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Comparing Time Domain and Frequency Domain Results

Posted Aug 23, 2024, 12:16 p.m. EDT Structural & Acoustics, Acoustics & Vibrations, Structural Dynamics 5 Replies

FDZR
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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

5 Replies Last Post Sep 9, 2024, 1:59 a.m. EDT
Acculution ApS Certified Consultant
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Posted: 1 year ago Aug 26, 2024, 6:21 a.m. EDT

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?
FDZR
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Posted: 1 year ago Aug 27, 2024, 4:17 p.m. EDT
Updated: 1 year ago Aug 27, 2024, 4:18 p.m. EDT

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: 1 year ago Aug 30, 2024, 4:24 a.m. EDT

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.
FDZR
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Posted: 1 year ago Sep 6, 2024, 2:23 p.m. EDT

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.

Thank you very much for your reply.

Actually I removed PML and used Low-Reflecting boundary condition and by that I realized that frequency domain and my time dependent study are more compatible for a range of frequencies. I also compare both of them for frequency domain and I saw perfect match particularly at low frequencies. However, I see in different sources particularly COMSOL documentation that PML can be used for transient problem. I'm a little bit confused about that. I noticed that changing parameters doesn't change and improve results when I use PML.

For the PML I'm using Polynomial with setting parameters of 1, and 3. Also I'm using more than 8 mapped elements for this layer.

>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. Thank you very much for your reply. Actually I removed PML and used Low-Reflecting boundary condition and by that I realized that frequency domain and my time dependent study are more compatible for a range of frequencies. I also compare both of them for frequency domain and I saw perfect match particularly at low frequencies. However, I see in different sources particularly COMSOL documentation that PML can be used for transient problem. I'm a little bit confused about that. I noticed that changing parameters doesn't change and improve results when I use PML. For the PML I'm using Polynomial with setting parameters of 1, and 3. Also I'm using more than 8 mapped elements for this layer.

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Acculution ApS Certified Consultant
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Posted: 1 year ago Sep 9, 2024, 1:59 a.m. EDT

Hmmm, maybe submit a support case. If the option for PML is there, and you are not seeing expected results, it would be good to know what is going on.

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Hmmm, maybe submit a support case. If the option for PML is there, and you are not seeing expected results, it would be good to know what is going on.

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