What is minimum number of nodes in porous zone?

schwaral · September 1, 2020, 07:32

Hi,

I try to simulate a porous area in a 2D case. I figured out, that the calculation starts to diverg, when I use 5 or less cells in the porous Domain. My goal would be to have less than five cells in the porous domain.

The residuals show me, that only the term for my UDS is not converging. NS-equations converge fine. How is that possible? I Use for UDS first order Upwind and for all other equations 2nd Order. Gradient = least square cell based, Scheme = SIMPLE.

To my understanding there should be no reason why the UDS equation starts to diverg first. I thought equations with 2nd order should diverg first if there are not enough cells.

karachun · September 1, 2020, 15:35

But mesh convergence mean that you need to reduce mesh size, not increase it! Yiu should refine your mesh and not coarse it.

schwaral · September 1, 2020, 16:04

Because I have a quite large 3D domain I am testing how many cells I need for a certain concentration loss in porous area using first a 2d domain. But it seems like the solver does not work when I go lower than around 10 cells. That does not make sense to me since the NS equations are solved just fine down to 3 cells in the porous area. I dont understand why the UDS behaves so much worse. I also found outdecreasing the timestep splves the problem. But this wont be helpful for my large 3d case.I found as well, that the UDS source term should be linearized.

September 1, 2020, 07:32	What is minimum number of nodes in porous zone?	#1
schwaral Member Deutschland Join Date: Jun 2020 Location: https://t.me/pump_upp Posts: 43 Rep Power: 5	Hi, I try to simulate a porous area in a 2D case. I figured out, that the calculation starts to diverg, when I use 5 or less cells in the porous Domain. My goal would be to have less than five cells in the porous domain. The residuals show me, that only the term for my UDS is not converging. NS-equations converge fine. How is that possible? I Use for UDS first order Upwind and for all other equations 2nd Order. Gradient = least square cell based, Scheme = SIMPLE. To my understanding there should be no reason why the UDS equation starts to diverg first. I thought equations with 2nd order should diverg first if there are not enough cells.

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September 1, 2020, 15:35		#2
karachun Senior Member Join Date: Nov 2015 Posts: 244 Rep Power: 11	But mesh convergence mean that you need to reduce mesh size, not increase it! Yiu should refine your mesh and not coarse it.

September 1, 2020, 16:04		#3
schwaral Member Deutschland Join Date: Jun 2020 Location: https://t.me/pump_upp Posts: 43 Rep Power: 5	Because I have a quite large 3D domain I am testing how many cells I need for a certain concentration loss in porous area using first a 2d domain. But it seems like the solver does not work when I go lower than around 10 cells. That does not make sense to me since the NS equations are solved just fine down to 3 cells in the porous area. I dont understand why the UDS behaves so much worse. I also found outdecreasing the timestep splves the problem. But this wont be helpful for my large 3d case.I found as well, that the UDS source term should be linearized.