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-   -   static vs. total pressure (http://www.cfd-online.com/Forums/fluent/40268-static-vs-total-pressure.html)

auf dem feld March 22, 2006 17:17

static vs. total pressure
 
I've some questionable results delivered by fluent. I plotted the contours of static pressure on a wall. Then I looked at the total pressure on the same wall expecting the exactly same results since all the velocities are zero at the wall (no slip)and therefore dynamic pressure should be zero BUT there is a non negligible difference between static and total pressure. what's wrong here?

Ahmed March 22, 2006 19:27

Re: static vs. total pressure
 
What Fluent and CFD codes call Static pressure is actually a vulgar way to refer to the Thermodynamic pressure calculated from the Navier Stokes equations. As you understand it, Static pressure, has nothing to do with fluid dynamics. You have to dive into their manuals and make some corrections, perhaps you can add your voice to mine and ask CFD vendors to use correct scientific terminology not just the industry chat.

mAx March 23, 2006 02:28

Re: static vs. total pressure
 
total pressure = operating pressure + static pressure + dynamic pressure so in your case, on your walls, the total pressure should be = operating pressure + static pressure

Philipp March 23, 2006 02:37

Re: static vs. total pressure
 
Hi,

without knowing anything about your geometry and in particular about your 'wall', I tend to say that you could, should (?, that really depends on the flow field) see a difference between static and total pressure. I guess that you noticed that Fluent defines the 'total pressure' as the sum of 'static pressure' and 'dynamic pressure'. You pointed out that the velocity is '0' at the wall (no slip condition + nothing can go 'through' the wall): well, I would say that this is "irrelevant" for the dynamic pressure. Much more important is the velocity component normal and 'slightly' away from the wall--if you picture this, the wall defines a 'stagnation point' (since nothing can get 'through' the wall)... take the "Pitot-Tube" as an example. Hope this helps a bit"

Philipp March 23, 2006 02:41

Re: static vs. total pressure
 
...and I have to "row" immediately back! Indeed, the pressure at the wall can only be the static pressure (plus eventually the operating pressure). ...what was I thinking....?

Philipp March 23, 2006 02:50

Re: static vs. total pressure
 
Hello mAx,

In my understanding, Fluent defines the 'total pressure' as a 'gauge pressure' with respect to the 'operating pressure'. Hence, 'total pressure' = 'static pressure' + 'dynamic pressure'. Same for the 'static pressure', which is a 'gauge pressure'. Only the 'absolute pressure' takes also the 'operating pressure' into account (e.g. 6.2. manuals, chapter 7.3.1).

mAx March 23, 2006 02:58

Re: static vs. total pressure
 
hi Philip, yes you're right total pressure = static + dynamic pressure absolute pressure = static + operating pressure but as I set always operating pressure at 0.....

Philipp Beierer March 23, 2006 03:09

Re: static vs. total pressure
 
...(with operating pressure = 0) you get: absolute pressure = static pressure and total pressure = static pressure + dynamic pressure. Anyway, at the wall of 'auf dem feld' (btw, interesting name ;-)) should be: static pressure = total pressure as the velocity is 0... and we're right back where we started!

Cheers

mAx March 23, 2006 03:27

Re: static vs. total pressure
 
I'm totally (or absolutely ;OP ) agree with you Philipp

zhoulin March 23, 2006 03:44

Re: static vs. total pressure
 
your total pressure = static pressure (which is static gauge pressure) + operating pressure.

please check

Zhoulin

auf dem feld March 23, 2006 09:45

Re: static vs. total pressure
 
as an update, I use operating pressure = 0 (for reasons that are defenetely discussed in other threads)

could anybody check one of his cases and plot static and total pressure on an arbitrary wall (vane, strut, pipe... whatever)?

thanks

Temel March 23, 2006 11:14

Re: static vs. total pressure
 
Hi. I also have a problem with total pressure. With any simple geometry such as nozzle, when I solved the problem I saw that in the exit plane total pressure is greater than inlet plane total pressure. there is no device to generate anything. Is that possible to be ? exit total pressure is greater than inlet total pressure? Boundary conditions inlet mass flow 69 kg/sn, flow direction normal to boundary , total temperature 300 K, outlet pressure 0 (gage) , operating conditions 101325 pa. Flow compressible, material is air , density is ideal gas. Help me???????

cfxstudent June 17, 2011 09:36

I imagine it's to do the calculation with Hybrid and conservative values for the static and total pressure, although I can't be sure

flachowski March 20, 2014 11:37

Simplification
 
All,

FLUENT regards pressure as such:

We are used to relating static pressure and gage static pressure back to atmospheric conditions. FLUENT allows us to change that mindset and relate gage static pressure to ANY user selected quantity. This selected quantity is the operating pressure.

In other words, your gage is referenced from FLUENT's operating pressure setting. Similar to the figure attached here; however, instead of P_atm think Operating Pressure or P_op.

http://www.engineeringarchives.com/i...catmpres_1.png

FLUENT's Terminology on left side of equal sign not to be confused with scientifically accurate terminology!

static pressure = Static Pressure [abs zero ref] - Operating Pressure
Absolute Pressure = Static Pressure [abs zero ref]
dynamic pressure = dynamic pressure (no change)
total pressure = static pressure + dynamic pressure

Note: total pressure in FLUENT is based on a difference of the operating pressure. Any time FLUENT says static pressure, they are referring to gage static pressure. If you set OP = 0 then, obviously you are now at abs zero reference!

