CFD Online Discussion Forums

CFD Online Discussion Forums (
-   FLUENT (
-   -   Setting pressure and velocity in inlet (

Asghari September 20, 2006 06:46

Setting pressure and velocity in inlet
Hi all;

I would like to set pressure and velocity in inlet ,synchronously. How is this possible?

Please lead me.


Asghari September 20, 2006 11:58

Re: setting guage pressure to zero in a inlet node
Really, in start of soloution , after 20 iteration , i finded gauge pressure in outlet approaches to zero and soloution diverges ,wheras, i expected at least in a inlet node,gauge pressure approach to zero due to i fixed operating pressure in a inlet node location. Why this happended? please help me .

Thanx alot.

Asghari September 21, 2006 03:40

Re: setting guage pressure to zero in a inlet node
I still am waiting for anybody who can help me, But if you can ,give me a udf or a method that can fix operating (absolute) pressure in a specified inlet location ( because of i use a equation such as state ideal gas equation for density) my problem will be solved .

Thank for your help in advance.

Jason September 21, 2006 09:54

Re: setting guage pressure to zero in a inlet node
From what I can tell, you're trying to use a Velocity inlet condition with the ideal gas law. This will not work... look at the Fluent documentation for their BCs (it specifically states that a Velocity inlet can not be used in a compressible case because it does not fix the total pressure).

Change your BCs... use a pressure inlet or a mass flow inlet.

Good luck, Jason

Asghari September 22, 2006 09:36

Re: setting guage pressure to zero in a inlet node
Thank you But, I am using udf for defining density in incompressible flow and i want to set velocity in inlet and pressure setting only in a inlet point.

User guide tells us:

" Operating pressure is significant for incompressible ideal gas flows because it directly determines the density: the incompressible ideal gas law computes density as density=(operating pressure)/(RT). Operating pressure is significant for low-Mach-number compressible flows because of its role in avoiding roundoff error problems. Operating pressure is less significant for higher-Mach-number compressible flows. The pressure changes in such flows are much larger than those in low-Mach-number compressible flows, so there is no real problem with roundoff error and there is therefore no real need to use gauge pressure. If the density is assumed constant or if it is derived from a profile function of temperature, the operating pressure is not used at all. "

But in my udf , also i use another dependent pressure equation for density defining, therefor, i need to absolute pressure for defining pressure.

For this purpose , i set 101325 value as operating pressure value in operating pressure panel & also i set location of this reference pressure in inlet and i do'nt need to constant total pressure in all of inlet , i would like to specify pressure only in a inlet point to 101325.(just as set in operating pressure menu).

But apparently this do'nt work and pressure in outlet domain is about 101325 and pressure in inlet is more than outlet (nearly 120000) and is'nt 101325 in a inlet position.

Why this happended ? Thank your help in advance.

san September 22, 2006 13:23

Re: setting guage pressure to zero in a inlet node

as per my understanding of the problem description,

since flow is incompressible, i.e density=constant pressure drop is negligible in your case (probabally)

at operating pressure specify static pressure. give inlet BC as velocity ( indirectly fixing mass flow) at exit give outlet static pressure.

i hope your problem will be solved if still ur problem is not solved please explain the physics involved.

bye san

All times are GMT -4. The time now is 04:15.