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Reynolds Number of a non-uniform duct

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Old   February 10, 2020, 04:10
Default Reynolds Number of a non-uniform duct
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Raza Javed
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Hello Everyone,


I have a question relating to the calculation of the Reynold number.



If I have a non-uniform duct or may be a pipe, and at the inlet of the pipe, the cross-section area will be different, and as the length of the pipe increases, the area and the shape of the pipe changes.


Now, in this case if I want to calculate the Reynolds number to check whether my flow will be laminar or Turbulent. Do I need to just consider the area of the pipe at the inlet and calculate the Reynolds number or is there any other way to calculate it in this case?


Thank you for your help.


Raza
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Old   February 11, 2020, 00:19
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Glenn Carlson
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Hi, Raza.

The critical Reynolds number dividing laminar and turbulent flow regimes was found from experiments with long round pipes. Flow in non-uniform ducts may show recirculation, separation, or other phenomena that don't arise for long straight pipe.

Therefore, the well-known Re.crit = 2000 should not be assumed correct for other shaped pipe. If a non uniform pipe has a long straight section, it begins to make sense to apply a Reynolds numbee in that section. But, otherwise, IMO, the Reynolds number is not particularly useful for non-uniform pipes.

Regards,
Glenn

Last edited by gcengineer; February 11, 2020 at 01:43.
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Old   February 11, 2020, 04:05
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Quote:
Originally Posted by gcengineer View Post
Hi, Raza.

The critical Reynolds number dividing laminar and turbulent flow regimes was found from experiments with long round pipes. Flow in non-uniform ducts may show recirculation, separation, or other phenomena that don't arise for long straight pipe.

Therefore, the well-known Re.crit = 2000 should not be assumed correct for other shaped pipe. If a non uniform pipe has a long straight section, it begins to make sense to apply a Reynolds numbee in that section. But, otherwise, IMO, the Reynolds number is not particularly useful for non-uniform pipes.

Regards,
Glenn

Hi Glenn,


Thank you so much for your reply.


As I am modelling the air Flow, in this case, if I want to check whether the flow will be laminar OR Turbulent, what could be the possible approach to do that?


Thank you.
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