocity (including the velocity of the line) or only the fluid velocity (ignoring any velocity of the line). Since VIVA is only called from the OrcaFlex static analysis, this data item only has any effect if the model starting velocity is non-zero. We recommend that the relative velocity should be used, since it is the total relative velocity that generates vortices.
The S-N data and stress concentration factor used by VIVA's fatigue calculations. The S-N curve is specified in the usual OrcaFlex way \begin{equation} N = a S^{-m} \end{equation} The values on the data form are those of $\log_{10} a$ and $m$.
For each section of the line, you must specify the VIVA section type and its properties. The OrcaFlex line type and length of the sections are displayed for information, but they are not editable here (they may be edited on the structure page on the OrcaFlex line data form).
The VIV diameter specifies the diameter used by the VIV model. Separate values can be specified for each section. The value specified is used for all nodes in that section. For a node at the intersection of two sections the VIV diameter of the following section is used. The VIV Diameter can be set to '~', which is taken to mean 'same as the section normal drag diameter'.
The VIVA section type may be one of the following:
For details of these VIVA data see the VIVA documentation.