If we want true total/stagnation pressure then we need to reference from absolute zero or vacuum conditions. Lets call this absolute total pressure (thanks to FLUENT's terrible terminology selection)

absolute total pressure = OP + static Pressure + dynamic pressure
OR
absolute total pressure = absolute pressure + dynamic pressure
OR
absolute total pressure = OP + total pressure

This formula IS NOT an option in FLUENT.
You must select FLUENT's total pressure and then add OP yourself as a custom defined function. Why hasn't FLUENT done this already. No clue. Obviously, if OP is set to zero, then this is not needed and total pressure is the same as our hydrodynamics books!

FLUENT should name their variables as such:
static pressure --> static gage(OP) pressure
dynamic pressure --> dynamic pressure (YAY they got one)
absolute pressure --> absolute static pressure (technically abs pressure is fine)
total pressure --> total gage(OP) pressure

MrDaimon February 3, 2015 06:08

If the node values are enabled, then you are considering also the velue of the total pressure in the neighbour cells, thus also the cells which are not set as "wall". If you switch off the node values, you'll get indeed that the static pressure is equal to the total one.

hadial February 26, 2016 11:32

problem with defining pressure in fluent
 
Hello every body :
I am simulating a flow in a channel, from experimental result I have 0.3 Mpa gauge in inlet, the question is , is it true to use this pressure ( I mean 0.3 Mpa ) as my SUPERSONIC/INLET GAUGE PRESSURE ? I am confused by the discussions here? should I change the operating condition pressure ?
your help would be appreciated

flachowski February 26, 2016 13:32

Response - @hadial
 
Quote:

Originally Posted by hadial (Post 587030)
Hello every body :
I am simulating a flow in a channel, from experimental result I have 0.3 Mpa gauge in inlet, the question is , is it true to use this pressure ( I mean 0.3 Mpa ) as my SUPERSONIC/INLET GAUGE PRESSURE ? I am confused by the discussions here? should I change the operating condition pressure ?
your help would be appreciated

@hadial,

I believe the root of your confusion is in the terminology I mentioned above. FLUENT uses "gage related" terminology and base it off of operating pressure. Engineers, use static/absolute terminology. Either way, both work.

We need more information to help, but here is a quick stab at it:
Assuming that your gage pressure as measured by some instrument is 0.3MPa, as you stated then I'll also assume that the device is measuring relative to atmospheric pressure (you need to confirm this). I am also assuming that you are using a Pressure Inlet condition and you will account for velocity effects at some other BC (e.g. massflow exit).

Then, your absolute pressure (Pabs = Pref + gage) will be
Pref = Patm = 0.101325 MPa
Pgage = 0.3MPa

Keep in mind that given a gage pressure, you are neglecting velocity effects in your channel (if you aren't sure, look at the measurement device you used and think about how it works).

Now you have 2 Options that will yield same results, but be careful when exporting and interpreting the solution!:

OPTION 1.
Set operating pressure = 0 MPa
Set Gauge Total Pressure = P_absolute = 0.101325 + 0.3 MPa
- You will notice when you export your results that static and total pressures will be referenced from absolute zero pressure (vacuum), which is what you set the operating pressure to.

OPTION 2.
Set operating pressure = 0.101325 MPa
Set Gauge Total Pressure = 0.3 MPa
- You will notice when you export your results that static and total pressures will be referenced from operating pressure (user defined). Refer to my previous post above.
Hope that helps.

hadial February 26, 2016 14:04

Quote:

Originally Posted by flachowski (Post 587053)
@hadial,

I believe the root of your confusion is in the terminology I mentioned above. FLUENT uses "gage related" terminology and base it off of operating pressure. Engineers, use static/absolute terminology. Either way, both work.

We need more information to help, but here is a quick stab at it:
Assuming that your gage pressure as measured by some instrument is 0.3MPa, as you stated then I'll also assume that the device is measuring relative to atmospheric pressure (you need to confirm this). I am also assuming that you are using a Pressure Inlet condition and you will account for velocity effects at some other BC (e.g. massflow exit).

Then, your absolute pressure (Pabs = Pref + gage) will be
Pref = Patm = 0.101325 MPa
Pgage = 0.3MPa

Keep in mind that given a gage pressure, you are neglecting velocity effects in your channel (if you aren't sure, look at the measurement device you used and think about how it works).

Now you have 2 Options that will yield same results, but be careful when exporting and interpreting the solution!:

OPTION 1.
Set operating pressure = 0 MPa
Set Gauge Total Pressure = P_absolute = 0.101325 + 0.3 MPa
- You will notice when you export your results that static and total pressures will be referenced from absolute zero pressure (vacuum), which is what you set the operating pressure to.

OPTION 2.
Set operating pressure = 0.101325 MPa
Set Gauge Total Pressure = 0.3 MPa
- You will notice when you export your results that static and total pressures will be referenced from operating pressure (user defined). Refer to my previous post above.
Hope that helps.

dear flachowski , thanks for your prompt answer:

1) the answer is yes, my pressure is measured by an instrument. and it is mentioned that the device is measuring relative to atmospheric pressure .( it is mentioned that it is a gauge pressure, btw I am not sure if they are the same thing)

2)I am using a mass flow inlet, and a pressure outlet BC. My fluid is a super heated steam and I am going to evaluate heat transfer along this channel, so the inlet pressure make a real difference in fluid properties
when I run my calculations the z velocity start to deconverge( I mean increases), I checked my mesh it is fine enough, so the problem should be somewhere in My BC.

I have no information for the outlet flow, so I do not make any changes in pressure outlet BC . can it make any problems ??

your kind help is appreciated
I will apply your suggestions to see how it helps


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