Different Meshes for Different Physics

physicsunited · August 26, 2018, 16:59

Dear fellow cfd-online members,

I am using a modified icoFoam setup, that couples transient passive scalar transport to the momentum equations.

My issue is that my momentum equations don't require a fine mesh, while my scalar transport problem requires a relatively very fine mesh (due to high Peclet numbers). This further constrains my time step choice (due to the Courant number and viscous stability constraints) and adds a significant number of time steps to my problem. These time steps do not add significant physical information to my problem, but are rather there due to numerical stability requirements. Basically in one of the equations I have to use very fine mesh while the other one works OK with a coarser mesh.

I was wondering whether it would be possible to solve my coupled system on different meshes (one for each set of equations), and project the velocity results from the coarse mesh to the finer mesh to be used in solving my scalar transport equation.

Is there anyway that I can assign different meshes for each of these equations and thus save computational time?

Best,
Pi-Man

piu58 · August 27, 2018, 00:40

You may always use a script which runs both simulations, one after the other. You need a tiny program in the programming language of your choice, which reads the velocity results of the coarse mesh and transfers it (with interpolation) to the finer one.

pete20r2 · August 27, 2018, 01:43

I'd be curious to see how you go, I suspect you will spend more time interpolating and IO operations than just solving on the fine mesh. Unless you do it inside the one solver with two meshes and keep the fields in memory while you solve I don't think you'll see the benefit.

GerhardHolzinger · August 27, 2018, 04:42

It can be done, see this post of mine, which contains a screenshot.

I took inspiration from the multi-region solvers, and the mapFields utility.

I have no idea, when the additional effort (the second mesh, the mapping of fields each time step) is worth it.

August 26, 2018, 16:59	Different Meshes for Different Physics	#1
physicsunited New Member Pi-Man Join Date: Feb 2018 Posts: 10 Rep Power: 8	Dear fellow cfd-online members, I am using a modified icoFoam setup, that couples transient passive scalar transport to the momentum equations. My issue is that my momentum equations don't require a fine mesh, while my scalar transport problem requires a relatively very fine mesh (due to high Peclet numbers). This further constrains my time step choice (due to the Courant number and viscous stability constraints) and adds a significant number of time steps to my problem. These time steps do not add significant physical information to my problem, but are rather there due to numerical stability requirements. Basically in one of the equations I have to use very fine mesh while the other one works OK with a coarser mesh. I was wondering whether it would be possible to solve my coupled system on different meshes (one for each set of equations), and project the velocity results from the coarse mesh to the finer mesh to be used in solving my scalar transport equation. Is there anyway that I can assign different meshes for each of these equations and thus save computational time? Best, Pi-Man

August 27, 2018, 00:40		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	You may always use a script which runs both simulations, one after the other. You need a tiny program in the programming language of your choice, which reads the velocity results of the coarse mesh and transfers it (with interpolation) to the finer one. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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August 27, 2018, 01:43		#3
pete20r2 Senior Member Peter Baskovich Join Date: Jul 2014 Posts: 127 Rep Power: 11	I'd be curious to see how you go, I suspect you will spend more time interpolating and IO operations than just solving on the fine mesh. Unless you do it inside the one solver with two meshes and keep the fields in memory while you solve I don't think you'll see the benefit.

August 27, 2018, 04:42		#4
GerhardHolzinger Senior Member Gerhard Holzinger Join Date: Feb 2012 Location: Austria Posts: 339 Rep Power: 28	It can be done, see this post of mine, which contains a screenshot. I took inspiration from the multi-region solvers, and the mapFields utility. I have no idea, when the additional effort (the second mesh, the mapping of fields each time step) is worth it.