End Time Specification
For transient analyses, Mechanical requires the end time specified in the setup to be respected. When coupling participants require their end time to be respected, the maximum allowable end time for the coupled analysis is the minimum of the end time specified by such participants.
Other participant systems, such as Fluent, can run past the end time specified in the setup. These participant systems have no effect on the allowable end time of the coupled analysis.
Ramping of Data-Transfer Loads
Mechanical has two types of ramping that can modify the loads obtained through data transfers in a coupled analysis. The two types of ramping are ramping over substeps,
and ramping over coupling steps. The ramping on your load will be determined by the interaction of ramping settings you have set between Mechanical as well as System Coupling. Both
Mechanical ramping behaviors are controlled by the Solution Command Object KBC, which can be set to 1 or 0.
In a steady-state analysis, the default setting is KBC = 0 (ramping on). In a transient analysis, the default setting is KBC = 1 (ramping off).
In System Coupling, substeps are unique to Mechanical, and are steps within a coupling iteration. Loads can be ramped over these substeps. The ramping factor applied to these
loads is based on the number of substeps, 
. At the 
substep, the ramping factor of 
is applied. This ramping is based on the initial value of the load at the end of the last step. When KBC = 0 and 
, ramping over substeps occurs. If KBC = 1 or 
, ramping over substeps does not occur.
In System Coupling, Mechanical has a second ramping option which ramps loads over the coupling steps. Regardless of what other ramping settings are on, System Coupling always transfers
the full load at the end of the coupling step, and then Mechanical applies a ramping factor to this full value at each coupling step. The ramping factor applied to the full
load at the coupling step is based on the number of coupling steps, 
. At the 
coupling step, the ramping factor of 
is applied. When KBC = 0 and 
, ramping over coupling steps occurs. If KBC = 1 or 
, ramping over coupling steps does not occur. Note that if you set ramping over coupling steps to occur in a transient analysis, loads received from
System Coupling will be ramped over all coupling steps, and so the full load will only be applied at the last coupling step. This situation is not physical, but may still be
useful when using a Transient Structural system to get steady-state results, for example when pre-stressing the structure for a further transient analysis.
Ramping of Loads Within Mechanical
Loads within the Mechanical system (that is, loads that are not transferred to Mechanical through the coupled analysis) are ramped linearly using the Step End Time specified in Mechanical. In a steady-state coupled analysis, ramping of these loads is controlled by the relationship between the Step End Time specified in Mechanical, and the number of coupling steps specified in System Coupling. For a steady-state analysis, each coupling step in System Coupling corresponds to 1 s of time in Mechanical.
When the number of seconds set for the Step End Time in Mechanical equals the number of coupling steps set in System Coupling, the load is ramped linearly across all steps in the coupled analysis.
When the number of seconds set for the Step End Time in Mechanical is less than the number of coupling steps set in System Coupling, the load is ramped linearly to the coupling step that matches the end time, and then the full load is applied for the remaining steps.
When the number of seconds set for the Step End Time in Mechanical is more than the number of coupling steps set in System Coupling, the load is ramped linearly, but it will not reach its full value. The final value applied will be the ramped value that corresponds to the last coupling step, which may cause inaccuracies in your simulation.
If Mechanical's Step End Time is set to
1s, this ramping will not occur.
Note that Mechanical's computational end time and its load-based end time are independent. The computational end time is equal to the number of coupling steps. The load-based end time controls the ramping behavior, and is set by the Step End Time option in the Mechanical interface.
Output Controls
When Mechanical is connected to System Coupling, behavior of the Output Controls is changed. For a normal Mechanical run, the Store Results At settings are applied per step and Specified Recurrence Rate is defined as the output frequency within a step at some substep frequency. When the Mechanical application is connected to System Coupling, these settings are applied across all steps, not within one step. This means that you cannot output results within a step. System Coupling simulations may run many steps, so these settings should be used to reduce the results frequency as needed.
For more information about how to use Mechanical for FSI analyses, see Fluid-Structure Interaction (FSI).