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Tubulent flow@ RE50,000 error

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Old   January 24, 2008, 03:28
Default Tubulent flow@ RE50,000 error
  #1
Tony
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HI,

I would like to have a simple analysis for a smooth pipe using ANSYS CFD/CFX software.

The parameters as follow: RE=50000 fluid density=997 (water) Diameter=0.048m dynamic viscosity=0.0008899 (water) Length=3m

and from the equation RE =(fluid density) * (Diameter) * (Mean velocity)/(Dynamic viscosity)

and Hence, we get the mean velocity as 0.93m/s.

We input the fully development profile(FDP) equation into the expression as follow:

FDP=Maximum velocity * (1 - (r/Rad))^(1/6.76)

the inlet we using the FDP as normal speed the domain we using the k-epsilon which k is 0.00888 and epsilon is 0.0495. The intensity (I) is 0.041376.

the outlet we using the pressure = 0Pa.

However, we get the solution from the CFX post as follow: the pressure gradient is around 262 Pa.

Compare with the Darcy-Weisbach equation, the pressure gradient is 186.47 Pa.

How can I reduce the percentage error between CFX result and theoretical result????

Many thanks

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Old   January 24, 2008, 16:30
Default Re: Tubulent flow@ RE50,000 error
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Glenn Horrocks
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Hi,

The first thing to check is mesh refinement. Check your mesh is fine enough.

Glenn Horrocks
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Old   January 24, 2008, 23:30
Default Re: Tubulent flow@ RE50,000 error
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Tony
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Hi,

thanks for your answering~

how to define the mesh is fine enough???

I tailor-made the centre of the pipe have more mesh...(the grids size is much smaller than grids near the pipe wall)... since the pressure gradient is seem at the centre of pipe along the pipe.

However, I have the same result.

Moreover, I would like to know the different between SST, K-epilson, BSL & SSG ??

The data input (the value of k, epsilon) at the inlet is the same or different with the different analysis (e.g. Shear Stress Transfer (SST), K-epsilon)?

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Old   January 28, 2008, 17:23
Default Re: Tubulent flow@ RE50,000 error
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Glenn Horrocks
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Hi,

The mesh at the wall is much more important than the internal mesh. It is the refinement of the wall mesh which will determine the accuracy of your simulation.

Do a search on this forum on mesh sensitivity check. It has been discussed several times.

Glenn Horrocks
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