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

outflow boundary contition

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 25, 2010, 09:28
Cool outflow boundary contition
  #1
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
Hi!
i need some help with an outflow bc. i am writing an 1D code for a flow in a duct. i am using 1D finite volume equations and an crank nikolson method.

at the last node "n" (exit flow node) i need a bc because i do not know the "n+1" node. i found that one proper boundary contrition is the
DT/dt + v*DT/dx =0 (1)
(I am solving only the energy equation).

what i'd like to ask is that in my me equation which is in form
A(n,n-1)Tn-1(t+dt) + A(n,n)Tn(t+dt) + A(n,n+1)Tn+1(t+dt) =
B(n,n-1)Tn-1(t) + B(n,n)Tn(t) + B(n,n+1)Tn+1(t)

i have to terms that i do not know the Tn+1(t+dt) and the Tn+1(t).

to do this i have to solve the (1) two times one implicit and one explicit so to replace the two above terms ?

thanks
kordou is offline   Reply With Quote

Old   December 8, 2010, 11:04
Default
  #2
New Member
 
zahra
Join Date: Sep 2010
Location: Tehran
Posts: 7
Rep Power: 6
khahar is on a distinguished road
Y don't U use CFD softwares?
khahar is offline   Reply With Quote

Old   December 8, 2010, 12:42
Default
  #3
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
I am using cfd. the purpose of the code that i make is to speedup the cfd calculations
kordou is offline   Reply With Quote

Old   December 9, 2010, 15:14
Default
  #4
New Member
 
zahra
Join Date: Sep 2010
Location: Tehran
Posts: 7
Rep Power: 6
khahar is on a distinguished road
which program U write codes with? I just know some fortran 90.
khahar is offline   Reply With Quote

Old   December 9, 2010, 17:49
Default
  #5
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
I am also using fortran 90.

i found that the vdT/Dx = 0 seems to work good. but i don't know if this is the best BC
kordou is offline   Reply With Quote

Old   December 19, 2010, 11:13
Default
  #6
New Member
 
zahra
Join Date: Sep 2010
Location: Tehran
Posts: 7
Rep Power: 6
khahar is on a distinguished road
Sorry. I don't know either.
khahar is offline   Reply With Quote

Old   December 20, 2010, 03:32
Default
  #7
Senior Member
 
Hamid Zoka
Join Date: Nov 2009
Posts: 162
Rep Power: 8
Hamidzoka is on a distinguished road
Dear Kordou;
it seems you need basic studies in CFD.
All T(t) terms,like Tn(t) or Tn+1(t) or Tn-1(t), are determined either from a previous time step olution or an initial guess.
-if you use an implicit scheme for the term of vdT/Dx , you will have two unknown terms for each point, namely Tn(t+dt) and Tn+1(t+dt). knowing the boundary condition for first and last points, (inlet and outlet) a set of equations will be earned. solution of this set will give you the right answer.
it is unconditionally stable but its solution needs more efforts.since a set of equations wil be solved simultaniously.
-if you use an explicit scheme, there is only one unknown term, namely Tn(t+dt), which is easily found at each point. but it is not unconditionally stable and may diverge for specific values of dt,dx and v.

regards
Hamidzoka is offline   Reply With Quote

Old   December 20, 2010, 15:52
Default
  #8
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
Dear Hamidzoka

At start thank you for the help
Indeed I have the basic knowledge of finite element method.

I have a code that uses the crank-nicolson method (both implicit and explicit) to solve a system of equations (the Energy equation) in the above form (1st post here). The terms in the right of the equation at time (t) are known from either a initial guess of from a previews time step.

My problem for the boundary condition (1) of the first post is not the term vDT/Dx. I calculate this as you suggest it. My problem is the that in one Boundary condition i have 2 terms, one that depends on time (DT/Dt) and the other that depents on space (vDT/Dx). If i apply only the second term as a boundady condition vDT/Dx=0 (as a read this is ok for steady state) which basically gives the same BC as DT/DX=0, my result are very good. But i have a transient code so (as i read) i have to also include the term DT/Dt (whicj is (T(n+1)-T(n))/dt as well.

but i have never worked on a BC both on time and space.

As i said i I do not have much knowledge on cfd theory, so sorry i have said something stupid.

thank you again
kordou is offline   Reply With Quote

Old   December 21, 2010, 02:17
Default
  #9
Senior Member
 
Hamid Zoka
Join Date: Nov 2009
Posts: 162
Rep Power: 8
Hamidzoka is on a distinguished road
Dear Kordou;
you are not permitted to descritize the energy equation in "n"th (exit flow) node as well as first (inlet flow).disctritization starts from node 2 to n-1.
since there is no n+1 or 0 node in domain!
lets focus on BC:
you have to choices for "n"th (exit flow) BC as the following:
1-temperature: Tn is specified
2-heat flux: Qn is specified
suppose that you 've selected the temperature. therefore Tn is known.
so, let review all temperature nodes at node "n-1":
Tn-2(t+dt) : is known from previous time step or an initial guess
Tn-2(t) : is known from previous time step or an initial guess
Tn-1(t+dt) : is unknown and should be computed
Tn-1(t) : is known from previous time step or an initial guess
Tn(t+dt)=Tn(t) : Tn is BC and assumed to be fixed. therefore it dies not change with time.(in case boundary condition of Tn varies with the time, its function should be known and therefore Tn(t+dt) is calculated using its function).
you will see there is no other unknown term.

does it help?

Regards
Hamidzoka
Hamidzoka is offline   Reply With Quote

Old   December 21, 2010, 04:25
Default
  #10
New Member
 
Join Date: Mar 2009
Posts: 19
Rep Power: 8
kordou is on a distinguished road
Dear Hamidzoka

Your answer is quite helpful!

thank you
kordou is offline   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
outflow boundary condition ravi_adepu FLUENT 7 March 26, 2014 17:35
outflow boundary condition Andy Chen FLOW-3D 4 July 27, 2013 02:41
outflow boundary condition Maziar Mahzari CFX 0 May 8, 2006 03:23
Trimmed cell and embedded refinement mesh conversion issues michele OpenFOAM Other Meshers: ICEM, Star, Ansys, Pointwise, GridPro, Ansa, ... 2 July 15, 2005 04:15
outflow boundary condition Guillaume Dulong FLUENT 1 September 13, 2004 05:12


All times are GMT -4. The time now is 21:08.