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December 27, 2002, 17:30 
atmospheric roughness height

#1 
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Hi,
I work with atmospheric boundary layer. wind profile of inlet boundary layer is : u=u_tau/kappa*ln(z/z_0) where roughness height z_0=0.07 meter (flat plane) Turbulence intensity profile : I_t=1/ln(z/z_0) I use a law of wall : u_plus=1/kappa*ln(z+/z+_0) In StarCD Methodology page 66, i found that y_p (height of the center of cell) should be 4 times larger than the roughness height. My questions : 1. I'm looking for publications on roughness height and CFD (witch grid can i take), could you help me? 2. Witch y+ range can i give? 3. Is this modelisation good? 4. Witch lenght scale can i take (i just know that L_30m=100m)? PS : I try a "power law" for the wind profil and i obtain bad results because i use low wind and power low is for strong wind Thanks 

December 27, 2002, 21:51 
Re: atmospheric roughness height

#2 
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For highRe ke models, the boundary layer has already been assumed a shape with the 'law of the wall' function. If you want to predict a resultant b/l, I think you should be calculating it directly with a lowRe ke model.


December 28, 2002, 03:49 
Re: atmospheric roughness height

#3 
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Hi,
My Reynolds number is near 10^5. I don't know that a low Re ke model is good. 

December 29, 2002, 22:52 
Re: atmospheric roughness height

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I've copied the following from www.adapcoonline.com:
 "The models for the RANS equation typically break down near the walls, where there is a very thin boundary layer with large velocity gradients. At high Re, this layer is so thin that it is difficult to get a sufficient number of fluid cells into this zone. Even if this is done, the forms of the ke equation suitable for the interior do not work very well at the wall, as the flow transitions to a laminar sublayer. STARCD offers three alternatives to deal with this situation. The usual procedure is to use an algebraic "Law of the wall" model in the first cell at the boundary. The boundary layer is assumed to be contained within this cell, and the shear stress at the wall, and the boundary conditions for the K and e equations are computed from the log law. For this to be applicable, the cell next to the wall must be big enough; the value of Y+ at the cell centroid should be greatar than 30. This is rarely a problem, except in the neighbourhood of separation points and reattachment regions. It can be checked easily by plotting the Y+ at the boundary in PROSTAR, after the solution. At the next level of complexity are the two layer models. In these models, a low Reynolds number representation of the turbulence equation is used in the viscosity influenced near wall region. Since transport equations must be solved in this region, there must be a sufficient grid density in the direction normal to the wall. Typically about 10 cells are placed in the region of Y+ < 30. This usually results in a large cell count, increasing the computational requirements. There is also a (one time) penality where STARCD has the determine the wall distances of the viscous layer cells. In these two layer models, the flow in the wall layer is treated by one of two ways; a one equation model where nearwall flow is simulated by a low Reynolds number model consisting of a transport equation for k and an algebraic prescription for the turbulence length scale; or a mixing length model where an algebraic expression is used for the turbulent viscosity. In either case, the solution is matched to that of the standard k e equations at the edge of the viscosityinfluenced region. The third alternative for dealing with wall effects is a Low Reynolds Number Ke model. The meshing requirements near the wall are similar to the two layer models. This model uses the standard transport equation for K, but augments the equation for e with an extra term which depends on the distance from the nearest wall. Thus in principle, this model is valid all the way down to the solid wall. The startup computational cost is higher, because the wall distances need to be computed for all the cells."  With the HiRe models, you'll need to adjust the nearwall cell layer (to be within the recommended 30100 Y+ value) before you get a nice answer. HiRe turb models will usually give bad results when accurate calculation of the nearwall flow is required, eg. conjugate heat transfer, which needs an accurate velocity profile to in turn get a good thermal b/l. ICEM even has a commercial product called 'Y+' that is supposed to automatically adjust your near wall cells according to your previous sim results. OR: you can just TREAT it as if you were calculating for a LowRe flow with the LowRe ke model. Problem now is that you've got to mesh it really fine at the walls. Doesn't say anywhere that it is wrong to use it if your (mean) flow Reynolds number is in the turbulent regime (in fact, they are the same, except for the a slight mod in the e equation). You could try the twolayer models too. 

December 30, 2002, 06:12 
Re: atmospheric roughness height

#5 
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Hi,
by the way, what profile for turbulence length scale do you use? 

December 30, 2002, 06:26 
Re: atmospheric roughness height

#6 
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Hi,
I use a linear profile for turbulence length scale but i don't know if that is a good modelisation 

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