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What you asked in January 2021

By Chloe Allison 23 February 2021

Welcome back to our monthly review of your support questions! Let’s take a look at the most frequently asked questions in January 2021. 

Mode shapes are a type of output which allows you to extract the steady-state deformation shapes of a model at specified frequencies. 

How do I view/export shape data?

Calculating a Mode Shape

To extract a mode shape, you need to use the shap command. For example:

Set up mode shape

You can extract mode shapes at multiple frequencies for multiple data arrays. 

Export to Post Processor

To view the mode shape you can output the shape data to a *.flxdato to view it in the Post Processor UI. For example: 

Post processor UI

Here is a video demonstrating how to view the mode shape in Post Processor.

Export to CSV

It is also possible to output the raw shape data to a *.csv for post processing in other applications. The data can be exported in the real and imaginary form. For example:

mode shape

The asterisks at the end of the cddo subcommand indicate the ijk region of the model that the shape data is to be extracted for. So, it is possible to only export the shape data for a specific area within the model. Inserting asterisks for the ijk region sets it to default which is the whole model. 

The shape data can also be exported in magnitude and phase. To do this, you must first covert the real and imaginary data to magnitude and phase using the polr subcommand. For example:

Displacement mode shape

How do I view what is happening inside a 3D model?

If you have a 3D model but are interested in viewing the wave propagation inside of it, there are a couple of methods to do this. You can view the inside of the model during runtime or in post process.

You can see the wave propagation inside as the model runs by plotting the required data array on a slice of the model using grph plot.

CAD

You can plot slices of the model using ijk indices, keypoints or co-ordinates. For example:

Frequently Asked Questions

However, remember to set up the number of viewing windows prior to plotting so that you can see all the plots at once (maximum of 8 plotting windows). For example:

You can also extract slices of a model in Post Processor after it has run using the Cutting Plane Tool. However, note that this will only plot the data array at the final stage of execution.

CAD

Why is my simulation producing NaNs?

A common error message that users see is that the ‘Data manager contains bad values’ and there are non-finite values in the mesh. 

Data Manager

This means that the model is going unstable and could be happening for numerous reasons:

  1. Bad CAD – If there are gaps in your CAD, this causes void elements in the model that can cause the model to go unstable. Check your CAD to ensure there are no gaps.
  2. Bad elements / Coarse meshing – Coarsely meshed models may have bad elements that cause instability. Preview your model to check the structure is accurately represented.
  3. Absorbing boundaries – Absorbing boundary conditions need to be placed at least 3 wavelengths away from the source to be effective. Try moving the absorbing boundary condition further away or try switching to impedance-matched boundaries.
Chloe Allison
Chloe Allison

Chloe Allison is an Application Engineer at OnScale. She received her MA in Electrical and Electronics Engineering from the University of Strathclyde. As part of our engineering team Chloe assists with developing applications, improving our existing software and providing technical support to our customers.