CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Update actual pressure

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   February 21, 2008, 17:23
Default Update actual pressure
  #1
Ollimarc
Guest
 
Posts: n/a
Hi everybody,

I want to update the pressure in the whole process by CEL expressions. How do I do that? When I say p=p+100 it says that p is already in use.

How can I manage this?

Thanks in advance Ollimarc
  Reply With Quote

Old   February 21, 2008, 18:30
Default Re: Update actual pressure
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Update what pressure? A boundary condition? Is it a transient simulation?

Glenn Horrocks
  Reply With Quote

Old   February 21, 2008, 18:46
Default Re: Update actual pressure
  #3
Ollimarc
Guest
 
Posts: n/a
Hi,

no. The variable 'p'. CFX PRE tells me verytime that this "object name is already in use".

Finally I ve got a user routine which controls my pressure at all and I want to realise this.

Is this possible?

Oliver Marc
  Reply With Quote

Old   February 22, 2008, 13:45
Default Re: Update actual pressure
  #4
CycLone
Guest
 
Posts: n/a
Pressure is the variable solved for in the Mass equation, which is coupled to the momentum equations. To modify the pressure, you need to add appropriate sources to the mass and momentum equations.

What exactly are you trying to accomplish by specifying the pressure?

-CycLone
  Reply With Quote

Old   February 23, 2008, 12:15
Default Re: Update actual pressure
  #5
Ollimarc
Guest
 
Posts: n/a
my final goal is to simulate a flow around a wing with an airspeed of mach 5 or more. but with this speed the pressure (and other values) must be corrected. therefore i wrote user fortran subroutines for this. these subroutines are ok and i can implement them into cfx. but the problem is to update the pressure (and then the other values). how can i do this?
  Reply With Quote

Old   February 24, 2008, 09:59
Default Re: Update actual pressure
  #6
andy20
Guest
 
Posts: n/a
"but with this speed the pressure (and other values) must be corrected. "

Why?

Sure, your flow is very high-speed, but you still want to solve the compressible Navier-Stokes don't you? That's what CFX will do. And when CFX does that it will solve for the pressure field, the velocity field, and your turbulence model.

In general, if you try and update the variables that CFX is trying to solve for whilst it is in the middle of trying to solve for them you are going to cause big problems!!

So what extra physics are you trying to include with these corrections?

If you tell us in more detail what physics your are trying to describe with the corrections, what type of corrections and updates you are trying to apply, and where you are trying to apply them, then people can give you more help.

Good luck, Andy
  Reply With Quote

Old   February 24, 2008, 11:11
Default Re: Update actual pressure
  #7
Ollimarc
Guest
 
Posts: n/a
i want to simulate a hypersonic airflow. this means an airflow of about mach 4 to mach 25 (which can be a space shuttle in reentry conditions). air chemistry will change intensly and therefore the behaviour of it. when calculating this in cfx, temperatures of about 35000 kelvin will occur. in real conditions, there is for example no existing o2 and n2 anymore and delivers a temperature of about 15000 kelvin. the user fortran for this conditions are ready and i can read out the values i want to. but i do not know how to write them back to the system. i now trying this by researching in mms. but i do not know.
  Reply With Quote

Old   February 24, 2008, 18:12
Default Re: Update actual pressure
  #8
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

CFX cannot model the physics of plasma flows. You will need to test whether a fully compressible simulation will be good enough for what you want to do, but I doubt it will work for the conditions you describe.

I don't think CFX is the software to use for your application.

Glenn Horrocks
  Reply With Quote

Old   February 24, 2008, 20:11
Default Re: Update actual pressure
  #9
Ollimarc
Guest
 
Posts: n/a
i know that cfx is not the best way to simulate what i want to do. but i decided to try this. i ve got user subroutines for equilibrium flow and non equilibrium flow which describes the gas (the massfraction for each species as well). i want to try and relate it with other known results.

do you know how to change the values in cfx?
  Reply With Quote

Old   February 25, 2008, 09:23
Default Re: Update actual pressure
  #10
andy20
Guest
 
Posts: n/a
For a single fluid model, CFX uses the density and viscocity to describe the gas. From this data, and the boundary conditions it *solves* to find pressure and velocity.

The density can be controlled by the user, but it is restricted to being a function of pressure and temperature only (see the CFX documentation for compressible flows). The pressure is solved for, and therefore cannot be adjusted (which is what you asked about originally).

So, for a single fluid model, I think the best you can do is to find a function rho = rho(p,T) which is reasonably representative of your gas.

Of course if you have large errors in your temperature models, for whatever reason, these may cause large errors in the density if you make rho depend on T as well as p.

You might have a little bit more freedom to represent the effect of changing species, etc. if you use a mulitphase model, but I don't know the details of the mixture models so I can't advise on this, and I still don't think this will do quite what you want.

Good luck... I think you might need it

Andy
  Reply With Quote

Old   February 25, 2008, 17:45
Default Re: Update actual pressure
  #11
CycLone
Guest
 
Posts: n/a
Why not add the dissociative reactions? This is after all what is going on.

-CycLone
  Reply With Quote

Old   February 27, 2008, 14:58
Default Re: Update actual pressure
  #12
Ollimarc
Guest
 
Posts: n/a
hey, thanks for your answer. actually i need the density and the temperature to find out the results. i get then the pressure, cp, molar mass, entropy, enthalpy and mass fractions. and i want to give the most back. therefore i need the density and only want to update the values above. i am now looking at the poke function in the mms section. maybe i find a solution there. i can read out the values, alter it... but not write back....
  Reply With Quote

Old   February 27, 2008, 15:00
Default Re: Update actual pressure
  #13
Ollimarc
Guest
 
Posts: n/a
yea, i have the equations. but i have to change the mass fractions of each species. i mean i have to update them, dont i? how did that work? how can i tell cfx to do so?
  Reply With Quote

Old   February 28, 2008, 14:39
Default Re: Update actual pressure
  #14
CycLone
Guest
 
Posts: n/a
Add sources to the mass fraction equations.
  Reply With Quote

Old   February 28, 2008, 20:06
Default Re: Update actual pressure
  #15
Ollimarc
Guest
 
Posts: n/a
how and where?
  Reply With Quote

Old   March 5, 2008, 11:48
Default Re: Update actual pressure
  #16
Ollimarc
Guest
 
Posts: n/a
Hey Cyclone,

can you help me? How can I add sources to the mass fraction equations which results in new, my desired, massfraction?

Regards Olli
  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
UDF to define or adjust pressure??? engahmed FLUENT 0 July 6, 2010 17:19
Setting pressure and velocity in inlet Asghari FLUENT 5 September 22, 2006 13:23
Neumann pressure BC and velocity field Antech Main CFD Forum 0 April 25, 2006 02:15
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13
Definition of pressure Ola Nordblom FLUENT 1 August 16, 2001 15:58


All times are GMT -4. The time now is 14:04.