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how to choose the roughness height

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Old   August 26, 2004, 17:57
Default how to choose the roughness height
  #1
icestar
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I'm doing research to find out the surface roughness effect to the tornado vortex by fluent.

I tried to change the value of the roughness height and constant. But the result didn't changed a lot.

I have no idea how to choose these two parameters.

Did someone have experence to use fluent to simulate the roughness efforts?

Thanks a lot.
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Old   September 20, 2004, 08:54
Default Re: how to choose the roughness height
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ruth
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I am currently running a thunderstorm model using fluent and also faces the same problem. Please give some advice if anyone knows.thanks
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Old   September 20, 2004, 13:23
Default Re: how to choose the roughness height
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Titiksh Patel
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Hi

Take care that roughness is defined in metres.

Titiksh
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Old   September 20, 2004, 13:28
Default Re: how to choose the roughness height
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icestar
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I notice that, but it didn't change a lot.
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Old   September 20, 2004, 23:48
Default Re: how to choose the roughness height
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wjruth
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Hi,

I am running a 2D axisymmetry thunderstorm using fluent and roughness height between 0.01m to 1.6m, roughness constant = 0.5 . The height of the max horizontal velocity should be affected by the roughness of the ground but my results is not as expected.I would like to ask if there is other parameter that would affect the roughess. thank you
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Old   September 21, 2004, 08:03
Default Re: how to choose the roughness height
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Titiksh Patel
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Hi,

I think the first grid distance from the wall may also be the criteria. It should be above the roughness height.

Titiksh
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Old   September 22, 2004, 22:17
Default Re: how to choose the roughness height
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wjruth
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Hi Titiksh, thanks for your advice. my biggest roughness height is 1.6m and I uses boundary layer for the grid near the ground. The 1st cell height from the ground is 2m and is uniform up till a height of 200m. But still the result doesn't show much changes. Am I doing the right thing? thank you for your time. wjruth
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Old   September 23, 2004, 09:26
Default Re: how to choose the roughness height
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Titiksh Patel
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Hi

Can u tell me what is the roughness height you are providing The bottom wall is plane, isn't it, and then you are providing equivalent roughness height. What changes you are expecting when you change height. Also you are saying that u are providing B.L. and then you are sayin that its uniform upto 200 metres, it seems little bit contradicting.

I will look at more details in mean time.

Regards, Titiksh
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Old   September 23, 2004, 22:33
Default Re: how to choose the roughness height
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wjruth
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Hi Titiksh,

mine is a 2D axisymmetry domain of a thunderstorm model and I am using boundary layer to create denser mesh near the ground (wall boundary). This BL has a height of 50m (within is 25 intervals of 2m height). Under the boundary condition panel of wall type, I used roughness height of 1.6m and roughness constant of 0.5. My concern is to find the height above ground of the max radial velocity when the inlet velocity flows, and hits the ground. This height above ground should change with different roughness (by changing the roughness height) of the ground. I have also read that wall Y+ will also affect the roughness effect but I still not sure how to get wall Y+ between 30 to 60. thank you for your attention to this problem.
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Old   September 25, 2004, 03:33
Default Re: how to choose the roughness height
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Hi ,

There are three step -

1. While creating the BL, make sure that your Y+ value is suited for the wall treatment you selected. See what Y+ you are getting with first cell height = 2 m . If you get y+ < 30 then increase your first cell height.

2. Effect of Roughness depends on the Non-dimensionalise factor Ks+. You Ks+ should be above 90 for rough wall. (see in Fluent Users Guide to see how to calculate Ks+)

3. Roughnes height should not be more than half of the first cell height. Your roughnes hight should fall below the center of first cell to see the effect of the roughness.

in your case it should be below 1 m if your Y+ is correct. so 1.6 is wrong value you are providing.
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Old   August 31, 2012, 09:00
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Sure this is a very old topic
but as there are some good point in it
BUMP

Last edited by morecfd; August 31, 2012 at 13:07.
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