[sadra.khodaei]

hi everybody!!
I want simulate a simple straight pipe line that has a water flow,and the result I need to be extract:
1.deformation becouse flow
2.modal solution(for structure)
3.harmonic solution(for structure)
is it kind of FSi?
I define 2 kind of physics solid and fluid!(in APDL)
but when I read solid physics for modal solution,my results are not different to pipe without flow!!!(modes that extract for pipe,flow and pipe with out flow are the same)

what can I do?

[evcelica]

Sorry I don't fully understand what you are trying to model or how you have tried to modeled it. Did you do a 2-way FSI using CFX and mechanical? Or are you just doing mechanical alone?

Why would the flow cause deformation? Internal pressure? weight of fluid?

Why would the fluid movement make a difference in the natural frequency of the two together in a straight pipe?

[sadra.khodaei]

Originally Posted by evcelica

Sorry I don't fully understand what you are trying to model or how you have tried to modeled it. Did you do a 2-way FSI using CFX and mechanical? Or are you just doing mechanical alone?

Why would the flow cause deformation? Internal pressure? weight of fluid?

Why would the fluid movement make a difference in the natural frequency of the two together in a straight pipe?

1.it's a simple model,a hollow cylinder that one end has no displacment(like a cantilver beam) with water flow in it!
2.both,internal pressure and weight of fluid
3.at the first I analys a pipe alone(without flow) and get some natural frequency and then with physic environment I model pipe with flow,and get exactly the same frequency!it should be change becouse the system changed(water flow added)[am I right?it should be change??]
my question is:my way is wrong?
I think this is 1-way fsi,should I use workbench or I can use APDL like before?
tnx

[evcelica]

You say "flow" but I think you mean with the extra mass of the water which would be stationary in your model?

So it is an open pipe at both ends? One end is fixed and you are solving for the natural frequency with and without the extra mass of the water? Are you actually modeling the water or just putting the force and pressure of the water on the pipe. If the latter, then of course the natural frequencies wouldn't change from without the additional forces.

How are you adding the "flow" or "water"?

[sadra.khodaei]

Originally Posted by evcelica

You say "flow" but I think you mean with the extra mass of the water which would be stationary in your model?

So it is an open pipe at both ends? One end is fixed and you are solving for the natural frequency with and without the extra mass of the water? Are you actually modeling the water or just putting the force and pressure of the water on the pipe. If the latter, then of course the natural frequencies wouldn't change from without the additional forces.

How are you adding the "flow" or "water"?

yeah both ends are open!
the pipe is 200m length that has water flow with 5 m/s velocity,gravity effect on fluid and solid
I didn't add force and pressure of the water on the pipe!is it necessary?

my model is hollow cylinder as a long pipe,and solid cylinder in it as fluid!

[evcelica]

200m!!! Well that's a hell of a cantilever beam!!!! I'm pretty sure natural frequency won't be your main concern if you are trying to support a 200m pipe at one end!

The fluid will change the natural frequency of some of the modes, but not others. For example the mode of the pipe oscillating axially or rotating about an axis probably would not change much. The ones where the fluid is moving as well would change. This sound like a simple problem that can be solved analytically instead of using a very complex 2-way fluid structure interaction involving vibration.

[sadra.khodaei]

Originally Posted by evcelica

200m!!! Well that's a hell of a cantilever beam!!!! I'm pretty sure natural frequency won't be your main concern if you are trying to support a 200m pipe at one end!

The fluid will change the natural frequency of some of the modes, but not others. For example the mode of the pipe oscillating axially or rotating about an axis probably would not change much. The ones where the fluid is moving as well would change. This sound like a simple problem that can be solved analytically instead of using a very complex 2-way fluid structure interaction involving vibration.

I model it in workbench to reduce my problem,and I change the dimmension of the pipe to 100cm,it's a 1-way fsi,
at the first cell is cfx that connected to static stractual that connected to modal analys!!
I import the pressure that cause by flow to the pipe wall,but my problem is changed:modal analys said that you should suppress the fluid body,so if I suppress it,the weight of fluid neglected so it can't change the natural frequency!!! and if I don't suppress it,it take error and said fluid(water) need elastical module to define!!

[evcelica]

Well I don't know the exact way to actually model it, but you can solve the problem quite easily by adding the weight of the water to the density of the pipe material. you just have to keep in mind what modes the extra density would affect (ones where the water is moved as well) and ones where it won't.

This is a CFX forum and your problem is in ANSYS mechanical, maybe some mechanical forums would be more help?