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April 2, 2019, 08:45 
Staggered mesh

#1 
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Mohamed Abubakr Ibrahim
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I am coding a cfd code using FV method, and I wonder if I have to use staggered mesh for simple algorithm. The problem is lamina 2d back step for incomprehensible flow.


April 3, 2019, 01:52 

#2  
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As far as I know, and correct me if I am wrong, the whole point of staggered mesh for incompressible is to preserve energy. If not, spurious energy will be generated, and it's reflected in your pressure field ('cauz pressure is directly relates to energy). Anyway, in a word, yes, it might be the easiest way for you to use staggered mesh for incompressible simulation. 

April 3, 2019, 02:10 

#3  
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Mohamed Abubakr Ibrahim
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I think this will lead to destroying energy (kinematic) energy and the solution may never converge. But what I was wondering about is, even for simple flow with good initial values, will checkerboard condition (That what I think you meant by pressure oscillation) happen? 

April 3, 2019, 02:18 

#4  
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BTW, thanks for the clarification on the pressure thing. 

April 3, 2019, 02:22 

#5 
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Mohamed Abubakr Ibrahim
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You are welcome and thanks for your help.


April 3, 2019, 04:09 

#6 
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Filippo Maria Denaro
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Is it necessary to use a staggered grid? The answer is no, you can use a nonstaggered grid provided that proper care in the formulation is in act
Is it useful to use a staggered grid? the answer is that can be simple and useful only for second order central discretization. The problem with the nonstaggered grid is in the possible decoupling of spurious solutions in the pressure field when a large stencil is used for the pressure. But to see this in terms of kinetic energy production you need to see the divergencefree constraint. Indeed, p*(Div v) is the production term in the balance equation for the kinetic energy. 

April 3, 2019, 04:22 

#7  
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Mohamed Abubakr Ibrahim
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Could you please clarify what kind of formulation you mean for usage of nonstaggered mesh? Thanks for correcting the energy generation point 

April 3, 2019, 04:39 

#8  
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Filippo Maria Denaro
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Upwind introduce a lot of numerical viscosity in the discretization of the convective term but is not used in the pressure equation that is elliptic. 

April 3, 2019, 04:44 

#9 
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Hello guys, some points were already arised during your discussion. Just to give you my point of view, staggered mesh will solve the checkboarding problem but they are not necessarily easier to implement since the staggering can create some confusion in your head. Moreover they are unsuitable for unstrucured domains or nonrectangular domain. Since you are in the very beginning consider to do a collocated arrangement which nowdays is the common practice. RhieChow interpolation in easy to perform and well documented. Regarding your last question, yes you need rhiechow even with upwind. The point is that you have to interpolate your momentum equation at cell faces in collocated arrangement (you don't in staggered) and this may cause spurious oscillations in the pressure field.


April 3, 2019, 04:52 

#10 
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Mohamed Abubakr Ibrahim
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Pressures equation ?! pressure correction equation in simple algorithm?
And I know about the false diffusion in the upwind scheme however I am currently not so focused on accuracy however I am targeting roboust simple implementation. 

April 3, 2019, 04:53 

#11 
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Filippo Maria Denaro
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Yes, RC interpolation is easy to implement and produce stable and unique pressure solutions. It is suitable to start using it. However, such artificial dissipation terms can affect the quality of the solution in some specific formulations like LES.
I developed a more sophisticated formulation that does not introduce dissipation. However, it is introduced in the framework of th Exact projection methods and requires some effort in the implementation and has some computational cost. https://www.researchgate.net/publica...ydriven_flows https://www.researchgate.net/publica...taggered_grids 

April 3, 2019, 04:56 

#12 
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Yes, pressurecorrection. At some point you have to compute the coeffs as rho*vol/aC and interpolate them at faces.
Upwind is a point to start. You have a lot of diffusion (espescially if the flow is tilted), but it is the most stable option you have. To get the 2nd order go for a linearupwind, still robust (more than central difference) 

April 3, 2019, 05:00 

#13  
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Mohamed Abubakr Ibrahim
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April 3, 2019, 05:10 

#14  
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cfd, simple algorithm 
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