# Total pressure drop in laminar channel flow (calculation of friction factor)

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 October 24, 2021, 12:16 Total pressure drop in laminar channel flow (calculation of friction factor) #1 New Member     HOU Xiao Join Date: Oct 2021 Location: Nantes, France Posts: 1 Rep Power: 0 Hello everyone, I'm calculating the total pressure drop at Re = 2000, for water at 300K in a pip D = 0.1 m, L = 2 m. And this is a verification for the other studies. The problem is, the theoretical result for friction factor is around f = 64/Re = 0.032, but the result is much higher (f = 0.0729). Could you help me to verify my settings as follows? ------------------------------------------------------------ Re = 2000, density: 998.2 kg/m^3, viscosity: 0.001003 kg/(m s) As a result: hydraulic-diameter: 0.1 m (for pip D = 0.1 m) inlet-velocity: 0.0201 m/s sandgrain roughness height = 0 ------------------------------------------------------------ Results: Area-Weighted Average Total Pressure: inlet: 0.49894214 Pa outlet: 0.20497435 Pa friction-factor: f = 0.0729 >> 0.032 ------------------------------------------------------------ Did I make a mistake of common sence or there are something special to be noticed?

 October 24, 2021, 17:48 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,689 Rep Power: 66 64/Re is for fully developed flow. Are you using streamwise periodic BC's or is this with a velocity inlet and outlet type calculation? You might need to take into account developing effects. oliver_houxiao likes this.

 October 2, 2023, 01:27 the result is not matching with theoretical calculation #3 New Member   Sumeet kotak Join Date: Jun 2015 Posts: 12 Rep Power: 10 I am facing the same issue. Simple fluid Flow problem Problem Description: Pipe diameter: 0.0478 m density: 998 kg/m^3 Velocity: 0.036 m/s theoretical calculation: Re: 1500 Friction factor f=64/Re delta P= 0.57 Pa Fluent Analysis: Inlet BC: Velocity inlet outlet BC: Pressure outlet 0 Pa (atmospheric condition) Model: laminar Result: Pressure at inlet: 1.23 Pa delta P = pressure inlet - pressure outlet = 1.23-0 = 1.23 Pa which is 115% higher than theoretical value. Need help Thanks in advance

 October 2, 2023, 01:57 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,689 Rep Power: 66 A velocity inlet is not a fully developed flow. The theoretical value for this case is NOT 64/Re. You can find in engineering handbooks the apparent friction factor for developing cases. Use the right formula! It should not match 64/Re. If anyone matches 64/Re for a developing flow then either they are a liar or they have messed up. If you want to match 64/Re then use periodic BC's so that you do have a fully developed flow, or apply the fully developed profile at the inlet, or make the pipe super long so that entrance effects can be ignored.

 October 5, 2023, 06:26 #5 New Member   Sumeet kotak Join Date: Jun 2015 Posts: 12 Rep Power: 10 Thanks for your response. Can you help me how to apply fully developed profile as velocity inlet ? Thanks in advance

 Tags fluent, friction factor, pip flow, pressure drop