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-   -   Comparing explicit/implicit convective BCs (http://www.cfd-online.com/Forums/main/10016-comparing-explicit-implicit-convective-bcs.html)

Quarkz October 4, 2005 21:39

Comparing explicit/implicit convective BCs
 
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


ganesh October 8, 2005 06:41

Re: Comparing explicit/implicit convective BCs
 
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.

Regards,

Ganesh


harish_0689@ August 3, 2011 10:11

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?


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