# Trouble Using Pressure In Expression For BC

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

 June 17, 2008, 10:01 Trouble Using Pressure In Expression For BC #1 Nolan Guest   Posts: n/a Hello, I am running CFX v.11 and trying to set a fuel injector to a certain pressure based on the mass flow rate of the incoming air. With Temperature and velocity specified, this pressure will define my fuel mass flow rate at the desired equivalence ratio. I am attempting to do this with built in CFX expressions, but when I try to use "p" in an expression, I get the following error in the .out file: -------------------------------------------------------- Error processing expression 'Relative Static Pressure'. The expression is invalid because: p is not available for use in this term -------------------------------------------------------- Even if I try something simple, like setting the pressure of the injector equal to the exit pressure of my domain, ie: psIn = areaAve(p)@outlet I get the same error...I have no such problems with setting temperature or velocity in this same manner. Any suggestions?

 June 17, 2008, 11:10 Re: Trouble Using Pressure In Expression For BC #2 CycLone Guest   Posts: n/a Hi Nolan, There are no solver rules preventing the use of pressure in this manner. I'm not sure why you are getting this error, I certainly can't reproduce it. That said, if your trying to specify the pressure based on mass flow rate, why do you need pressure in your expression? Also, to prevent instability, you may want to include a relaxation in your pressure condition. Create an expression for PsIn as a function of mass flow rate where PsIn is your desired pressure and let PsOld and PsSpec be the current pressure and the specified pressure respectively. PsSpec = PsOld + (Psin - PsOld)*PsRelax Set PsRelax to a value between zero and one and use PsSpec as you boundary condition value. -CycLone

 June 17, 2008, 11:46 Re: Trouble Using Pressure In Expression For BC #3 Nolan Guest   Posts: n/a Thanks for the quick reply... I'm not really sure why I can't use pressure at all in an expression that is then used to set my 'Relative Pressure' BC...has me completely stumped. We typically use pressure (while holding fuel temp and velocity fixed) at the face of our supersonic injectors as a control for their flowrate. Since mass flow will scale directly with this pressure, this function would take the injector pressure (from the previous timestep) and multiply it by the ratio of (desired injector equivalence ratio/equivalence ratio from previous timestep). Equivalence ratio is 1.0 for stoiciometric flow...and it is based on the inlet air mass flow and fuel flow rate on the injector. For example, say at a given timestep the currently calculated ER for a given injector is 0.5 (using the fuel flow rate, inlet air mass flow rate, and the stoich. ratio for the given fuel). If my desired ER is 1.0, the pressure will be doubled on the next iteration. I felt this would probably cause instability, so thanks for the recommendation to use relaxation to slowing run the pressure up. My problem still persists, I cannot define my pressure BC at my injector using the "p" variable in anyway. I even tried using "pabs" to no avail. Like I said, if I wanted to set the fuel temperature to some average (say tinj = areaAve(T)@inlet) it has no problem. Hmmmm...very stumped.

 June 17, 2008, 19:29 Re: Trouble Using Pressure In Expression For BC #4 CycLone Guest   Posts: n/a It may not be available for supersonic inlets. But if you inlet is supersonic, you have control over the pressure, temperature and velocity, so you should be able to compute the values needed to achieve your mass flow rate.

 June 23, 2008, 16:10 Re: Trouble Using Pressure In Expression For BC #5 Nolan Guest   Posts: n/a Just as an update, I received some help from ANSYS tech support on this issue and they were able to reproduce the problem. They told me it was a bug in the solver, and posted a .ccl file to load in with my run as a workaround. Here is the ccl: -------------------------------------------- RULES: PARAMETER: Relative Static Pressure Parameter Type = Real Solver Name = PRES Quantity Type = Pressure Dependency List = ANY Profile Variables List = p,Pressure Dynamic Reread Item = Yes END END -------------------------------------------- It is loaded with -ccl *.ccl added at the command line to your run. Hope it helps...

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Giuki FLUENT 1 July 19, 2011 11:35 lzgwhy Main CFD Forum 1 January 11, 2011 19:44 manaliac Main CFD Forum 2 November 29, 2010 07:25 engahmed Main CFD Forum 0 June 16, 2010 11:38 tania FLUENT 11 March 23, 2004 09:51

All times are GMT -4. The time now is 00:19.