CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > CFX

Help setting up pressure function across opening

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

LinkBack Thread Tools Search this Thread Display Modes
Old   October 17, 2007, 09:09
Default Help setting up pressure function across opening
Posts: n/a

I've been working on a problem with a fan for a while and can only come up with a numerical solution within 40% of the experimental results. I've done grid independence testing, looked at the sensitivity of the residuals, changed the turbulence models and tried different boundary conditions, but still can't get a better agreement than 40% which just isn't good enough.

I've been speaking with one of my lecturers today and he's suggested that the problem may be in the airflow entry point into the fan. Currently I have the set up of the fan in an open circular duct (pipe) using a mass flow on the outlet and an opening in the shape of a half circle on the front (see I have set the boundary condition as an opening with reference pressure 1 atm, and relative pressure on the surface as 0Pa. I'm using opening pressure and direction and flow normal to boundary for the opening. The suggestion is that due to the shape of the inlet not all the air flow will flow with constant velocity through the fan. The air entering from the side of the duct will turn in with a higher velocity, so there may be a false representation of the actual mass flow entering into the fan compared with the real situation of the fan in the experiment.

I would like to try putting a variance of pressure across the opening and wondered if anyone else has tried this or has any other suggestions. I definitely have a discrepancy between the mass flow in the numerical experiment and in the actual experiment. The numerical model is showing the fan to be incredibly efficient, but this can't be a real situation even allowing for losses which would be present in the 'real world' situation.

Any suggestions anyone can give would be great on how to put a variance of pressure, or whether this would even be the correct approach. Thanks in advance.

  Reply With Quote

Old   October 17, 2007, 12:29
Default Re: Help setting up pressure function across openi
Posts: n/a
Hi Jenny,

If you have set a specific mass flow outlet condition, and the only way for air to enter is through the opening boundary condition, then as long as your solution is converged and imbalances are small, the mass flows should be identical. Are you saying that some air circumvents the fan through a tip clearance, and this is smalller/larger than it should be?

It also wasn't clear to me from the picture how you are modelling your fan - is it with a rotating domain (with frozen rotor interfaces), or with momentum sources?

Could you elaborate further?


  Reply With Quote

Old   October 17, 2007, 17:27
Default Re: Help setting up pressure function across openi
Posts: n/a
Hi Johnson,

Thank you for your reply. I'll try and elaborate a little better on the physical set up of the system. I've added some text to the diagram at to clarify the parts of the geometry.

Basically I have used the frozen rotor approach with interfaces between the spherical "inlet" and the small rotating cylinder containing the fan, and between the rotating cylinder and the stationary outlet duct.

The problem I'm having is that when I use the mass flow measurements from the actual experiment they are giving me a pressure reading much lower than the experimental value. They even become negative as the volume flow rate increases through the duct, so since I am using gauge pressure somehow there seems to be too much mass flowing through the system than it is capable of handling if I use the experimental value. We were trying to work out how this would happen.

In my model you can see that the inlet has a flat plane of zero pressure across the front of the rotating cylinder containing the fan. What we suspect might be happening is due to the fact some air must essentially turn to get into the duct it needs to speed up to get in, so in reality the pressure is not constant across this plane as we are forcing it to be. So basically it looks to the system like a denser amount of air is coming in at these areas. If this makes sense?

It was just a way we thought might explain the increased mass flow. I wanted to test the theory by putting a variation of pressure across this plane, so it isn't just a relative pressure of 0Pa.

I hope that makes sense. Thanks in advance for any light you can throw on this.

Best wishes, Jenny
  Reply With Quote

Old   October 18, 2007, 00:16
Default Re: Help setting up pressure function across openi
Posts: n/a
Hello Jenny.

I feel the way you have considered open atmosphere is creating problem. Try following things..

1) Instead a hemisphere have a complete sphere (minus pipe) to consider atmosphere AND

2) Increase size of this sphere (This will take care of pressure variation as you reach near to fan)

3) Define opening boundary on this whole sphere

I hope this should help.

Best luck,

  Reply With Quote


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
Setting Pressure Boundary Conditions in ANSYS CFX Pre saisanthoshm88 CFX 21 February 22, 2017 16:50
Does star cd takes reference pressure? monica Siemens 1 April 19, 2007 11:26
Neumann pressure BC and velocity field Antech Main CFD Forum 0 April 25, 2006 02:15
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 05:13
Hydrostatic pressure in 2-phase flow modeling (CFX4.2) HB &DS CFX 0 January 9, 2000 13:19

All times are GMT -4. The time now is 12:50.