CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

Comparing explicit/implicit convective BCs

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

LinkBack Thread Tools Display Modes
Old   October 4, 2005, 21:39
Default Comparing explicit/implicit convective BCs
Posts: n/a
when imposing a convective BC at the outlet, given by du/dt+(c)du/dx=0, it is possible to discretize as

explicit u(n+1,i+0.5) = u(n,i+0.5) - c*(dt/dx)*(u(n,i+0.5)-u(n,i))

or implicit u(n+1,i+0.5) = u(n,i+0.5) - c*(dt/dx)*(u(n+1,i+0.5)-u(n+1,i))

n refers to current(known) time step i refers to right extreme cell centered node. i+0.5 refers to right extreme(boundary) face location c = space-averaged streamwise exit velocity

if i am using an implicit scheme, does it mean that i must use an implicit BC & vice versa? i've tried both for my implicit scheme but preliminary tests don't show much difference....

anyone care to comment? thanks

  Reply With Quote

Old   October 8, 2005, 06:41
Default Re: Comparing explicit/implicit convective BCs
Posts: n/a
Dear Quarkz,

It is not necessary that if you are using an implicit scheme for time stepping your bcs should also be implicit. An explicit boundary condition is good enough. The advantage however seems to appear in fine grid computations. While the use of an explicit bc seems to cause a saturation in the residue fall ( and hence 'unclean' convergence, if you can call it so), the implicit boundary conditions show a smmoth convergence to steady state. This property of damping associated with implicit b.c. can be explouted in 2d and 3d EULER computations. Improvements in viscous flows are not very significant. Thus use of implicit schemes accelerate convergence and coupled with implicit bc's (which can dampen the errors) lead to a better and smoother convergence.

Hope this helps.



  Reply With Quote

Old   August 3, 2011, 10:11
New Member
Join Date: Jun 2011
Posts: 15
Rep Power: 6
harish_0689@ is on a distinguished road
Hello, can any one plz explain me what is exactly meant by the Implicit boundary condition and how to implement it in my code of c-d nozzle?
harish_0689@ is offline   Reply With Quote


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
Dealing with BC's in OF 1.6 vkrastev OpenFOAM Running, Solving & CFD 5 September 4, 2012 11:58
Convective / Conductive Heat Transfer in Hypersonic flows enigma Main CFD Forum 2 November 1, 2009 23:53
Convective schemes for vector fields edoardo OpenFOAM Running, Solving & CFD 6 February 16, 2009 12:05
convective bc star Main CFD Forum 0 March 24, 2005 09:35
Need Assistance: BCs for k-e model implementation Dean Schrage Main CFD Forum 3 August 11, 2001 01:02

All times are GMT -4. The time now is 03:32.