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Cannot get similar results based on two methods of friction factor calculation

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Old   January 14, 2016, 10:37
Default Cannot get similar results based on two methods of friction factor calculation
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Dantong Shi
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Hi.

I'm using periodic conditions in Fluent with specify mass flow rate to calculate a single phase laminar flow with heat transfer. After simulation, I want to calculate the Fanning friction factor. I'm using the following two methods:

1. Based on shear stress. I used the area averaged shear stress T (constant density) from the fluent report. f = T/(0.5*density*velocity^2).

2. Based on pressure gradient. After calculation, the periodic conditions panel would give the pressure gradient dp/dx. And f=-dp/dx*dh/(2*density*velocity^2).

But I got very different results from this two methods. The result of the second one is about two times than the first one. It should be similar.

My geometry is a sinusoid wavy fin in 2D. I also tried my simulation in a flat plate, and it doesn't show this problem.

My geometry is in the attachment.
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Old   January 14, 2016, 22:40
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Lucky
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Make sure you've properly integrated the shear stress.

The proper length scale in 3D (usually the hydraulic diameter Dh) is not quite the same in 2D.

The best way to check is to apply a force balance to relate the wall shear stress to the bulk pressure gradient and then compare that to what you have for friction factor. It is critical that the correct length scale is chosen to ensure the force balance holds.
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