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February 10, 2014, 11:05 
turbulence kinetic energy at the nearwall node

#1 
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Ke Wu
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I applied standard Kepsilon model with standard wall function
I'm modelling air flow around 3d building I only apply a single layer in the ground and building surface Is it one thin layer enough? I only applied one because I didn't use low reynolds Kepsilon model I set the inflation layer in 0.3m Now I wanna find out the turbulence kinetic energy at the nearwall node One thing I'm confused is that if the nearwall node is 0.3m above the ground Thanks 

February 10, 2014, 16:57 

#2 
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You should use more than just one layer. I never do less than 3 Layers, usually more.
In Fluent kepsilon implies a zero gradient condition normal to the wall for k. Therefore the values of k on the walls are equal to the first node values, thus k is physically zero on the wall due to noslip condition. In general you can view the first node values of all quantities by deselecting node values under contours. 

February 10, 2014, 17:09 

#3  
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Ke Wu
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Quote:
How can it be We need k for value of y plus 

February 10, 2014, 17:35 

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Nope, it is a zero gradient condition. This does not mean that k is zero but that it does not change in the surface normal direction. Check out http://aerojet.engr.ucdavis.edu/flue...ug/node512.htm .


February 10, 2014, 17:57 

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Thanks for your reply In the viscous sublayer, u plus = y plus I wonder why the error is significant if y plus is located in this region Why we normally take y plus in the region between 30 and 300 (for example) Is effect wall roughness excluded between 30 and 300 

February 10, 2014, 19:12 

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Remember that kepsilon model is only valid for fully turbulent flow. Therefore the first node has to be far enough from the wall where flow is fully turbulent. Then wall functions are needed to bridge the cell values with the wall values. If the first node is to close too the wall, you are using the turbulence model in a non valid region.
Wall roughness is generally not excluded but its influence depends strongly on the flow itself, e.g. it might be covered by the laminar sublayer. 

February 10, 2014, 19:25 

#7  
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Ke Wu
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Thanks very much I'm much more clear now, and still get some doubts to be clarified If k is zero gradient and equal to the value of first node Does it mean the velocity magnitude is nonzero at wall If it is, how can we explain noslip condition at wall Does noslip condition only confine tangential velocity is zero, not the normal component of velocity In case we use low reynolads Kepsilon models, is laminarsublayer solved too cause low value of y plus required for this model. Does it mean the numerical results from low reynolads kepsilon model is more accurate than standard kepsilon model with wall function 

February 10, 2014, 22:15 

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