## General Mechanical

#### What is the best Element type to use for a Pipe conveying fluid with a crack model on Ansys WB?

• ZaidH
Subscriber

I am trying to model a pipe conveying fluid with different crack depths and locations, changing the depth should show a decent amount of change in the frequency but in my case there is barely any change in the frequency the deeper the crack is.. I assume it could be because of the element type? If anyone can help with this or any other suggestions other than Element Type I would really appreciate it.

So if its the element type what is the best one to use, if not what could be the reason for the almost no change in frequency while the crack depth is increasing?

• Erik Kostson
Ansys Employee
Hi

Assuming here you want to study the change in eigen-frequencies (modes) of a 3D structure with 3D elements (which is the correct way of doing this). The solid186/187 family so the ones we mesh with in a Modal System in mechanical are fine then.
So in theory it should be possible to predict some changes in the frequencies of the crack/notch depth increases - say the extreme case where the crack is completely through the whole thickness in the centre (along length) of a cantilever beam, then the cairlever (that is now split in the middle) will have much higher frequency since its length is half now (since it is cracked at the middle).

To get some confidence in the modelling, one can try and replicate some results from papers or studies online:

All the best

Erik
• ZaidH
Subscriber
Thought my model is pipe and conveying fluid, I have to use Fluent-> Static Structural -> Modal, using the simply supported boundary conditions, everything starts off good.
Comparing my results with one of the studies was kind of accurate till we reach higher crack depths, since while I am modelling it the natural frequency barely decreases but in the article it decreases way more.
So if I understood you correctly the element type used automatically is the best for the model, what could be causing this issue?
• Erik Kostson
Ansys Employee
Hi

I think the standard 3D elements (18X are those to use for the structure yes.

It is not possible to say exactly why this is .

Can you though explain what the aim is of the study, that is do you want to see the change in eigenfrequency due to crack depth? Also why then do you need the fluid flow (to account for the fluid mass we can use acoustic elements)?

Also seems to be doing similar thing perhaps you can share some experience and help each other out since we (ansys employees) cannot really help out with why the model is inaccurate really and why it deviates from tests (one would need to see and understand the model and the measurements tests one compares with and we can not really do that - beyond this forum which is to help and give some modelling advice which is here that you are using the correct elements)
• ZaidH
Subscriber
ITs a test to see the crack effect on a pipe that has a fluid flow with different speeds, using fluent to transfer the pressure from the flow into the pipe.
Just the fact that the eigenfrequency isn't changing by much while the crack is almost cutting through the pipe is the issue I can't find an answer to and to be honest not sure where to look for an answer for this.

• ZaidH
Subscriber
Do you have any suggestion where I can find an answer to why my model isn't changing the natural frequencies with the crack depth increasing ?
• Erik Kostson
Ansys Employee
Hi

Unfortunately I do not (what you are doing is quite difficult I would say and can be difficult to get help ) perhaps another user as suggested who is doing something similar can provide some ideas and you can discuss and help each other out perhaps.

As suggestion one can list out all of the possible differences that might cause this discrepancy and then try to investigate them one by one - that is all I can suggest.
All the best

Erik
• ZaidH
Subscriber