Timestep Size |
 
|
Timestep Size |
|
Clashing is a complex and highly non-linear phenomenon, so it is imperative that you use a temporal discretisation which is sufficiently refined to accurately capture the occurrence of clashing and the subsequent structural response. It is generally advisable to use a variable time step in analyses that include clashing. This allows relatively small time steps to accurately capture the contact and subsequent structural response, while allowing relatively large time steps to be used during periods where no interaction occurs. Various options relating to time stepping are provided (as described in the following paragraphs), but these inputs are meaningless if your analysis uses a fixed time step. Note also that because relative velocities are required as part of the time step optimisation, these are estimated from displacements if you are running a static analysis.
The Maximum Time Step input limits the time step used during clashing. Naturally this must be somewhere between the (overall) maximum and minimum analysis time steps specified for the analysis, and a relatively small value (for example 0.01s) would generally be recommended in order to accurately capture the relative impact between two adjacent lines. Flexcom continually monitors the relative velocity of approaching lines with a view to estimating a time of impact. The time step is gradually reduced in anticipation of contact, remains at a relatively small value while the lines are in contact, and only increases when some finite separation has developed after the lines have separated. The magnitude of this separation is dictated by the Threshold Clearance input. The Threshold Clearance is specified in terms of a percentage of the average contact diameter of the elements which come into contact.
In certain circumstances, it may not be desirable to persist with a very small time step while clashing is taking place, particularly if the interaction between two lines is likely to be constant rather than intermittent. A good example might be a static current analysis where a cross-current loading would tend to induce (and maintain) contact between two adjacent lines. A recommended procedure for performing this type of analysis would be to use a variable time step (whether the analysis is performed statically or quasi-statically), which will allow Flexcom to monitor the relative velocities and reduce the time step in advance of contact. If the lines tend to remain in contact after the initial impact, the time step will continue to be very fine (resulting in a very inefficient analysis). In such situations, it would be advisable to use the Successive Solutions option, allowing the time step to be increased after contact has been firmly established (up to the maximum analysis time step).
•*TIME is used to define time parameters for an analysis.
•*CLASHING SOLUTION is used to specify solution parameters associated with clashing.