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Old   April 12, 2012, 14:04
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Hi all,
I'm currently writting up my disertation an am getting slightly confused about a few aspects of the wall functions.

I generally understand the different velocity profiles within a turbulent boundary layer and the physical principles (viscous sublayer, buffer layer, law of the wall).
However, the problem arises when I start to think about how the wall functions employ the use of these profiles. From my understanding, the law of the wall can be used to approximate the profile for values of Y+ > 30. What happens about the rest of the fluid below this point i.e. the viscous sub layer which doesn't exhibit this profile, as this is where the molecular viscous forces are dominant and surely contribute greatly to the skin friction. Does the wall function then assume a linear relationship between y and the fluid velocity to approximate the velocity profile below Y+ = 30

Any help would be great,

Kind Regards,

Kit
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Old   April 13, 2012, 02:03
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Quote:
Does the wall function then assume a linear relationship between y and the fluid velocity to approximate the velocity profile below Y+ = 30
No. Wall function (log law) does not assume the linear relationship by definition. However, two points are important to consider.

1) Standard wall functions ceases to be valid in linear profile.

2) Commercial codes assume the log profile above Y+ = 11.06 where both profile intersect each other.


So if you want to use the standard wall function then try to keep the Y+ alteast 11.06 otherwise your results wont be having any physical significance.

The solution to above problem is the use of scalable wall function for K-epsilon based models (Available in CFX and Fluent) which keeps the Y+ above 11.06.

OR

Use the automatic wall treatment (aka hybrid wall function or all Y+ wall function) which blends the both profiles and uses the linear profile for Y+ below 5, wall function for Y+ greater than 30 and mix of both in between Y+ = 5-30.
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Old   April 13, 2012, 04:37
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Hi there,

thanks for your reply. So is the law of the wall applied to the fluid in the cell next to the wall? even though it is not valid? or is the wall function turned off for the fluid in this cell?

Regards,

Kit
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