# Reynolds Number of a non-uniform duct

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 February 10, 2020, 03:10 Reynolds Number of a non-uniform duct #1 Senior Member   Raza Javed Join Date: Apr 2019 Location: Germany Posts: 183 Rep Power: 7 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

 February 10, 2020, 23:19 #2 Member   Glenn Carlson, PE, PhD (ret) Join Date: Oct 2012 Location: US Posts: 49 Rep Power: 13 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 00:43.

February 11, 2020, 03:05
#3
Senior Member

Raza Javed
Join Date: Apr 2019
Location: Germany
Posts: 183
Rep Power: 7
Quote:
 Originally Posted by gcengineer 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,

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.

 Tags laminar, reynolds number, turbulence