Enforced motion and nodal force locations

If you use the Mode Acceleration data recovery method, enforced motion excitations, or DDAM excitations, you must specify the locations where you will excite the structure before you solve the solution. These excitation locations determine where Simcenter Nastran creates attachment and constraint modes.

Note:

If you use the Mode Displacement data recovery method, you can apply excitations to any degree of freedom; there is no need to define excitation locations.

You select the geometry (polygon edge, curve, face, and so on) on which the excitation will take place, and define up to six component directions in which it will be applied. In the solver, each Force location and Enforced Motion location consists of the nodes associated with the geometry that you select as well as the component directions that you select.

If you do not apply actual excitations to the excitation locations, the response evaluations will treat them as free degrees of freedom with zero external forces applied to them.

Excitation location type	Description
Nodal Force Location	A nodal force location is required if you plan to use Mode Acceleration as the Analysis Type in your event. When you solve the solution, for each node in the nodal force location, an attachment mode is generated for each DOF set to Nodal Force. For example, if your nodal force location has 5 nodes and DOF2 and DOF3 are set to Nodal Force, 10 attachment modes are generated.
Enforced Motion Location	An enforced motion location is required if you want to apply an enforced motion excitation (displacement, velocity, or acceleration), velocity impact excitation, or DDAM excitation. It is also required for evaluating transmissibility. Enforced motion is typically used for calculating response to ground motion, such as a seismic analysis. When you solve the solution, for each node in the enforced motion location, Simcenter Nastran generates a constraint mode for each DOF set to Enforced. For example, if your enforced motion location has 5 nodes and DOF2 and DOF3 are set to Enforced, 10 constraint modes are generated. Note: For any DOF not set to Enforced (that is, “free”), you must fix that DOF with another constraint, such as with a User Defined constraint.Constraint modes capture the base motion in the directions you define for the enforced motion excitation. Also, an equivalent attachment mode is generated for each enforced DOF.

Note:

To identify a scalable distributed-load excitation (such as dynamic pressure) for either mode displacement or mode acceleration dynamic calculations, you must define loads in the Dynamics Subcase when you define the boundary conditions for a response dynamics solution. Each dynamic load generates a load set, which determines the load distribution that you can scale to define a transient, frequency, or random excitation. The solver calculates attachment modes and load distribution results for each load set.

Where do I find it?

Enforced motion location:

• Choose Home tab→Loads and Conditions group→Enforced Motion Location .

• Simulation Navigator, right-click Constraints → New Constraint → Enforced Motion Location

Nodal force location:

• Choose Home tab→Loads and Conditions group→Nodal Force Location .

• Simulation Navigator, Dynamics subcase → right-click Loads → New Load → Nodal Force Location

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Enforced motion and nodal force locations, Simcenter 3D 2021.1 Series

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