# Update actual pressure

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 February 21, 2008, 17:23 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

 February 21, 2008, 18:30 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

 February 21, 2008, 18:46 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

 February 22, 2008, 13:45 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

 February 23, 2008, 12:15 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?

 February 24, 2008, 09:59 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

 February 24, 2008, 11:11 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.

 February 24, 2008, 18:12 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

 February 24, 2008, 20:11 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?

 February 25, 2008, 09:23 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

 February 25, 2008, 17:45 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

 February 27, 2008, 14:58 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....

 February 27, 2008, 15:00 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?

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

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

 March 5, 2008, 11:48 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

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