[Nateqian]

Hello everyone, I have a question about the boundary condition. There is a transient analysis about damping pressure pulsation. The model I analysis is a tube which is expanding in the middle of the tube. The inlet boundary I set is a sine function of pressure. And the outlet boundary I set is still a constant pressure. I want to solve the outlet pressure that to know the rate of pressure decay. I want to ask the outlet boundary I set is right or not. I have attached a file of my model. My English is poor, I don't know what I said is clear or not. Thank you for helping me.

[ghorrocks]

You cannot tell whether your boundary conditions are correct based on what you have said. The boundary conditions are correctly applied if the accurately model the situation in the real system at that point. In the real system what is at your inlet and outlet points?

[Nateqian]

Originally Posted by ghorrocks

You cannot tell whether your boundary conditions are correct based on what you have said. The boundary conditions are correctly applied if the accurately model the situation in the real system at that point. In the real system what is at your inlet and outlet points?

Thank you for your reply. The model I analyzed is a part of the hydraulic system. The upstream of the model is the pump which the inlet connected to. And the downstream of the model is hydraulic tube. The result I want to get is the attenuation of the pressure pulsation. Now I set the inlet boundary for total pressure: 11±1 MPa, and the outlet boundary for static pressure: 11 MPa. During the analysis, I monitor the pressure of the outlet. Then calculate the attenuation by comparing the monitor value with the inlet pressure pulsation.
But I don't know whether the pressure boundary of the outlet can affect the monitor or not. And if the setting is right, I want to know where the monitor point to set up is reasonable.
Waiting for your reply. Thank you very much.

[ghorrocks]

You are using an outlet boundary where you define the pressure and you are using the outlet pressure as the output result. Can you see the problem with this configuration?

Are you doing a compressible flow model? If not why not? Is the geometry deformable? Where does the attenuation come from? Is it just the flow losses?

You need to do a lot of thinking before proceeding with this simulation. In particular:
* You need to consider whether your boundary conditions reflect pressure waves and will ruin your results.
* You need to think about an output result which actually measures something useful.
* This model requires careful validation as it is also a transient model.

[Nateqian]

Originally Posted by ghorrocks

You are using an outlet boundary where you define the pressure and you are using the outlet pressure as the output result. Can you see the problem with this configuration?

Are you doing a compressible flow model? If not why not? Is the geometry deformable? Where does the attenuation come from? Is it just the flow losses?

You need to do a lot of thinking before proceeding with this simulation. In particular:
* You need to consider whether your boundary conditions reflect pressure waves and will ruin your results.
* You need to think about an output result which actually measures something useful.
* This model requires careful validation as it is also a transient model.

Thank you very much. It's really a big help for me. The model's function like the muffler of the car. Now it also used in the hydraulic system. The material of the model is hydraulic oil, so it can be regarded as incompressible. And the the geometry is non deformable. During the analysis, the flow losses can be ignored.
Now I will take your advice to have a careful deliberation.
Can I extend the outlet tube to avoid the interference of the outlet boundary's reflection. Or is there other usefull measure to avoid this?
I know my question may be annoying. I hope you can still give me some advices. Thank you very much. Best wishes!

[ghorrocks]

Your question is not annoying - the forum is here to help people and if it is helping you then everything is good.

The muffler in a car works by using the compressibility of the exhaust gases to absorb some frequencies. You need some form of compliance to reduce the wave. That compliance could come from compressibility of a fluid, movement of the structure or other actions.

[Nateqian]

Originally Posted by ghorrocks

Your question is not annoying - the forum is here to help people and if it is helping you then everything is good.

The muffler in a car works by using the compressibility of the exhaust gases to absorb some frequencies. You need some form of compliance to reduce the wave. That compliance could come from compressibility of a fluid, movement of the structure or other actions.

Sorry for my late reply. Thank you very much for your detailed reply. I will consider this problem carefully.