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Originally Posted by fabrio
about the image posted by me, I have a question.
In the explanation text, it can be read that the Cd is in relaction on exposed area!
So, in the last case (C), the area it is equal to zero.
Now, if the area is zero, also the Cd is zero.
But, how can to konw the effective drag impact for last solution?
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This has kind of thrown me for a loop...
Form drag = Cd*A
Total Drag
Cd = cd + Cd,i
Profile Drag
cd = cd,f + cd,p + cd,w
Since cars aren't experiencing wave drag, they just combat profile drag (cd,f + cd,p)...that is skin friction drag and pressure drag.
Since that flat plate is operating in viscous flow, it will be subject to shear stress...therefore it will have skin friction drag.
cd,f = Df/Q∞*S
where S is surface area, not frontal area.
That plate is dealing with pressure drag due to the exit duct.
Cp = p-p∞/q∞
I believe cd,p requires the integration of all pressure coefficients across the surface area of the body and then the horizontal component solved. I'm about 2% sure on this one, though...
The Cl is determined from the pressure coefficients the same way, but lift is determined normal (perpendicular) to the body. I imagine the duct creates some form of lift, so I don't know how else this could be determined outside of experimentation??? The Cd,i is then determined from the Cl via
Cd,i = Cl^2/pi e AR
Sum it all up to form Cd and then we come to an area of contention...A. I believe in flat plates that A is determined from surface area. I take that from
here:
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where A is a suitable body area, either the surface area of the body or the area of the body when projected onto a plane normal to the flow
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So form drag = Cd * A
That's all I could come up with...a long and convoluted answer...
In conclusion, I'm not 100% sure...but those are all the tools I have to work with. I look forward to anyone knowing the
true answer posting a response...I hope this isn't a common pitfall of aerodynamics: applying equations where they don't belong.
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Originally Posted by MetroMPG
We know that air flowing in/out of holes in a surface can directly affect Cd without changing projected area- eg. vacuum holes/slots in airplane wings; "active" pneumatic aerodynamic devices at the rear of some experimental vehicles.
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What about form drag, though?
Form drag = 0.002 * 0
Form drag = 0?
That doesn't seem right, for some reason. Maybe it is?
- LostCause