Oscillating velocity and porous zones

alberto · August 16, 2008, 20:54

I noticed that in simulations with porous zones, the velocity oscillates at the exit of the porous zone. Moreover something unexpected happens also close to the inlet, as shown in the picture.

The system is a 2D channel, 1m long (diameter 0.07), with a porous zone in the first 0.5m (porosity = 0.5, d = 10^7, f = 0). The inlet velocity is 1m/s. All solver settings are taken from the rhoSimplePorousFoam implicit tutorial case.

The result shown here is obtained with a coarse grid. The behaviour improves significantly with a very grid with cells of some millimeter, but some issue is still present at the end of the porous zone, as shown below (cell 0.001 x 0.001).

Is this a known issue or are there particular requirements for the mesh around the porous zone?

With kind regards,
Alberto

alberto · August 16, 2008, 20:59

Sorry for the typo in

"The behaviour improves significantly with a very grid with cells..."

which should be:

"The behaviour improves significantly with a very fine grid with cells..."

dbxmcf · August 17, 2008, 16:16

Hi, here is just a guess, when the density changes significantly for the inlet and outlet, a shock wave case probably occurs, to overcome this, some type of artificial viscosity should be added to the momentum equation, see Von Neumann and Richtmyer (1950) and Jameson et al

alberto · August 18, 2008, 15:25

Thanks Roy,

I agree because the porous zone introduces a discontinuity in the resistance at the borders of the porous zone. However I think the right way to solve this is to work on the interpolation schemes/solution algorithms to manage the shock.

What gives me something to think about is the too high sensitivity to the mesh refinement. Even on a very refined mesh, the velocity profile changes significantly.

For example:

Grid with squared cells of 3mm:

Grid with squared cells of 1mm:

Regards,
Alberto

RobertoCirolini · October 28, 2016, 03:14

Hi,

I am sorry for re-opening this thread but I have found the same problem in my case.

I am really new in OpenFoam and I have been looking for solutions for my problem and I do not really see any other options but editing the solution/schemes algorithm.

I am simulating a flow passing through a flat porous media and when I see my results I also see the oscillation of velocity profile before it enters in to porous zone. Alberto suggested that we can manage those shock working on interpolation schemes/solution algorithms. I would like to know if it is possible to minimize those oscillations just changing my solver or my configurations either in fvSchemes or fvSolutions.

I am now working with SimpleFoam and I have already changed the relaxations factors, refined the mesh near the porous zone and also changed the time steps considering the CFL-Number.

I would be very thankful hearing any suggestions!!

Best regards,

Roberto

August 16, 2008, 20:54	I noticed that in simulations	#1
alberto Senior Member Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36	I noticed that in simulations with porous zones, the velocity oscillates at the exit of the porous zone. Moreover something unexpected happens also close to the inlet, as shown in the picture. The system is a 2D channel, 1m long (diameter 0.07), with a porous zone in the first 0.5m (porosity = 0.5, d = 10^7, f = 0). The inlet velocity is 1m/s. All solver settings are taken from the rhoSimplePorousFoam implicit tutorial case. The result shown here is obtained with a coarse grid. The behaviour improves significantly with a very grid with cells of some millimeter, but some issue is still present at the end of the porous zone, as shown below (cell 0.001 x 0.001). Is this a known issue or are there particular requirements for the mesh around the porous zone? With kind regards, Alberto __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

August 16, 2008, 20:59	Sorry for the typo in "The	#2
alberto Senior Member Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36	Sorry for the typo in "The behaviour improves significantly with a very grid with cells..." which should be: "The behaviour improves significantly with a very fine grid with cells..." __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

August 17, 2008, 16:16	Hi, here is just a guess, when	#3
dbxmcf Member roy fokker Join Date: Mar 2009 Posts: 44 Rep Power: 17	Hi, here is just a guess, when the density changes significantly for the inlet and outlet, a shock wave case probably occurs, to overcome this, some type of artificial viscosity should be added to the momentum equation, see Von Neumann and Richtmyer (1950) and Jameson et al

August 18, 2008, 15:25	Thanks Roy, I agree because	#4
alberto Senior Member Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,912 Rep Power: 36	Thanks Roy, I agree because the porous zone introduces a discontinuity in the resistance at the borders of the porous zone. However I think the right way to solve this is to work on the interpolation schemes/solution algorithms to manage the shock. What gives me something to think about is the too high sensitivity to the mesh refinement. Even on a very refined mesh, the velocity profile changes significantly. For example: Grid with squared cells of 3mm: Grid with squared cells of 1mm: Regards, Alberto __________________ Alberto Passalacqua GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541) OpenQBMM - An open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

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October 28, 2016, 03:14		#5
RobertoCirolini New Member Roberto Join Date: May 2016 Posts: 17 Rep Power: 9	Hi, I am sorry for re-opening this thread but I have found the same problem in my case. I am really new in OpenFoam and I have been looking for solutions for my problem and I do not really see any other options but editing the solution/schemes algorithm. I am simulating a flow passing through a flat porous media and when I see my results I also see the oscillation of velocity profile before it enters in to porous zone. Alberto suggested that we can manage those shock working on interpolation schemes/solution algorithms. I would like to know if it is possible to minimize those oscillations just changing my solver or my configurations either in fvSchemes or fvSolutions. I am now working with SimpleFoam and I have already changed the relaxations factors, refined the mesh near the porous zone and also changed the time steps considering the CFL-Number. I would be very thankful hearing any suggestions!! Best regards, Roberto