VIV Effects |
 
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VIV Effects |
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This section explains the automatic interface to Shear7, which allows you to iteratively perform a VIV analysis of the downstream structure as part of the overall wake interference computations. This produces enhanced drag coefficients may be more accurately determined based on the reduced current profile.
You can include the effect of VIV-enhanced drag on one or both of the upstream and downstream structures using the VIV Drag facility described earlier (refer to VIV Drag for further details). However, Flexcom provides a further level of sophistication for the downstream structure analysis, as follows.
In performing a Shear7 analysis of either structure, you input a current velocity distribution, which is generally the ‘undisturbed’ current distribution you also input into Flexcom. In addition, the modes shapes generated by Flexcom for input to Shear7 might (optionally) have been calculated on the basis of this current velocity distribution (and the resulting distribution of tension in the structure). You can tell Flexcom to repeatedly recalculate the modal solution for the downstream structure and to rerun Shear7 based on this solution as the wake interference analysis proceeds, to reflect the actual current velocity ‘seen’ by that structure and the resulting tension distribution in the structure. In Flexcom parlance this capability is termed VIV Effects, to distinguish it from the VIV Drag capability described earlier.
To summarise, the procedure is as follows:
1.At each iteration at each solution time, once the static configuration is determined, the program automatically performs a modal analysis of the downstream structure (or of both structures if you are using a combined model).
2.The results of this analysis are used to generate an mds file for input to Shear7.
3.Flexcom automatically executes Shear7, and reads the drag amplification factors from the Shear7 output file.
4.The drag amplification factors are used to compute enhanced drag coefficients for the downstream structure.
5.The program solves again to determine the downstream structure static configuration, and so on until convergence is achieved at all solution times.
For Flexcom to be able to operate in this way, you need to prepare (in advance) a modal analysis input file with the relevant data for both the modal analysis itself and the Shear mds file generation. The name of this file forms part of the VIV Effects specification. If your two structures are in the same model, you may need to give careful consideration in preparing this file, to ensure that Flexcom includes the correct modes in the Shear7 file.
It is worth noting that this process is computationally intensive and greatly increases the run-times for wake interference analyses. Also, in order to include the effect of VIV on your downstream structure analysis in this way, you must have Shear7 installed on your PC. Shear7 is not supplied with Flexcom.
•*VIV EFFECTS is used to instruct Flexcom to continuously run Modes/Shear7 analyses of the downstream structure during the wake interference analysis.