Frequency Response Analysis

Brian_20847

Hi expert,

I want to ask about Frequency Response Analysis. What is the difference between Direct Frequency Response and Modal Frequency Response? When should use it?

viniciusgomes

Hello Pham Tuan, 

The Modal FRF uses normal modes for calculation and is recommended when many different excitation frequencies are required and the Direct FRF solves directly the equation of motion, and it is more efficient when you need to cover a smaller frequency range. 

Please refer to the attached document for more details. 

Thanks,
Vinicius

viniciusgomes

Hello Pham Tuan, 

The Modal FRF uses normal modes for calculation and is recommended when many different excitation frequencies are required and the Direct FRF solves directly the equation of motion, and it is more efficient when you need to cover a smaller frequency range. 

Please refer to the attached document for more details. 

Thanks,
Vinicius

Brian_20847

Thank you for your information. I am doing Direct Frequency Response Analysis of a Flat Plate Tutorial in version 2022. I can't choose SID. In addition, can you explain more detail about LANCOZ method and AMSES method?

viniciusgomes

Hello,

 

Can you please check if the card image of the set you have created is defined as SET_GRID? 

You can also define the output requests through the model browser:

Regarding AMSES and Lanczos, in short, Lanczos is the most traditional and industry proven method to get the eigenvalue and eigenvector and is more accurate. AMSES solution is much faster than Lanczos, but the results are approximate. Accuracy of the lower modes is very high; therefore we recommend using AMSES for solutions with a large number of modes and big models (more than 1 million DOFs). 

You can read more details about it in our documentation: https://2022.help.altair.com/2022/hwsolvers/os/topics/solvers/os/solvers_os_intro_c.htm#ariaid-title9 

Thank you,

Vinicius

Brian_20847

Hi Vinicius,

Thank your for your information.

viniciusgomes

Brian_20847 said:

Hi Vinicius,

Thank your for your information.

Sure, no problem. If possible, please mark the question as resolved to let us know we found the solution and help others.

Thanks!

Brian_20847

Hi Vinicius,

I am doing modal frequency response analysis, I want to have a dynamic force (time-dependent force (Fsinwt)) for unit-load instead of using static force. How to create dynamic force (Fsinwt)? 

Thank you.

viniciusgomes

Brian_20847 said:

Hi Vinicius,

I am doing modal frequency response analysis, I want to have a dynamic force (time-dependent force (Fsinwt)) for unit-load instead of using static force. How to create dynamic force (Fsinwt)? 

Thank you.

Hello Pham Tuan, 

If you want a time-dependent force, you should not use modal frequency response, but modal TRANSIENT response. This type of analysis is in time domain. The steup in general ir pretty similar to the freq. response with only a few different cards. The TABLED1 card will now represent the variation of the force in time domain. Please check the attached material, there is a presentation and a tutorial for this type of analysis. 

Thank you,

Vinicius

Brian_20847

Vinicius Gomes said:

Hello Pham Tuan, 

If you want a time-dependent force, you should not use modal frequency response, but modal TRANSIENT response. This type of analysis is in time domain. The steup in general ir pretty similar to the freq. response with only a few different cards. The TABLED1 card will now represent the variation of the force in time domain. Please check the attached material, there is a presentation and a tutorial for this type of analysis. 

Thank you,

Vinicius

Hi Vinicius.

Can you explain in more detail several types of damping and methods? I attached an image of this issue!

Thank you.