Learning Forum

[selenaii]

I'm trying to perform transient analysis structure for wing and found out that for modal analysis, needs to be carried out to find frequency of interest. I can't figure which frequency to use tho as there are 6 modes and they can be increased.

[peteroznewman]

When you create a Modal analysis, the default number of modes requested is 6 but you could request 50 or 100 modes.
The highest frequency of interest depends on the purpose of the transient structural analysis.
Why are you doing a transient analysis?
What is the input load to the transient analysis?
What is the frequency content of that input load?

[selenaii]

Hey, so it is my final year project, I have to design a eing for MALE UAV. The input load is from FLUENT by performing flow over wing simulation. i have imported that load to structural analysis. My static structural analysis are done and now I need to perform the transient one.
In Fluent, I created the max design condition for the wing and then took that pressure profile and observed wether my recommended fibre scheme for compostie wing eps cored can work under those conditions.
It works for static and I need help to perform transient.

[peteroznewman]

Okay, you are well along the way. I understand the 1-way FSI where you pull pressure from a Steady-State Fluent solution and apply that as a load in a Static Structural model.
Are you proposing a 2-way Transient simulation where on each time step, the pressure from Fluent will be sent to Transient Structural, and the deformations from that will be sent back to Fluent? That is possible on a Research license. You cant do 2-way FSI on the Student license. Are you on a Research license?
If the Fluent Transient model starts from the Steady-State solution, what is the disturbance that will cause vibration?
For small linear elastic deformations of the wing in Transient Structural, it is best to have a Modal solution linked into the Setup cell of the Transient Structural analysis.
Perform the Modal analysis and request 200 modes. Look at the animation of each mode. At some high mode, rather than the whole wing moving, a panel between the ribs will start to participate. Do you need this to be included in the fluid flow simulation?
There is a practical consideration for how high the modal frequency you can afford to include in the simulation. If you want to include a mode at 500 Hz, you need 20 time steps to represent that motion, so the time step would have to be 1/10000 or 0.0001 seconds. If the first mode of the wing was 10 Hz and you want the simulation to run for 100 cycles of the first mode that means the simulation needs to have an end time of 10 seconds, which means the simulation will need 100,000 time steps. How long does one time step take to solve the Fluent model then the Mechanical model? If it is 1 minute, then the simulation will end in 69 days!
Disclaimer: I don't do 2-way FSI, so the above is an example of the kind of evaluation you might want to make, and in addition to the time, there is a consideration of disk space.

[selenaii]

It is a 1 way FSI. I will try what you are suggesting regarding the modal analysis one.