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

compressible boundary conditions

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 5, 2006, 23:07
Default compressible boundary conditions
  #1
vivian
Guest
 
Posts: n/a
Hi everybody, I am writing a finite volume code for unsteady 2D laminar flow. It uses for numerical simulations of resonant oscillations in a tube which has one inflow and three walls(no outflow).The boundary conditions that I use is prescribed velocity u=u0*sin(wt) at the inlet. It is like fluid entering a closed end box. The tube is filled with air under normal circumstances (T0=291.789,P0=101325pa).The density can be get from the equation of state. The pressure at the boundary is extrapolated. My code follows the SIMPLE method for compressible flow. The mass residual and total residual do not converge at one time step(dt=0.000027). I don't know how to set pressure boundary conditions in this problem. In SIMPLE method, only relative pressure is not enough. The absolute pressure is needed.Because the density is got from the equation of state P/(Rc*T). How can I know the absolute pressure? What boundary conditions should I use for this problem? And the SIMPLE method can solve this kind of problems ? Any message from you is appreciated. Thank you!

  Reply With Quote

Old   April 7, 2006, 04:36
Default Re: compressible boundary conditions
  #2
George
Guest
 
Posts: n/a
Sorry for the delay. In the pressure correction subroutine you do not have a reference pressure.That means that the pressure is calculated as follws: P=P+URFP*P' and not P=P+URFP*(P'-P'(REF)) as in incompressible flows. This is the reason that you can use the equation of state to calculate density without having a reference density as well. Hope that I have helped you a bit.If you have any more questions,feel free.
  Reply With Quote

Old   April 9, 2006, 11:29
Default Re: compressible boundary conditions
  #3
vivian
Guest
 
Posts: n/a
Hi George, thank you for your reply. You mean the absolute pressure can be calculated as P=P+URFP*P'. How is UREP defined? In my pressure correction subroutine, the pressure is calculated as follows: P=P+ (P'-P'(REF)). Because the pressure correction equation has the Neumann boundary condition in SIMPLE methods. Only relative pressure variety (P'-P'(REF)) can be got. How can the absolute pressure variety P' be calculated? Does any special pressure boundary condition be needed besides velocity inlet boundary condition? Does the P'(REF) be needed to calculate first, then the P'= (P'-P'(REF)) + P'(REF)? Or other method is used. And the velocity u=u0*sin (wt) can be used at the inlet as boundary condition, right? Thank you !

  Reply With Quote

Old   April 10, 2006, 05:06
Default Re: compressible boundary conditions
  #4
George
Guest
 
Posts: n/a
URFP is the underrelaxation factor for the pressure correction.You should set the P(REF)=0.0 , because in compressible flows the pressure as you say is the absolute one.That is why you calculate pressure with the formula: P=P+URFP*P' . As long as the velocity boundary condition is concerned,the answer is yes,you can use it,no problem!

  Reply With Quote

Old   April 10, 2006, 08:50
Default Re: compressible boundary conditions
  #5
vivian
Guest
 
Posts: n/a
When I set the P'(REF)=0.0, I have to find this reference point at which the value of absolute pressure should be known. Which point is chosen to be the reference point is the difficulty for me. My problem has the resemblance to your internal combustion engine problem. How did you find the reference point in your problem? I think not all the points in the calculation region can be chosen as the reference point (be set P'(REF)=0.0). Can you give me some advice about my problem? Thanks a lot!

  Reply With Quote

Old   April 10, 2006, 09:43
Default Re: compressible boundary conditions
  #6
George
Guest
 
Posts: n/a
You do not need the reference point if you calculate the absolute pressure.The only thing you need is to specify the absolute pressure at the inflow or/and at the outflow,for example Pin=101325 Pa and Pout=99000 Pa
  Reply With Quote

Old   April 11, 2006, 23:47
Default Re: compressible boundary conditions
  #7
vivian
Guest
 
Posts: n/a
Hi, George Pin and Pout are not known in my problem if I give the inlet velocity. I don't know how to specify the absolute pressure at the inflow or/and at the outflow if I use the inlet velocity boundary condition. Any message from you is appreciated. Thank you!

  Reply With Quote

Old   April 12, 2006, 16:53
Default Re: compressible boundary conditions
  #8
George
Guest
 
Posts: n/a
It should be known,otherwise you could not tell what the density of the incoming fluid would be.You can specify the pressure at the inlet,but this boundary condition is not what drives the fluid in the chamber,but the velocity condition that you have set
  Reply With Quote

Old   April 24, 2006, 06:23
Default Re: compressible boundary conditions
  #9
vivian
Guest
 
Posts: n/a
Hi, George ,I followed your advice and thought one way to specify the inlet pressure. I used the compressible boundary condition, prescribing the total pressure, the total pressure and the inlet flow direction.

I specified the static pressure Pin=P0+Pd*cos(wt) at the inlet and the inlet velocity Uin=u0*sin(wt).Then the total pressure is calculated from the static pressure and inlet velocity. The static temperature is known. The total temperature can be calculated.

But I met two difficulties. First, in certain time-step, the residuals fluctuate dramatically. The code seemed not converge. In most time-step, it was OK. Second, when the inlet velocity specified Uin=u0*sin(wt), I think the velocity wave in the middle of the tube would be the sine wave. Now, in my code, the velocity wave was sine wave when the inlet velocity was positive. When the inlet velocity was negative (3.14+2*k*3.14< t <2*3.14+2*k*3.14,k=1,2,3&hellip;&hellip;.), the velocity was nearly zero.

Can you give me some advice about my pressure boundary condition and my difficulties?

Thanks a lot!

vivian

  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
Impinging Jet Boundary Conditions Anindya Main CFD Forum 24 January 11, 2012 13:40
mesh file for flow over a circular cylinder Ardalan Main CFD Forum 6 April 17, 2010 23:40
Proper Pressure Boundary Conditions for Buoyant Flow mchurchf OpenFOAM 0 March 25, 2010 13:16
mass flow in is not equal to mass flow out saii CFX 2 September 18, 2009 08:07
boundary conditions in compressible flow mohnish FLUENT 3 March 9, 2007 03:58


All times are GMT -4. The time now is 11:39.