CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Bearing simulation outlet pressure

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 31, 2018, 06:32
Default Bearing simulation outlet pressure
  #1
New Member
 
Join Date: Jun 2016
Posts: 1
Rep Power: 0
Tierce is on a distinguished road
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): https://www.dropbox.com/s/1z8k9f8fdr...tlet.wbpz?dl=0

Best regards
Attached Images
File Type: png geom.png (65.1 KB, 11 views)
File Type: png mesh.png (46.6 KB, 10 views)
File Type: png mesh_clearance.png (56.4 KB, 12 views)
File Type: jpg sol_mon.jpg (141.8 KB, 10 views)

Last edited by Tierce; October 31, 2018 at 07:43.
Tierce is offline   Reply With Quote

Old   October 31, 2018, 17:00
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,538
Rep Power: 128
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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.
Tierce likes this.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Tags
boundary condition, cfx, outlet, pressure

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
simple model, difficult outlet Eric CFX 7 May 23, 2014 09:13
Pressure Outlet for phase change simulation dinesh FLUENT 0 November 22, 2013 00:50
Pressure outlet boundary condition fluent_newbie FLUENT 0 December 2, 2011 00:51
VOF Outlet boundary condition in cfd - ace JM Main CFD Forum 0 December 15, 2006 09:07
Neumann pressure BC and velocity field Antech Main CFD Forum 0 April 25, 2006 03:15


All times are GMT -4. The time now is 10:47.