Quote:
Originally Posted by LostCause
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:
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.
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 |
LostCause,I'll try and muddy the waters.The Coefficient of Aerodynamic Drag,encompasses all elements of drag for the body.---------------------------On submarines and aircraft they will use the fully wetted area of the structure,whereas in automobiles,the 1/2,in the drag force equation,converts the drag formula from wetted area to frontal area.This goes back to the "mirroring-effect",where the air actually sees two of your cars,joined at the wheels,one right side up,the other upside down, "below" the ground plane.------------------------ With the Cd,profile drag,skin friction induced drag, internal drag,and interference drag,etc.,are all contained within the Cd.----------------------- And Cd is only a function of shape.So like in my public displays,I have a very large New Beetle model,and a very small New Beetle model.They have identical Cds(given proper Reynold's Numbers),however vary greatly in frontal areas.And remember that a vehicles "drag-factor" is the product of the Cd times it's frontal area,or,CdA.------------------------- With respect to profile drag,it is singularly the largest component of drag for a car,and its something we can do something about,as we can alter it simply by changing the car's shape.------------------------- Pressure drag is a function of profile drag.The pressure behind a 2-to-1 teardrop is essentially the same as the pressure ahead of it.It's"stagnation" pressure is identical to it's "base" pressure acting on it's aftbody.----------------------------- For shapes other than teardrops,separated flow will create a "wake" of turbulence behind the structure which,from the law of conservation of energy dictates,must have a lower pressure than the forward stagnation pressure.The net difference in pressure is the" Delta-P" across the structure which represents it's "pressure drag".------------------------------------- The teardrop has NO form drag,or profile drag,or pressure drag,only skin friction drag,because it has no separation.If it yaws or pitches it may experience induced drag until it straightens back out,otherwise it has the lowest drag of any form yet investigated,excepting perhaps the "Morrelli" form.Thats a different thread.------------------ Hope this helps!