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*Clashing |
 
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*Clashing
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*Clashing |
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To specify regions where clashing may occur, and suitable contact stiffness and damping values.
Refer to Line Clashing for further information on this feature.
A single line of data which may be repeated as often as necessary.
FIRST=First Set Name, SECOND=Second Set Name, STIFFNESS=Stiffness [, DAMPING=Damping]
Damping defaults to zero if omitted.
Input: |
Description |
First Element Set: |
The first element set to be monitored for clashing. Clashing may occur between this set and the corresponding second element set. See Note (a). |
Second Element Set: |
The second element set to be monitored for clashing. Clashing may occur between this set and the corresponding first element set. See Note (a). |
Contact Stiffness: |
The contact stiffness used in the clashing model. This input is mandatory. There is no default value. See Note (b). |
Contact Damping: |
The contact damping. This input is optional. By default, no damping effect is modelled. |
(a)Monitoring of clashing introduces a significant computational overhead, as all elements in the first set are checked for contact with all elements in the second set, for all the clashing regions that you have defined. When defining contact regions, you should be careful that you specify reasonable data to prevent excessive runtimes, by ensuring that the program is not checking for contact between points that will never approach. It is generally recommended that similar spatial discretisations be used for any element sets which may come into contact with each other. The choice of which element set is designated as first or second should be irrelevant – it is merely a convenience for the internal operation of the contact modelling algorithm.
(b)It may be difficult to quantify what value of contact stiffness represents the physical reality. Generally speaking, the higher the stiffness value chosen, the greater the impact will be, and the smaller the time step that will be required for a robust solution. So you should avoid using excessively high stiffness values, which may lead to convergence difficulties. Conversely, the stiffness value effectively determines the maximum clashing force that may be modelled – the maximum reaction force is equal to the contact stiffness times the average contact radius of the elements which interact with each other. In other words, the maximum relative penetration of the contact elements should not be allowed to exceed the average contact radius. If this occurs, the lines will simply pass though each other. Although in practice this situation would be quite rare, to prevent the situation from occurring, you should ensure that the contact stiffness is sufficiently high. While it is difficult to provide meaningful guidance as to what value might represent a “reasonable” level for all cases, a value of approximately 100 kN/m (or equivalent in Imperial units) is tentatively suggested as a starting point if you are unsure.