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-   -   difference between st. pressure and tot pressure (http://www.cfd-online.com/Forums/cfx/26783-difference-between-st-pressure-tot-pressure.html)

jai December 10, 2008 00:14

difference between st. pressure and tot pressure
 
what is the difference between total pressure and static pressure and how cfx-pre taken these value in opening type boundary condition

The boss of total loss December 10, 2008 07:15

Re: difference between st. pressure and tot pressu
 
What the @$%! are you doing with CFD if you don't know the difference between Total and Static conditions?

If you have access to CFD-online you can also type wikipedia or better READ A BOOK!!!!


CycLone December 10, 2008 17:53

Re: difference between st. pressure and tot pressu
 
Total Pressure represents the pressure you would acheive if you decelerated the fluid isentropically, recovering the dynamic pressure. It is also referred to as Stagnation Pressure. For an incompressible fluid, the total pressure is related to the static pressure by:

Ptotal = Pstatic + Pdynamic

where

Pdynamic = 1/2*Density*Velocity^2

For a compressible fluid it is a little more complicated.

Since the static pressure varies with velocity, it is not a very good inlet boundary condition because it doesn't capture the total momentum of the fluid. Furthermore, in the absence of losses, total pressure is conserved (looking at the total pressure field is a good way to identify where losses are occurring).

Say for instance you have air entering a device from a room and you need a boundary condition representing the pressure at the inlet. In the room, the velocity is very small, so we can assume the dynamic pressure is zero and therefore:

Ptotal = Pstatic

If you used Pstatic = atmosphere as the boundary condition for your model it would be wrong, because at the inlet the velocity is higher and therefore the dynamic pressure needs to be subtracted. A better option is to set the inlet Total Pressure equal to atmosphere, which will result in the correct physical behavior.

As a final note, remember that the pressure (total or static) is reported relative to your domain pressure. So in the above example, if your domain pressure is 1 [atm], the boundary condition would actually be Ptotal = 0 [atm].

-CycLone

Ahmed December 11, 2008 10:33

Re: difference between st. pressure and tot pressu
 
@ boss of total loss

GET A LIFE!

The boss of total loss December 11, 2008 11:11

Re: difference between st. pressure and tot pressu
 
READ A BOOK!!!

Adam December 11, 2008 12:42

Re: difference between st. pressure and tot pressu
 
I'd have to agree somewhat with Mr Boss. If you don't know the difference between total and static pressure, CFD isn't really the thing you should be doing.

I just hope that it was an English barrier so that he couldn't pose a proper question.

Ahmed December 11, 2008 13:37

Re: difference between st. pressure and tot pressu
 
May be he does not have access to library. Boss of total loss is so insensitive piece of shit. This is a public forum, he can post what he wants. You have the option to ignore the question if you dont like it. You dont have to be rude to others.


The boss of total loss December 12, 2008 04:11

Re: difference between st. pressure and tot pressu
 
Dear Ahmed,

I asked around and I'm not that insensitive. Thing is that I do want to invest some of my time answering questions. But when people do not invest even the slightest amount of time to figure out the absolute basics of fluid dynamics then it's not really encouraging me and a lot of others to but down my work for some minutes and help people with CFD. It's not only about having access to a library (jai does have access to internet so...), but in CFX you can actually type F1 to get some help. Apart from that: I don't think my remark to encourage people to dive deeper into theory is on the same level as stating somebody is a piece of shit.

The Boss of Total Loss. (aka: a sensitive piece of poo-poo :,-( )


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