Negative Contact Reactions |
 
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Negative Contact Reactions |
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For the case of nodes which already have boundary conditions applied due to rigid seabed contact at an earlier solution time, then as part of the seabed monitoring process, Flexcom checks the value of the reaction in the normal direction. If the reaction is positive, this means the node is “pressing down” on the seabed and the boundary condition should be retained. If on the other hand the reaction is negative, then the seabed is “holding onto” the node and the boundary condition should be removed. Again this process is done prior to solving the equations of motion, and again a situation can occasionally arise that convergence is deemed to be achieved when a reaction at a seabed node is negative, and the node would be released if a further iteration were to occur.
You can use the Negative Reaction Threshold entry to prevent this occurring. If you input any threshold value, it is an instruction to Flexcom to check for negative reactions at all seabed nodes as part of the convergence checking after solving the equations of motion. Furthermore, if the magnitude of any reaction is greater than the threshold you specify here, then convergence cannot be deemed to have been achieved, regardless of the maximum convergence measure calculated in the normal fashion. The rationale for specifying a threshold is to prevent unnecessary iterating in the case for example of infinitesimal negative reactions. Note that the threshold value you specify is a magnitude – that is, it should be a positive rather than a negative number.
•*TOLERANCE is used to define the analysis convergence tolerance measure and related data, including the Negative Reaction Threshold input.