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Sufficient resolution of the flame front in RANS

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Old   February 22, 2021, 03:23
Default Sufficient resolution of the flame front in RANS
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Guillaume Beardsell
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Hi,

I am interested in running port-fuel injection IC engine simulations at higher pressures, and we know that the (laminar) flame thickness generally decreases when pressure is increased. I was wondering if there are some general guidelines as to what mesh size should be used for RANS in order to properly capture the flame front.

It seems like there are a few things to consider:

1) The flame front is thickened in RANS through the turbulent conductivity.
2) Converge needs a certain number of grid points per flame thickness to capture the flame front properly (?)
3) The turbulent flame brush thickness decreases when the laminar flame thickness is decreased. (?)

Any help/thoughts would be greatly appreciated!
Thank you,
Guillaume
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Old   February 22, 2021, 07:25
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Originally Posted by magicbretzel View Post
Hi,

I am interested in running port-fuel injection IC engine simulations at higher pressures, and we know that the (laminar) flame thickness generally decreases when pressure is increased. I was wondering if there are some general guidelines as to what mesh size should be used for RANS in order to properly capture the flame front.

It seems like there are a few things to consider:

1) The flame front is thickened in RANS through the turbulent conductivity.
2) Converge needs a certain number of grid points per flame thickness to capture the flame front properly (?)
3) The turbulent flame brush thickness decreases when the laminar flame thickness is decreased. (?)

Any help/thoughts would be greatly appreciated!
Thank you,
Guillaume

Hello Guillaume,


You are correct, the turbulent flame thickness would increase with turbulent diffusivity. You can check if you are resolving the RANS length scales by comparing the minimum grid resolution with the length_scale reported in turbulence.out file.


Ideally you should run a grid sensitivity study (this can easily be done using different grid scale parameter).



Thanks,
Nitesh
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Old   February 22, 2021, 11:40
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Originally Posted by nitesh.attal View Post
Hello Guillaume,


You are correct, the turbulent flame thickness would increase with turbulent diffusivity. You can check if you are resolving the RANS length scales by comparing the minimum grid resolution with the length_scale reported in turbulence.out file.


Ideally you should run a grid sensitivity study (this can easily be done using different grid scale parameter).



Thanks,
Nitesh
Hi Nitesh,

Thank you for your answer! Could you elaborate more on comparing the grid resolution to the length scale reported in turbulence.out? My understanding is that the reported length scale represents the size of the energetic/largest subgrid scale eddies. However, I am confused about how to compare the two. Do we want the reported length scale to be roughly the same size as the grid size so that we have a "continuous" energy cascade? Does that mean that if the reported length scale is smaller than the grid size, we need to refine the mesh?

Best regards,
Guillaume
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Old   February 22, 2021, 13:32
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Originally Posted by magicbretzel View Post
Hi Nitesh,

Thank you for your answer! Could you elaborate more on comparing the grid resolution to the length scale reported in turbulence.out? My understanding is that the reported length scale represents the size of the energetic/largest subgrid scale eddies. However, I am confused about how to compare the two. Do we want the reported length scale to be roughly the same size as the grid size so that we have a "continuous" energy cascade? Does that mean that if the reported length scale is smaller than the grid size, we need to refine the mesh?

Best regards,
Guillaume

You need to have few grid points across the length_scale to resolve it. Basically, try to get have 2-4 cells across the length_scale reported in turbulence.out file.



Thanks,
Nitesh
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