# boundary conditions and artificial viscosity

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 March 31, 2014, 11:56 boundary conditions and artificial viscosity #1 Senior Member   Joachim Join Date: Mar 2012 Location: Atlanta Posts: 140 Rep Power: 7 Hey everyone! I am using the Jameson dissipation terms in my solver, and I was wondering what to do at the boundaries. I read in some papers that the dissipation was sometime set to zero at the wall. Why is that so? why shouldn't I compute the value of the dissipation term at the wall assuming that u = 0 (viscous)? I guess that at the inlet/outlet, I can simply compute the dissipation terms from the boundary values. Any suggestions?? Thanks! Joachim

March 31, 2014, 12:20
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by Joachim Hey everyone! I am using the Jameson dissipation terms in my solver, and I was wondering what to do at the boundaries. I read in some papers that the dissipation was sometime set to zero at the wall. Why is that so? why shouldn't I compute the value of the dissipation term at the wall assuming that u = 0 (viscous)? I guess that at the inlet/outlet, I can simply compute the dissipation terms from the boundary values. Any suggestions?? Thanks! Joachim

artificial dissipation is just a trick to stabilize the numerical solution across zones where strong gradients are present and can generate oscillations, so it should be added only locally

 March 31, 2014, 13:18 #3 Senior Member   Joachim Join Date: Mar 2012 Location: Atlanta Posts: 140 Rep Power: 7 Thanks for your answer! However, I am using a central scheme for my convective terms (+RK order 4), so that I need fourth order artificial viscosity to remove any odd/even decoupling.

March 31, 2014, 13:26
#4
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Filippo Maria Denaro
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Quote:
 Originally Posted by Joachim Thanks for your answer! However, I am using a central scheme for my convective terms (+RK order 4), so that I need fourth order artificial viscosity to remove any odd/even decoupling.

but artificial dissipation is modulated by the local gradient intensity... do you have strong gradients as BC.s?

 March 31, 2014, 13:32 #5 Senior Member   Joachim Join Date: Mar 2012 Location: Atlanta Posts: 140 Rep Power: 7 well, I am computing the flow over a laminar flat plate right now, so I have gradients at the wall...

March 31, 2014, 13:51
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Filippo Maria Denaro
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Quote:
 Originally Posted by Joachim well, I am computing the flow over a laminar flat plate right now, so I have gradients at the wall...

This is not clear ... when you solve laminar flow, there is no unresolvable gradient in such case, therefore the artificial dissipation does not make sense...

 March 31, 2014, 13:54 #7 Senior Member   Joachim Join Date: Mar 2012 Location: Atlanta Posts: 140 Rep Power: 7 I have the values at time step n, and I explicitly compute the fluxes to get the state vector at time step n+1. However, I don't know if I should include the artificial viscosity at the wall or not...

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