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Modal Analysis Introduction – Solve Feature Update

By Cyprien Rusu 25 May 2021

Modal Analysis is now available in OnScale Solve!

We’re very excited to tell you about why modal analysis is essential to understanding the vibration characteristics of your mechanical systems, and its critical nature in holistic dynamic simulations/testing.

Let’s start by understanding what modal analysis does, and then perform such an analysis in OnScale Solve.

 

What is modal analysis?

In finite element analysis (FEA), engineers divide simulations into 2 large categories: static simulations and dynamic simulations.

While static simulations allow you to understand a system when it doesn’t move and is at rest, dynamic simulation allows you to understand its behavior when it moves or the load applied to it changes depending on time.

Consider complex mechanical systems like cars or pumps, for example. Their main usage is inherently dynamic! Thus, engineers need to understand what happens when their systems are subjected to vibrations of time-dependent loads.

Understand the Vibration Characteristics of Your SystemAll systems have specific frequencies where they create detrimental vibrations. We call them natural frequencies.

Those frequencies are very special, because once your system is excited to one of their natural frequencies, it enters into resonance. Here, it starts to vibrate more and more, and we risk the system breaking.

Of course, no one wants that to ever happen! So, that’s why engineers calculate natural frequencies in order to better avoid them.

In some cases, the whole system requires redesigning because of a nasty vibration, causing a potential destructive resonance. Luckily, we have tools now to find these points of failure long before production, and, surprisingly, without ever making a physical prototype.

 

Now, can you guess what modal analysis does?

Yes! It calculates these natural frequencies!

Most vitally, you get a list of all the natural frequencies that will make your system vibrate as the first output of a modal analysis.

But that’s not all!

Modal simulation also provides the mode shape for vibrations of your system.

A mode shape for a natural frequency is the visual shape of how the model would vibrate and deform at that specific frequency. OnScale Solve produces easy to understand mode shapes to help you quickly paint a picture of the vibration characteristics – unleashing the full potential of your beast of a design.

Pump HousingNow, some beginners have a typical misunderstanding. They think that those mode shapes display the actual calculated deformation that the system will experience. In their mind, this provides a certain deformation value that they can use to assess the displacements and stresses that will happen.

This is not the case!

This is very important: Modal analysis only provides the list of modes and an idea of how the system would deform at those modes, but it doesn’t actually calculate the deformation.

This technicality arises because we apply no load in a modal analysis, only restraints. Therefore, there is no physical way to make the model actually vibrate in the first place.

To calculate the actual displacements under a dynamic load which excites the model to one of its natural frequencies, engineers need more advanced dynamic simulations, such as transient or frequency response simulation.

These simulations are now in development and will be coming in OnScale Solve too.

 

How to perform a modal simulation in OnScale Solve

Now that you understand the applications of modal simulation, it’s time to practice and use OnScale Solve to extract the natural frequencies and the mode shapes of a mechanical system!

You can download the model we used here to follow along:

Or copy the following model from Onshape – Model: PumpHousing – Step File

https://cad.onshape.com/documents/4266ddb7ddf182151590b458/w/daaba38ae5f27d65a6e37937/e/c5ba7203627e660d72eeabe4

This is a pump housing, and we want to know its natural frequencies and mode shapes in order to make sure it doesn’t vibrate too much during its operation and cause a catastrophic failure.

Let’s start by importing the model and assigning some material. We’ll use “structural steel“ for the sake of simplicity.

Material Library: Pump Housing SteelAfter that, let’s apply some restraint boundary conditions to fix our pump housing. It is important to fix it under approximately real operating conditions! For the sake of the tutorial, this doesn’t reflect exact real conditions.

Let’s fix, for example, the surfaces at the bottom and on the right of the housing.

 Modal Analysis: BCThere are no loads applied in a modal analysis, so we are set to move past setting the boundary conditions!

Now we just need to go into the simulator tab and into the new Analysis section on the right. Select Mechanical Analysis Type “Modal” and keep the default values. Set to calculate the first 10 modes of vibration. Of course, many more can be calculated using further core hours as required!

Solve: Simulation MenuWe are now ready to perform our modal simulation! Not too bad of a workflow, right?

Mesh, estimate, and then click “run” to launch the simulation in the cloud!

Solve: MeshedOnce the simulation is finished, load the results to obtain the 10 calculated natural frequencies and the corresponding mode shapes.

 

Results and Modes

UI WalkthroughIn OnScale Solve post-process, you can animate each of the mode shapes to understand better how the system would vibrate at each of the 10 calculated natural frequencies.

You can also open the results in Jupyter Notebook if you want to download the result files in VTU format to post-process it more in depth with Paraview.

Modal Analysis: ModesNow you know how to perform a modal analysis with OnScale Solve!

We hope you go and try it on your own, with your own model! If you have any questions, we always are happy to help you or walk you through our latest features!

 

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Cyprien Rusu
Cyprien Rusu

Cyprien Rusu is our Director of Engineering for Asia at OnScale. He has a extensive background in FEM, Technical Marketing, Sales and Support. Cyprien recieved his MS in Civil Engineering from Tsinghua University. At OnScale he is a trusted advisor for our client base in Asia, while creating and providing OnScale training.

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