turbulence kinetic energy at the near-wall node

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 February 10, 2014, 11:05 turbulence kinetic energy at the near-wall node #1 Senior Member   Ke Wu Join Date: Jan 2012 Posts: 179 Rep Power: 7 I applied standard K-epsilon 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 K-epsilon model I set the inflation layer in 0.3m Now I wanna find out the turbulence kinetic energy at the near-wall node One thing I'm confused is that if the near-wall node is 0.3m above the ground Thanks

 February 10, 2014, 16:57 #2 Senior Member   Join Date: Feb 2013 Location: Germany Posts: 190 Rep Power: 16 You should use more than just one layer. I never do less than 3 Layers, usually more. In Fluent k-epsilon 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 no-slip 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:
 Originally Posted by kad You should use more than just one layer. I never do less than 3 Layers, usually more. In Fluent k-epsilon 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 no-slip condition. In general you can view the first node values of all quantities by deselecting node values under contours.
Do you mean k in the first node is zero

How can it be

We need k for value of y plus

 February 10, 2014, 17:35 #4 Senior Member   Join Date: Feb 2013 Location: Germany Posts: 190 Rep Power: 16 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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Ke Wu
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Quote:
 Originally Posted by kad 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 .

In the viscous sub-layer, 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 #6 Senior Member   Join Date: Feb 2013 Location: Germany Posts: 190 Rep Power: 16 Remember that k-epsilon 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
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Ke Wu
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Quote:
 Originally Posted by kad Remember that k-epsilon 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.

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 non-zero at wall

If it is, how can we explain no-slip condition at wall

Does no-slip condition only confine tangential velocity is zero, not the normal component of velocity

In case we use low reynolads K-epsilon models, is laminar-sublayer solved too cause low value of y plus required for this model.

Does it mean the numerical results from low reynolads k-epsilon model is more accurate than standard k-epsilon model with wall function

February 10, 2014, 22:15
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Quote:
 Originally Posted by itsme_kit Hi Kad 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 non-zero at wall If it is, how can we explain no-slip condition at wall Does no-slip condition only confine tangential velocity is zero, not the normal component of velocity Normal and tangential velocity is zero on walls. Like it is stated in the link above a modified k equation is solved in the near wall cells. Transport of k can still occur over the internal faces of a boundary cell so it isn't usually zero. In case we use low reynolads K-epsilon models, is laminar-sublayer solved too cause low value of y plus required for this model. Yes Does it mean the numerical results from low reynolads k-epsilon model is more accurate than standard k-epsilon model with wall function In general low Re models should give better results, but mesh requirements (especially wall resolution) are much more strict.
My general advice would be to remesh your geometry with a good resolution of the boundary layer and at least 5+ layers. Then use k-omega-sst as it overcomes some k-epsilon (and standard k-omega) weaknesses and can be used for y+< 30.

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