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Contradiction in COMSOL's Documents

Posted Dec 9, 2014, 11:59 a.m. EST Fluid & Heat, Computational Fluid Dynamics (CFD), Microfluidics, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.4 4 Replies

Marwan Nafea
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Hi All,

I have found a contradiction in COMSOL's manual about the same example that has different names in version 3.5 and 4.4.

In 3.5 the PDF document states that Reynolds number = diameter of the pipe*density*velocity/dynamic viscosity
In 4.4 the PDF document states that Reynolds number = length of the pipe*density*velocity/dynamic viscosity

So which one is correct?

Please refer to the attachments.

Thanks.
Marwan
4 Replies Last Post Dec 11, 2014, 12:51 p.m. EST
Lasse Murtomäki
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Posted: 9 years ago Dec 10, 2014, 1:55 a.m. EST
Diameter is correct.
Diameter is correct.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 9 years ago Dec 10, 2014, 2:07 a.m. EST
Hi

the Reynolds number is one of the "Engineering" characteristics numbers, where the definition of some parameter are "it depends on ..."

"L" (the "characteristic length") in the Reynolds definition is often referred as the "hydraulic diameter" (check Wiki)

I would use the pipe diameter, COMSOL calculates as an independent variable the "cell Reynolds number" where L is replaced by the mesh average size "h", this gives you an information on the mesh cell density limit

--
Good luck
Ivar
Hi the Reynolds number is one of the "Engineering" characteristics numbers, where the definition of some parameter are "it depends on ..." "L" (the "characteristic length") in the Reynolds definition is often referred as the "hydraulic diameter" (check Wiki) I would use the pipe diameter, COMSOL calculates as an independent variable the "cell Reynolds number" where L is replaced by the mesh average size "h", this gives you an information on the mesh cell density limit -- Good luck Ivar
Henrik Sönnerlind COMSOL Employee

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Posted: 9 years ago Dec 10, 2014, 2:18 a.m. EST
Hi,

In the version 4.4 document, it actually states that "L is a characteristic dimension of the device". This is taken as the cross section width of the rectangular channel. So there is no contradiction. The Reynolds number is also the same (0.001) in both models, since the same length is used (even though it was designated 'd' in the 3.5 version).

There is however a typo in the 4.4 version (fixed in 5.0), stating that the channel width is 10 mm. It should be 10 microns. The correct value (10^-5) is used in the expression for Re, though.

Regards,
Henrik
Hi, In the version 4.4 document, it actually states that "L is a characteristic dimension of the device". This is taken as the cross section width of the rectangular channel. So there is no contradiction. The Reynolds number is also the same (0.001) in both models, since the same length is used (even though it was designated 'd' in the 3.5 version). There is however a typo in the 4.4 version (fixed in 5.0), stating that the channel width is 10 mm. It should be 10 microns. The correct value (10^-5) is used in the expression for Re, though. Regards, Henrik
Marwan Nafea
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Posted: 9 years ago Dec 11, 2014, 12:51 p.m. EST
Thank you so much everyone.
Thank you so much everyone.

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