Coefficient of Drag for Air and Water

kilroy · January 29, 2014, 12:59

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

truffaldino · January 29, 2014, 13:42

for incompressible flows the answer is yes

kilroy · January 29, 2014, 13:50

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)?

truffaldino · January 29, 2014, 14:35

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.

January 29, 2014, 12:59	Coefficient of Drag for Air and Water	#1
kilroy Senior Member Join Date: Mar 2013 Location: USA Posts: 120 Rep Power: 13	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

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January 29, 2014, 13:42		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	for incompressible flows the answer is yes

January 29, 2014, 13:50		#3
kilroy Senior Member Join Date: Mar 2013 Location: USA Posts: 120 Rep Power: 13	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, 14:35		#4
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	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.