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-   -   What happen with the scale of pressure? (http://www.cfd-online.com/Forums/fluent/49509-what-happen-scale-pressure.html)

Kostas Pan October 14, 2008 15:11

What happen with the scale of pressure?
 
i have problem with the scale of static pressure of a flow.

it is show me big negative values of pressures. For example: In a pipe i put inlet pressure 40000Pa(approximately 0.4 atm) and outlet pressure 200000Pa(approximately 2 atm) and fluent show me values of pressure until -700000Pa (approximately 7 atm).

How can it be occured this? If we have inlet pressure 0.4 atm then the minimum value can take the fluid is 0 atm.

devesh October 15, 2008 03:48

Re: What happen with the scale of pressure?
 
Hi,

The Inlet pressure you are giving is Static or Total Pressure ? is the same case for Outlet condition ? Did you check the Total/Static/ Dynamic pressure values after getting the solution ?

you provided Pinlet = 0.4 atm, Poulet = 2 atm. in that case direction of flow will be completly reverse. In that Pinlet(0.4 atm is Total pressure) will be decomposed into two component:- Static Pressure & Dynamic Pressure. Please check those values. Are they give 0.4 atm by summing up Static & Dynamic pressure in the solution.

I guess that will be helpfull, please fell free to ask if I can you help you in better way


Kostas Pan October 15, 2008 05:24

Re: What happen with the scale of pressure?
 
hi devesh.

In the box of Pressure inlet of Boundary contitions there are two options: "Gauge total pressure" and "Initial gauge prssure". I use the second option for static inlet pressure. This is right?

Still, i want to ask you, why is there the "operation condition". if i put a value of prssure in the operation condition, this value where will apply? I don't unstarstand where useful this.

devesh October 15, 2008 06:05

What happen with the scale of pressure?
 
Hi Kostas, Operating Pressure is Standard Pressure(i.e. 1 atm by default). Now what-ever pressure you put, that is Gauge pressure. You put 0.4 atm in Gauge Total Pressure (total pressure= static pressure + dynamic pressure).

Fluent works on Absolute Pressure(i.e. Operating pressure + gauge pressure). If you want to know more about Pressure , Please go through FLuent User Guide, that will help you alot.

By chance if u want to run your analysis, you put 0.4 atm in Gauge total Pressure. "Initial gauge Pressure" is for initial guessing & is used in case of Compressible Flow.

best of luck......


Kostas Pan October 15, 2008 11:16

Re: What happen with the scale of pressure?
 
Ok. i understand!

In the box of operating condition there are X: and Y:. What mean that? That on this point we will have the operation condition? If define aq point which find out of geometry what mean?

devesh October 16, 2008 03:07

Re: What happen with the scale of pressure?
 
Hi, I think you are talking about Reference Pressure Location !....If you have been scaled your geometry & if this is not a very complex flow condition then leave it as default value of Zero.

If I am correct to understand your problem, then it is the location from where solver consider it as a reference point to adjust the Numerical Pressure with respect to surrounding cell centre.

If I am misleading something, pls correct me.



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