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-   -   B.C Pressure inlet/outlet (http://www.cfd-online.com/Forums/fluent/119219-b-c-pressure-inlet-outlet.html)

omar87st June 12, 2013 08:31

B.C Pressure inlet/outlet
 
Hello,

Is there any difference between a pressure outlet/inlet if we set it to 0 Pa?

I have a case where the calculation converge way better when i change a pressure outlet (0Pa) to pressure inlet (0Pa)

Thank you.

Omar

delaneyluke June 14, 2013 10:13

You'll have to be more specific. What are you modeling? What is driving your flow if your pressure differential is zero?
Regards
Luke

omar87st June 14, 2013 15:26

Thanks for answering, i have 2D wind tunnel car simulation with a vellocity inlet and all the others edges are: walls -> for the car, and pressure outlet for all the others .
The think is when i run the calculation like that it does not converge verry good "the continuity does not"
And when i replace one of the edges from pressure outlet "0Pa" to pressure inlet "0Pa", the model converge verry well and verry fast...

Thanks
Omar

Barzanoni June 16, 2013 06:56

Hi
if the other boundaries are something like as free stream ( or pressure far fild in the compressible flow), the pressure inlet with 0 Pa lead to better and faster convergence. I had some experience like this.

You should be care about no entering flow from these boundaries.

omar87st June 16, 2013 14:48

Hi, Thanks for answering
I only have pressure outlets and a vellocity inlet ...
I don't know why it's happening, i will take it as a rule until i find the difference between the pressure inlet/outlet at 0 Pa !

Thanks
Omar

delaneyluke June 17, 2013 15:55

A pressure inlet/outlet at 0 Pa is technically the same (differs by direction of flow). You have to make sure that you do not have air entering in through the pressure inlets.
The reason your simulation works well with the pressure inlet and not the outlet is cause there might be air entering your domain from these surfaces.
Do you get a lot of reversed flow when you pressure outlets?

Alternately you could use the farfield BC's, which are meant for your kind of problem

Regards
Luke

mohamed rabie July 13, 2013 18:53

hi,
when you set the B.C as pressure inlet, it is total pressure
but when you set it as pressure outlet it is static pressure

flow_CH July 14, 2013 08:32

Quote:

Originally Posted by mohamed rabie (Post 439597)
hi,
when you set the B.C as pressure inlet, it is total pressure
but when you set it as pressure outlet it is static pressure

I don't think so. Total pressure= static pressure+dynamic pressure and absolute pressure=gauge pressure+operating pressure. Every time we use the gauge pressure=0, it means that absolute pressure=operating pressure and it means, the outside of boundary is atmosphere(if we use the default for operating pressure=101325 pa)

mohamed rabie July 19, 2013 18:49

Quote:

Originally Posted by flow_CH (Post 439632)
I don't think so. Total pressure= static pressure+dynamic pressure and absolute pressure=gauge pressure+operating pressure. Every time we use the gauge pressure=0, it means that absolute pressure=operating pressure and it means, the outside of boundary is atmosphere(if we use the default for operating pressure=101325 pa)

mmmmmm,

no problem at all, it is right, but the default in fluent is to take the pressure inlet value as total, but take the pressure outlet value as static , you can take a view in the fluent user guide.
and kindly please reply me,
I will wait for your reply
thanks


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