# Coefficient of Drag for Air and Water

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 January 29, 2014, 13:59 Coefficient of Drag for Air and Water #1 Senior Member     Join Date: Mar 2013 Location: USA Posts: 120 Rep Power: 4 Hello, Let's say I did a air wind tunnel test on a cylinder and get a drag coefficient for a certain Reynolds number. If I did the same test only by replacing air with water and keep the Reynolds number constant, will I get the same drag coefficient? Thank you very much for your time. kilroy

 January 29, 2014, 14:42 #2 Senior Member     Join Date: Jan 2011 Posts: 235 Blog Entries: 5 Rep Power: 8 for incompressible flows the answer is yes

 January 29, 2014, 14:50 #3 Senior Member     Join Date: Mar 2013 Location: USA Posts: 120 Rep Power: 4 truffaldino, thank you for your response. I agree that the pressure effect will be the same. But how about the viscous effects. Water and Air have different viscosity. Will not that make a difference (especially for lower Reynolds numbers)?

 January 29, 2014, 15:35 #4 Senior Member     Join Date: Jan 2011 Posts: 235 Blog Entries: 5 Rep Power: 8 This is whole idea of the reynolds number to express all the coefficients in terms of one dimensionless parameter. The reynolds number is the only parameter entering the dimensionless NS equation, so do not worry about different components of drag: Pressure to viscous drag ratio also depend on Re only.

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