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-   -   Setting of Roughness Constant and Height (https://www.cfd-online.com/Forums/fluent/43417-setting-roughness-constant-height.html)

 Sing January 20, 2007 05:47

Setting of Roughness Constant and Height

Hi,

I am simulating a case for the air flowing over a building in a wind domain, the ground of the wind domain consists of three areas which are short grass, concrete and water.

As the wind enters the domain, the wind will first flow over water, then grass and finally reaches the concrete land.

I am trying to set different surface roughness for these three surfaces, and i have searched over the internet and obtained the roughness length for these three layers.

However, all i can set in Fluent are 'Roughness Constant' and 'Roughness Height'. How can i set the roughness of these three surfaces in fluent by using the roughness length i have obtained form the net?

Plz help!!

thx Sing

 Sing January 21, 2007 08:53

Re: Setting of Roughness Constant and Height

Oh~ I have just read a journal saying that it's a common practice for engineers to set 'Roughness Length' equal to 'Roughness Height', however, it is totally wrong!!

For Fluent, the correct relation between 'Roughness Height','Roughness Constant' and 'Roughness Length' should be

Roughness Height = (9.793 * Roughness Length) / Roughness Constant

Roughness Constant accounts for the uniformity of the particles on the surface, default value of 0.5 indicates a uniform surface, rough surface should have a value between 0.5 and 1. Thus, for normal surface, 0.5 should be appropriate.

 Goutam August 1, 2014 09:37

Quote:
 Originally Posted by Sing ;138395 Oh~ I have just read a journal saying that it's a common practice for engineers to set 'Roughness Length' equal to 'Roughness Height', however, it is totally wrong!! For Fluent, the correct relation between 'Roughness Height','Roughness Constant' and 'Roughness Length' should be Roughness Height = (9.793 * Roughness Length) / Roughness Constant Roughness Constant accounts for the uniformity of the particles on the surface, default value of 0.5 indicates a uniform surface, rough surface should have a value between 0.5 and 1. Thus, for normal surface, 0.5 should be appropriate. I am not sure if this is totally correct, does anyone know more about this?
Did you find the answer? You said, Roughness Height = (9.793 * Roughness Length) / Roughness Constant. What is Roughness Length?

From Moody Chart, we know the relative roughness. What is the relation between relative roughness and roughness height?

Thanks

 dandcrae April 25, 2015 13:56

Relative roughness usually means the roughness height / hydraulic diameter for internal flow. Not sure how it would be defined for external flow.

 ugli91 April 28, 2015 09:24

that equation for calculating laminar boundary of cell. An example for fully grass ground; each grass have 0.03m Roughness Lengt. then your roughnest height (ks) calculating with that equlation. But there is other critical thing about defining roughnes is your boundary meshs height. if half of your mesh height (Yp) smaller than ks fluent couldnt solve roughness correctly. So that we using roughness constant to keep ks < Yp equation.

 akhme August 25, 2015 18:08

Quote:
 Originally Posted by Sing ;138395 Oh~ I have just read a journal saying that it's a common practice for engineers to set 'Roughness Length' equal to 'Roughness Height', however, it is totally wrong!! For Fluent, the correct relation between 'Roughness Height','Roughness Constant' and 'Roughness Length' should be Roughness Height = (9.793 * Roughness Length) / Roughness Constant Roughness Constant accounts for the uniformity of the particles on the surface, default value of 0.5 indicates a uniform surface, rough surface should have a value between 0.5 and 1. Thus, for normal surface, 0.5 should be appropriate. I am not sure if this is totally correct, does anyone know more about this?
What journal is this?

 atikah121 January 21, 2016 09:53