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Tony January 24, 2008 04:28

Tubulent flow@ RE50,000 error

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

Glenn Horrocks January 24, 2008 17:30

Re: Tubulent flow@ RE50,000 error

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

Glenn Horrocks

Tony January 25, 2008 00:30

Re: Tubulent flow@ RE50,000 error

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)?

Glenn Horrocks January 28, 2008 18:23

Re: Tubulent flow@ RE50,000 error

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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