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-   -   when use lowReynolds Rans models (http://www.cfd-online.com/Forums/openfoam/63867-when-use-lowreynolds-rans-models.html)

erge April 22, 2009 08:26

when use lowReynolds Rans models
 
Hi everybody.

I have a question about when one can use the lowReynolds Rans models. I think it depends on the y+ value of the mesh that one is using. But what is the critical value of y+ for using lowReynolds/highReynolds models?

santos April 22, 2009 10:53

Hi Eric,

I normally use LowRe models when I am interested in accurate wall-related calculations, such as wall shear stress or mass transfer coefficient. In order to use these models, your y+ should be below 1. For standard Re models, your y+ should be above 30.

Regards,
Jose Santos

erge April 27, 2009 05:27

Thank you for your aswer.

There is still one thing i don't understand. You say that the y+ value should be below 1 for lowRe models and y+ should be over 30 for standard Re models. What type of model should one use when 1<y+<30? It might be a stupid question but im very new to OpenFOAM. :)

//Eric

santos April 27, 2009 05:53

In order to maintain the accuracy of your results, I think you should try to keep your y+ in the range I mentioned. If your y+ is between 1 and 30, you need to either refine or coarsen your mesh, depending on the type of turbulence model you want to use.

Regards,
Jose Santos

harishg May 5, 2009 13:17

The wall function boundary conditions are valid only for yplus > 30 and hence it fixes that upper limit. The yplus < 1 is necessary to obtain grid independent solution when the model is integrated into the viscous layer. This fixes the lower limit. To use the low Re models, yplus should be less than 5 but when it is not less than 1, it might not ensure grid independent solution. Also not all models guarantee a grid independent solution when yplus is < 1. Check which RANS model has been most successfully employed for the kind of flow you are modeling and employ that model.


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