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high levels of sub-grid viscosity

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Old   March 24, 2015, 06:27
Default high levels of sub-grid viscosity
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Hi, everyone. Recently, I am working with DES. There are 2 problems that I would like to ask for help here.
First, "very low mesh count for inlet velocity, have the levels of sub-grid viscosity been checked", the question is how can I check the levels of sub-grid viscosity in Fluent?
Second, "high levels of sub-grid viscosity could account for the non-turbulent appearance in the wake of the blunt body", the question is what does the high levels of sub-grid viscosity mean?

Any feedback is very welcome. Thanks!
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Old   March 25, 2015, 01:14
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Originally Posted by csuperfect View Post
Hi, everyone. Recently, I am working with DES. There are 2 problems that I would like to ask for help here.
First, "very low mesh count for inlet velocity, have the levels of sub-grid viscosity been checked", the question is how can I check the levels of sub-grid viscosity in Fluent?
Second, "high levels of sub-grid viscosity could account for the non-turbulent appearance in the wake of the blunt body", the question is what does the high levels of sub-grid viscosity mean?

Any feedback is very welcome. Thanks!
I don't what it is for DES, but for LES the variable is "subgrid turbulent viscosity" in the GUI. Not sure what it is in TUI.

A high subgrid turbulent viscosity is an indicator that the subgrid scale model is having a large influence on the flow (which can be attributed to mesh being too coarse, i.e. under-resolved DES/LES).
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Old   March 26, 2015, 03:53
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Originally Posted by LuckyTran View Post
I don't what it is for DES, but for LES the variable is "subgrid turbulent viscosity" in the GUI. Not sure what it is in TUI.

A high subgrid turbulent viscosity is an indicator that the subgrid scale model is having a large influence on the flow (which can be attributed to mesh being too coarse, i.e. under-resolved DES/LES).
Yes, I also got it through some references. Thank you very much.
By the way, do I need to refine the mesh in three directions? If that, the total mesh number will increase sharply. Is it OK that I just improve the mesh resolution in the streamwise direction?
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Old   March 26, 2015, 10:34
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Originally Posted by csuperfect View Post
Yes, I also got it through some references. Thank you very much.
By the way, do I need to refine the mesh in three directions? If that, the total mesh number will increase sharply. Is it OK that I just improve the mesh resolution in the streamwise direction?
Because of the way the filter width is calculated (from cell volume), reducing the cell volume will tend to decrease the filter width, so you can refine the grid in any direction to achieve that; but it is still best to refine in all three dimensions and if that is not feasible then refine in direction of strongest gradients. Usually the strongest gradients are normal to the streamwise direction. Unfortunately refining the grid in the streamwise direction will do the least to help you with your problem.
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Old   March 27, 2015, 02:01
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Originally Posted by LuckyTran View Post
Because of the way the filter width is calculated (from cell volume), reducing the cell volume will tend to decrease the filter width, so you can refine the grid in any direction to achieve that; but it is still best to refine in all three dimensions and if that is not feasible then refine in direction of strongest gradients. Usually the strongest gradients are normal to the streamwise direction. Unfortunately refining the grid in the streamwise direction will do the least to help you with your problem.
Thank you very much. I will try it.
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Old   March 27, 2015, 20:19
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Originally Posted by LuckyTran View Post
Because of the way the filter width is calculated (from cell volume), reducing the cell volume will tend to decrease the filter width, so you can refine the grid in any direction to achieve that; but it is still best to refine in all three dimensions and if that is not feasible then refine in direction of strongest gradients. Usually the strongest gradients are normal to the streamwise direction. Unfortunately refining the grid in the streamwise direction will do the least to help you with your problem.
When I refine the mesh in all three dimensions, can you give me some help or references to ensure the mesh is OK?
By the way, in simulations, the grid-independent validation is needed to be carried out, How can I draw the Coarse mesh, Medium mesh and Fine mesh?
Is it neccesary to keep the height of the first grid the same near wall surface, and I just reduce/increase the nodes/ratio?

Thank you very much!
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