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Goal of this tutorial
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Step 1: MSC Nastran
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preparation
To be able to use the Eigenfrequency Optimisation a MSC Nastran version with a valid license must be installed on the same machine.
The
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path to the MSC Nastran executable can be set
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manually in the application settings:
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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 |
Step 2: Open an already existing optimisation model / Set up a model as you are used to
The Eigenfrequency Optimisation is activated additionally to static events. Thus, you the model can set up your model as every other optimisation and before you start the optimisation the Eigenfrequency Optimisation gets activated with the Advanced User Settings.
For this Tutorial a beam model is used. The model is already completely set-up (Design Space, Interfaces, Loads & Boundary Conditions, Material assignment, optimisation parameters and static Events) for a static optimisation and in the following the Eigenfrequency Optimisation gets activated.
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Step 3:
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The following 3 Advanced User Setting commands are mandatory to start a Eigenfrequency Optimisation. With these a additional event for the Eigenfrequencies is added.
event.EventFA.strategy=conditionBasedFrequencyAnalysis event.EventFA.frequencyTarget=210 event.EventFA.safetyFactor=1 |
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Frequency Event Creation
A frequency Event should only consists of displacement constraints without any loads or moments.
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Step 4: Frequency Constraint
As soon as one Event exists with only displacement constraint the frequency constraint can directly set in the GD Scenario:
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In this case the Frequency Constraint is set to 210 Hz as the target for the first occurring eigenfrequency of the part. This value need to be adjusted to your according requirements of the part. As an optional input the range and the number of frequencies which are taken into account for the optimisation can be modified. By Per default the first 10 eigenmodes are considered to an upper frequency limit of eigenfrequencies in a range up to 3 times the frequency target.
event.EventFA.frequencyMax=630 |
Step 4: Advanced User Settings to copy Boundary Conditions from an already existing Event
Since the Eigenfrequency Optimisation isn’t integrated into the GUI yet, Boundary Conditions (Fixations) can be copied from a static event. Therefore, the following command needs to be added:
event.EventFA.condition.\.include = event.Event_1.condition |
Event_1 is the static Event from which the BCs will be copied.
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constraint value are considered. Based on these eigenmodes the part is stiffened to push the the first eigenfrequency upwards.
Step 5: Starting the Optimisation
The optimisation can be started as usual. In the background MSC Nastran will be called to calculate the eigenfrequencies and eigenmodes.
Step 6: Post Processing
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Currently, the influence of the eigenfrequency calculation is displayed in the enveloped plot of the Failure Criterion (even if this isn’t based on a stress value)
In the Post Processing the first eigenfrequency for each iteration can be displayed in a chart.
The optimization achievement index for the frequency Event (Event_2) shows the influence of the eigenfrequencies on the design creation. Purple to red areas show the areas which need to grow to increase the eigenfrequency of the part.
To check the final eigenfrequencies of the part, please run a Re-Analysis with Apex structures.
Creation of a Point Mass
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Step 7: Creating Point Masses
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for the Eigenfrequency Optimisation
Point masses Masses play an important role in the calculation of eigenfrequencies. Therefore, Point Masses can be created with the Advanced User Settingsdirectly in Apex GD.
A In this case a Point Mass can only be attached to surfaces where a Force is already applied to. The geometry name has to be linked correctly. Furthermore, the position has to be entered with three values (x,y,z) regarding the global coordinate system (These can be also reused from a remote force).
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Advanced User Settings to create a Point Mass:
pointMass.pointMass1.geometryName=ModelLoadEvent_Force_-_Moment_1 |
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Be aware that the Unit System in the Advanced User Settings is SI. Thus, you have to enter the position of the point in meters and the mass in kg! |
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Step 8: Modify the static load
If acceleration is used, this may result in loads being considered twice. Modify the Force to a very low value, thus the force is close to no impact. The acceleration acting on the Point Mass will automatically generate a static load.
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of 0.1 kg is attached to the same spot (0; 402.50; 25.00) as the remote Force.
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By default it is connected with a compliant connector (RBE3). Point Masses are after the creation automatically considered for a frequency optimisation. They are also considered for static load cases if an acceleration is active. For Frequency Optimisations the connector can also be changed to a rigid one (RBE2) with the Advanced User Settings.
Please have a look at all known limitations!
The complete MSC Apex GD project with all results can be downloaded here:
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