SimScale CAE Forum

Confused about Frequency Analysis of an Airfoil

I have just had a look at the “Frequency Analysis of an Airfoil” example, after trying to simulate a cantilever beam to find its harmonic frequencies. So the setup seems fine, you choose the material properties and the fixed supports. There is no need to choose a force because natural frequency is not dependent on driving force. Now comes the results. It shows a displacement magnitude, (seems to be for a 4th harmonic), despite displacement being a function of force and not something universal. Furthermore, where are the harmonic frequencies? I thought that is what the simulation does, calculates the harmonics.

https://www.simscale.com/docs/tutorials/frequency-analysis-of-an-airfoil/

The simulation description says, Determine the natural frequencies of constrained or free structures. The results include eigenfrequencies and eigenmodes.

Hey @roy_g!

Are you only concerned about the displacement values? Do not pay too much attention on that as the values are random and have (AFAIK) no physical meaning whatsoever.

What you usually do is to perform a frequency analysis to get the natural frequencies of the your model and then use the harmonic analysis module and specify a range of interest (other approaches possible) to see where you have a large displacement for instance. Does that somehow answer your question?

If I missed something or explained something wrong, I am tagging the FEA experts @fea_squad as well as my colleague @rszoeke here.

All the best,

Jousef

@jousefm I could not find that natural frequency in the results. Where is it stated?

Hi @roy_g,

After you run the frequency analysis you find the frequency results in the log file. The results will be listed in the middle of the log not at the end (as some might expect).

Simscale creates a plot (graph) of the natural frequencies. The graph really is not useful but you can get the frequencies off the graph.

In the solution fields you can plot the mode shapes. Select the Displacement and then animate the displacements. Each step of the animation is a different frequency. Remember that the displacement values show the correct shape but not the correct magnitude.

I hope this helps.
Christopher

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Just an update, I calculated the natural frequency of the cantilever using a hand calculation, there is a 10% difference between the hand calculation and the simulation, which I guess is reasonable.

Question @cjquijano, say the beam cross section is 8x5 mm, how can I choose whether the vibration is in the direction of the 5 mm or the 8 mm? These are two distinct modes of vibration but the frequency analysis does not let me choose a direction (or a steady state load).

Hi @roy_g,

I am not sure what you mean by “how can I choose whether the vibration is in the direction of the 5 mm or the 8 mm?”

When you perform a frequency analysis you typically calculate multiple frequencies (the first 10 for example). For a simply supported rectangular beam the first frequency will be simple bending about the thinnest cross section. In the image below that would be F1. The frequency to bend in the F2 direction will be higher. Along with the mode shapes associated with F1 and F2, you will also have torsional modes shapes as well as higher order bending mode shapes.

image

To view your mode shapes for the computed natural frequencies you can do that in the post processor. The image below shows the steps to view the mode shapes in the post processor. You will need to set the default scaling factor for the deformation to a small number… try .1 for example.

In your post you mention a frequency analysis with a steady state load. It is my understanding that this capability is not available in Simscale at this time.

Did this help at all?
Christopher

2 Likes

Thank you!

Hi…It just gives you the recurrence at which the mode happens, and the state of the mode. The genuine relocations rely upon the info excitation, as has been said beforehand in different posts. In a hypothetically frictionless/dampingless world the relocations would be limitless, in light of the fact that by definition, these are the full frequencies of your framework and all excitation at that recurrence gets enhanced. By this very reality, under arbitrary vibration, the framework inclines toward these frequencies. Also, the main two material properties that matter in this computation is the Young’s modulus and thickness.