Very High Drag Coefficients?

spggodd · February 2, 2016, 05:04

Hi,

I'm not having much interest in my post in the Fluent forum so I thought I'd try this more general forum.

Please can you take a look at the following link:

http://www.cfd-online.com/Forums/flu...tml#post583347

Many Thanks
Steve

dray19 · February 2, 2016, 15:21

I haven't used fluent to calculate drag coefficient, but from my experience if you use fluent to directly calculate some dimensionless number the results are off.

Have you tried calculating drag coefficient using the following equation
Cd=(2F)/(rho U^2 A)

F - drag force - which can be broken up into friction force (skin friction) and form force (pressure difference)
rho - density
U - velocity
A - Surface area

spggodd · February 2, 2016, 15:44

When you say 2F do you mean the sum of the pressure and viscous drag?

spggodd · February 2, 2016, 15:47

Also, I am using that formula using the total force output from fluent in the X direction.

This gives me a Cd value and then I am converting it to N/m^2 to match the paper I am comparing against by multiplying by the dynamic pressure.

The paper then give the formula: F = Cd* S / 1000 where S is area and Cd* is the modified drag coefficient.
However, I'm not sure why he has measured drag in N/m^2, is this common?

dray19 · February 2, 2016, 16:22

Coefficient of Drag is a dimensionless number, but after looking more at the paper you are referring to. It looks like their Cd is actually drag force per area.

spggodd · February 2, 2016, 16:38

Yes, I was taking the dimensionless Cd from Fluent and multiplying it by the dynamic pressure to give it units of N/m^2.

However, do you think the paper is suggesting some more simple such as taking the Force/Area?

February 2, 2016, 05:04	Very High Drag Coefficients?	#1
spggodd Member Steven Goddard Join Date: Mar 2015 Posts: 34 Rep Power: 11	Hi, I'm not having much interest in my post in the Fluent forum so I thought I'd try this more general forum. Please can you take a look at the following link: http://www.cfd-online.com/Forums/flu...tml#post583347 Many Thanks Steve

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February 2, 2016, 15:21		#2
dray19 New Member Dustin Ray Join Date: Jan 2016 Posts: 6 Rep Power: 10	I haven't used fluent to calculate drag coefficient, but from my experience if you use fluent to directly calculate some dimensionless number the results are off. Have you tried calculating drag coefficient using the following equation Cd=(2F)/(rho U^2 A) F - drag force - which can be broken up into friction force (skin friction) and form force (pressure difference) rho - density U - velocity A - Surface area

February 2, 2016, 15:44		#3
spggodd Member Steven Goddard Join Date: Mar 2015 Posts: 34 Rep Power: 11	When you say 2F do you mean the sum of the pressure and viscous drag?

February 2, 2016, 15:47		#4
spggodd Member Steven Goddard Join Date: Mar 2015 Posts: 34 Rep Power: 11	Also, I am using that formula using the total force output from fluent in the X direction. This gives me a Cd value and then I am converting it to N/m^2 to match the paper I am comparing against by multiplying by the dynamic pressure. The paper then give the formula: F = Cd* S / 1000 where S is area and Cd* is the modified drag coefficient. However, I'm not sure why he has measured drag in N/m^2, is this common?

February 2, 2016, 16:22		#5
dray19 New Member Dustin Ray Join Date: Jan 2016 Posts: 6 Rep Power: 10	Coefficient of Drag is a dimensionless number, but after looking more at the paper you are referring to. It looks like their Cd is actually drag force per area.

February 2, 2016, 16:38		#6
spggodd Member Steven Goddard Join Date: Mar 2015 Posts: 34 Rep Power: 11	Yes, I was taking the dimensionless Cd from Fluent and multiplying it by the dynamic pressure to give it units of N/m^2. However, do you think the paper is suggesting some more simple such as taking the Force/Area?