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Roughness Height and Meshing Form?

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Old   August 17, 2012, 17:06
Default Roughness Height and Meshing Form?
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I have already simulated an open channel flow with the Roughness Height=0, the results are validated
Now I would like to model the the same flow, with a Roughness Height=7mm

do I need to change the meshing form?
I'm sure that the height of the first grids on the bottom is much bigger than 7mm
so do I need to use a new finer grid form, something smaller than 7 mm in the height?
(Its a turbulent flow)
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Old   August 17, 2012, 18:26
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CFD Online Home -> Online tools -> Y+ estimation or click on http://www.cfd-online.com/Tools/yplus.php
Estimated wall distance is y with this tool.
Y+ for the cell adjacent to the wall is between is 1 for enhenced or 30 for standard turbulent model (See documentation)
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Old   August 18, 2012, 02:57
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great tools there
thanks to CFD online
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Old   August 18, 2012, 03:01
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so in that Y+, the height of the cells in adjacent to the wall?

Here I have a Manning's coefficient and so a Roughness Height
but in this page (http://www.cfd-online.com/Tools/yplus.php) there is no Roughness Height!


Also have no idea about the "Boundary layer length"
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Old   August 18, 2012, 03:34
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This tool is for an estimation of Y+ because you need only an estimate to do a numerical computation. In the turbulence theory, the surfaces do not have roughness and that's why there is no roughness height in these type of calculators. However, the value of the roughness height has to be filled in wall boundary condition panel at the bottom. Maybe you can can find empirical formula with roughness height.

The boundary layer length is the length (in the same direction of the flow) of your channel . (If you have a flow over a plate, it's the length of the plate. If you have a flow in a pipe, it's the length of the pipe and not the diameter).
I hope that helps.
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Old   August 18, 2012, 08:29
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so I must calculate both the Y+ value and Roughness Height, and choose between these two, actually the one with a smaller value

"The boundary layer length is the length"
there is a bend in the channel, so boundary layer length is the length of whole channel or just straight channels before and after the bend?


after all
to clear things here and start my simulations
I think I must repeat my question
lets just forget about the Y+ value
and lets say I'm modelling a river with some grass growing in the bottom of it
so there is a Manning's value of about 0.04, Now lets say the Roughness Height would be about 7 millimeters (using the empirical formulas)
do i need to create a grid form with the mesh height <7 mm in the adjacent to the walls?
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Old   August 18, 2012, 16:19
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You don’t choose between Y+ and Roughness Height. You have to use both values Y+ (for the mesh) and Roughness Height (for boundary condition).

First, you choose Y+ = 1 or 30 for the cell adjacent to the wall and you compute y with the value of Y+ chosen.
Second, you use Roughness Height for the setting of the wall boundary condition in FLUENT.

The boundary layer length is the length of whole channel. It’s for an estimate because the theory used by the calculators is based on a straight plate.

I hope that it's not too confusing.
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Old   August 18, 2012, 19:51
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Remark: As FLUENT computes with the cell center, the value of y calculated is the same value as the center cell y-coordinate. Consequently, the height of the cell adjacent to the wall is twice the value of the y calculated.
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