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Old   October 31, 2018, 05:32
Default Bearing simulation outlet pressure
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Hello everyone.

I have a question regarding my CFD simulation.

My goal is to use the FLUID218 element to get the bearing characteristics of my gas bearings. For that purpose, I need the fluid pressure at the inlet and the outlet of the bearing. Common practice is to use the supply pressure as the inlet pressure and the ambient pressure (1 bar) at the outlet. But I am not sure if this is correct.

Therefore, I set up a CFD simulation of the bearing using measured boundary conditions to check the pressure. To measure the boundary conditions, I took a manufactured bearing and measured the total pressure and the mass flow rate of the fluid right before the inlet.

For my setup in CFX, I used a total pressure inlet and applied the supply pressure of 9.5 bar. Since the mass flow balance must always be constant, I took the mass flow (which I measured right in front of the inlet) and applied it on the outlet as a mass flow boundary condition.

The simulation converges; the RMS residuals are:
Momentum u: 3.2E-06
Momentum v: 4.7E-06
Momentum w: 6.5E-06
Mass: 8.2E-10

But when I look at my results, the pressure at the outlet equals about 5 bar, meaning that the pressure is far from 1 bar. Now I wonder if:
- Is something in my setup wrong (for example symmetry)?

- Is it possible that the pressure does not drop to 1 bar at the outlet (maybe because of the small geometry?

- Am I unable to accurately model the geometry because of the small bearing clearance (15 micron) and large geometry changes leading to a unrealistically small pressure drop?

- Can I use the measured mass flow at the inlet and apply it at the outlet (because of mass conversation)?

Bearing geometry data is as follows:
- Length: 18 mm (9 mm simulated because of symmetry)
- Rotor diameter: 3,17 mm
- Bearing clearance: 15 mu

I would really appreciate any help. I added images of my geometry, the mesh and the solution monitors. Additionally, I uploaded the file (could not attach it):

Best regards
Attached Images
File Type: png geom.png (65.1 KB, 12 views)
File Type: png mesh.png (46.6 KB, 12 views)
File Type: png mesh_clearance.png (56.4 KB, 15 views)
File Type: jpg sol_mon.jpg (141.8 KB, 12 views)

Last edited by Tierce; October 31, 2018 at 06:43.
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Old   October 31, 2018, 16:00
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Glenn Horrocks
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The problem is obvious when you look at your mesh. Your mesh is FAR too coarse to capture this flow. You need a minimum of 8, and preferably 10 of more elements across a flow path to capture it for highly viscous flows (normally laminar fully developed flows with no boundary layers). In your outer hoop there appears to be 2 elements across the flow cross section and in the bearing it is 4. This is far too coarse.

You need to refine the mesh in the thin flow passages to at least 8 and preferably more than 10 elements exist.
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boundary condition, cfx, outlet, pressure

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