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-   -   Outflow Boundary Condition (http://www.cfd-online.com/Forums/fluent/28505-outflow-boundary-condition.html)

J. Weiler May 6, 2001 11:13

Outflow Boundary Condition
 
In order to find the pressure drop through a 180 degree bend, I have chosen to use a mass flow inlet and an outflow (for which the pressure is unknown). The Fluent help files suggested using the Outflow boundary condition when pressure is a desired result. When I run the simulation for two mass flow rates (0.02 and 0.05 kg/s) I get zero static pressure at the outlet where the flow is fully developed, and my pressure drop is just the new pressure value at the inlet. However, when I run it with a mass flow rate of 0.10 kg/s I get 2 Pa at the fully developed outlet. I can still check for pressure drop between the inlet and the outlet, but I am just wondering why in some cases it yields zero static and in other cases it doesn't. I have extended the length of my outlet pipe to 40x the hyd. diameter, so the flow is fully developed. I welcome any advice. Thanks Justin

Jin-Wook LEE May 7, 2001 02:58

Re: Outflow Boundary Condition
 
Absolute value for the pressure is meaningless for SIMPLE-family algorithm. Only the difference is meaningful.

If you want to set the outlet pressure as reference value, just use pressure-outlet, instead of outflow. Two boundary conditions, outflow and pressure-outlet, resutl in nearly same solution, when there is no reverse flow at the flow-exit.

Sincerely, Jinwook


J. Weiler May 7, 2001 06:58

Re: Outflow Boundary Condition
 
Now I see what you are getting at. So if I set the outlet as a pressure outlet instead (with zero gauge pressure), the static drop will just be whatever value occurs at the inlet.

Just one more question. I have used line and point surfaces to plot (XY Plot) values along the inlet and outlet of the piping, but is there any way to compute actual values at localized areas of concern?

Thanks for the help.

Justin

Devy May 7, 2001 20:09

Re: Outflow Boundary Condition
 
Hi,

In the fluent mannul,the pressure input at subsonic massinlet is just used to initialize the flow field.

If you don't know the value of pressure at outflow, how can you change the outflow boundary to pressure-out?Would you like to tell me how to define the value at the pressure-out boundary?

Thank you.


Jin-Wook LEE May 10, 2001 05:06

Re: Outflow Boundary Condition
 
Of, course you can. You can create line or area anywhere you are interested. At Fluent, you can create the line or surface at the 'Surface Panel'.

Sincerely, Jinwook


Jin-Wook LEE May 10, 2001 05:11

Re: Outflow Boundary Condition
 
Your input(gauge pressure) for pressure-outlet is only reference value. Pressure in the computational domain is the difference between pres.-outlet and your interested region. This is same for every code which uses SIMPLE-family algorithm.

Sincerely, Jinwook



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