# modifing equations in the solver

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 May 15, 2012, 14:37 modifing equations in the solver #1 New Member   Join Date: Aug 2011 Posts: 13 Rep Power: 8 Sponsored Links Hello Ive a question about the non-linear term (convection term) in N.S equations : (u*nabla)u I want to modify this term into : ((u+u0)*nabla)(u+u0) I mean u=u+u0 where u0 is defind as some known function , can someone please give me a direction on how to do this? ThankYou

 May 16, 2012, 01:50 #2 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 14 Did you derive what the discretized equation should look like? Maybe you can treat all the terms that are not u*nabla u as a source? By the way, what is the physics in this approach? It seems a bit weird here? Maybe you want to use u0 as initialization?

 May 16, 2012, 08:12 #3 New Member   Join Date: Aug 2011 Posts: 13 Rep Power: 8 This form of equation represent the nonlinear dynamics of the perturbations only and u0 is the basic flow, so u is vector of perturbations and p is the fluctuated pressure only. in the convective term ((u+u0)*nabla)(u+u0): (u0*nabla)u0 is identically zero and the basic pressure plus the laplace term for u0 constitute the basic velocity u0, so also these terms falls , and then you get the dynamics of the perturbations Thanks for the advice I will try to understand your suggestion

May 16, 2012, 08:34
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Hi bernard
Quote:
 Maybe you can treat all the terms that are not u*nabla u as a source
I believe that is not possible because there are terms like '(u0*nabla)u' and
'(u*nabla)u0' ,and these terms cannot be treated as source terms, because they are depended on 'u'.
Thus they have to be treated as implicit terms like (u*nabla)u.
So, how do we implement these modifications?
Should we modify the matrix only or also the PISO loop?

Thanks

 May 16, 2012, 09:27 look #5 Member   Lev Join Date: Dec 2010 Posts: 31 Rep Power: 8 Hey, guys have a look there http://www.cfd-online.com/Forums/ope...tml#post361417 Regards Lev

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