Frequency Optimisation

The eigenfrequencies of a part play an important role in the process of creating a new design.

The eigenfrequency optimisation is added as an new function in this release. The option can be activated for a separate event and therefore additionally to static load cases.

The eigenfrequencies for each iteration of the optimisation process are calculated by MSC Nastran. Therefore, MSC Nastran has to be installed on the same machine and a valid license is required.

The path to the MSC Nastran executable can be set manually in the application settings:

Please use nast20233(.exe) (or the matching executable for your installed nastran version) for frequency analysis.

If you want to run the optimisation also via the command line you need the set up the following environment variable

EMENDATE_SERVICE_NASTRAN

C:\Program Files\MSC.Software\MSC_Nastran\2023.2\bin\nast20232.exe

Initializing the frequency optimisation:

In a first step a frequency Event has to be created. A frequency Event should only consists of displacement constraints without any loads or moments.

The frequency constraint can directly set in the GD Scenario:

Frequency Constraint

The Frequency Constraint (in Hz) is the target for the first occurring eigenfrequency of the part. Per default the first 10 eigenfrequencies in a range up to 3 times the frequency constraint value are considered. Based on these eigenmodes the part is stiffened to push the the first eigenfrequency upwards.

As an optional input the range and the number of frequencies which are taken into account for the optimisation can be modified with the Advanced User Settings.

Post Processing

The calculated first eigenfrequency over the iterations can be observed with a chart in the Post Processing. The frequency Event does not create any visible plots for the Failure Criterion or Stress on the geometry. But the influence of the frequency Event can be observed with the optimization achievement index which shows where material will be added (>0) to increase the first eigenfrequency of the part.

Known Limitations

Symmetry

Symmetry and a Frequency Event cannot be set up together. Some of the Eigenfrequency modes will be asymmetric.

Frequency Constraint

If the Frequency Constraint is higher than the first calculated Eigenfrequency of the whole Design Space in the first Iteration, this can lead to the case that no material will be reduced. This can be resolved by reducing the Frequency Constraint or by entering the following command to the Advanced User Settings:

schedule.iterations.~.operator.optloop_loop_analysis_Event-2_extractLocalFailures.useMetricGoalFactor=1

Point Masses

It has been found that comparable heavy point masses can cause that the optimisation does not achieve the Frequency Constraint. To address this the impact value can be set up to a value greater than 1. This can be done with the following Advanced User Settings command:

event.Event_2.impact=1.5

pointMass.PointMassInertia_Point_Mass_1.type=RBE2

You might also be interested in this tutorial:

Eigenfrequency Optimisation