[alphaduykhang]

Hi, i am trying to simulate squeeze film damping using cfx in ansys. But when i find some help in ansys, i only see squeeze film using cfd!!! So, with cfx, how can i simulate the air acting with both stiffness and damping effect? In cfx, I use the ideal gas, laminar flow, iso-thermal, no slip flow occur at the planar boundaries conditions, but when i take result, the natural frequency in ambient air is lower than the natural frequency without air!!!! How it can be? My model acts as it isn't any stiffness.
Please help me.... This is links my project with cfx.
https://www.dropbox.com/sh/z32o1pi93...xXP-_PiDhXO0Ha

[ghorrocks]

What do you mean by a simulation with air? Is there air entrained in the oil? Or is the oil cavitating? Or is the bearing running with air as the working fluid?

[alphaduykhang]

The bearing running with air as the working fluid, but the air only has damping but it doesn't have spring coefficient, which is necessary for squeeze film. I was try many times but it is the same!!!!!!!!!!

[ghorrocks]

CFX is a Navier Stokes solver - so it is modelling the detailed flow of the air. If you are not getting the expected behaviour in the fluid then you have not set it up in a configuration which results in spring behaviour.

I do not have time to look at your dropbox project. Can you post some image of what you are modelling and the output file?

[alphaduykhang]

Their are some images in cfx ... I tried with this problem 1 month, but i can't answer where is wrong?










You can also find them here: https://www.dropbox.com/sh/pxbkkwsdl...QKETgE9MxNvh0a

[ghorrocks]

You are not going to get much fluid motion which generates stiffness or damping with a model like that. Don't you need to add some of the inlet and outlet end, and the ends as well?

[alphaduykhang]

Yah, I see your advise, but when i change the force to increase the displacement as increase damping and stiffness, and the result doesn't change: The natural frequency in the ambient air doesn't have air stiffness.
So, I am crazy with model ... please help me....

[ghorrocks]

I do not think you understood my post. The stiffness of the bearing comes from the air being constrained in the pressure region - so you need to constrain the air in the same way as the real bearing. This means the correct inlet and outlet conditions, and the side conditions correctly formed. Likewise for the damping - this will come about from the air moving in and out of the pressure region. So you need to model the region on the side of the bearing where the air can escape to.

[alphaduykhang]

Yes, I don't understand because i use opening condition for air film. Does it mean that air can move in or out as inlet or outlet?

[ghorrocks]

Yes, an opening allows air to move in and out. But it is the resistance/inertance to this motion which creates the springiness and damping. You cannot easily put these higher order flow features in a simple boundary condition. That is why you need to move your boundary conditions outwards to somewhere the conditions are more easily specified.

[alphaduykhang]

Sorry ghorrocks, I delay responding your message because i tried to add your inlet and outlet in my model. But, i think it doesn't work. When i insert the inlet and outlet as your suggestion, the natural frequency in ambient air decrease more than before@@
I tried change boundary conditions, but results are same@@ Is there any guide for me? thanks....
I tried to find some example about squeeze film in cfx, but i surprised when i couldn't find any thing...