Comparing Time Domain and Frequency Domain Results

Fatemeh Delzendehrooy

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Hello,

I'm simulating a linear phononic crystal using a time-dependent study and a frequency domain study. For the time domain, I'm applying a sin displacement as u=u0sin(2pif0t), and for the frequency domain, I apply predescribed displacement of u0.

To convert my Time Domain results into Frequency Domain, I used FFT. However, when I compare the amplitude of output displacement from the Time Depependt + FFT study, it is very different with the Frequency Domain at the exact input frequency (f0). For Time Dependent + FFT study, I'm using the summation of complex amplitudes at harmonic frequencies where each amplitude is calculated as follows: sqrt(real(comp2.uo)^2+imag(comp2.uo)^2)

I would be grateful if someone could guide me regarding this issue.

Thanks


3 Replies Last Post 2024年8月30日 GMT+2 10:24
Acculution ApS Certified Consultant

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Posted: 4 days ago 2024年8月26日 GMT+2 12:21

Not sure, but in the time-dependent study you have a real signal, so should you not look only at the real part of the complex signal? Do you have enough signal time to get rid of the transient in the time-domain, to get to the steady-state behavior, as to only include that in your FFT?

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Not sure, but in the time-dependent study you have a real signal, so should you not look only at the real part of the complex signal? Do you have enough signal time to get rid of the transient in the time-domain, to get to the steady-state behavior, as to only include that in your FFT?

Fatemeh Delzendehrooy

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Posted: 3 days ago 2024年8月27日 GMT+2 22:17
Updated: 3 days ago 2024年8月27日 GMT+2 22:18

Thank you for your reply,

In calculation of my FFT output I consider both real and imaginary part of solution.

I'm considering results at least for 50-100 cycles

I removed PML and noticed that Frequency Domain and Time Domain results match with each other, But with this method the output amplitude is always higher than input which basically gives no transmission loss. For definition of PML I'm using the same criteria mentioned in https://doc.comsol.com/6.2/doc/com.comsol.help.aco/aco_ug_pressure.05.134.html#1552378 but I can see that changing parameters doesn't show any change in Time Domain results.

Thank you for your reply, In calculation of my FFT output I consider both real and imaginary part of solution. I'm considering results at least for 50-100 cycles I removed PML and noticed that Frequency Domain and Time Domain results match with each other, But with this method the output amplitude is always higher than input which basically gives no transmission loss. For definition of PML I'm using the same criteria mentioned in https://doc.comsol.com/6.2/doc/com.comsol.help.aco/aco_ug_pressure.05.134.html#1552378 but I can see that changing parameters doesn't show any change in Time Domain results.

Acculution ApS Certified Consultant

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Posted: 10 hours ago 2024年8月30日 GMT+2 10:24

I think you should upload the files. If the results are similar when you remove the PML, visualize the fields and make sure you have propagation in both time and freq domain.

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
I think you should upload the files. If the results are similar when you remove the PML, visualize the fields and make sure you have propagation in both time and freq domain.

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