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Old 04-18-2008, 03:08 PM   #61 (permalink)
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I'm beginning the question the whole interference drag issue simply because the underbody doesn't act as a traditional duct. There may be interference drag between the sides and underbody (which always exists) that can be mitigated with radiused corners, but there should be little in the way of underside and road interference.

On a calm day, the road surface is essentially moving at freestream velocity, therefore the boundary layer can't be thicker than the ride height. The lower your ride height is, the thinner the boundary layer. If the road was static, then I could understand that interference might exist...but that's not reality.

Compression certainly exists, but as Trebuchet03 noted, it is not significant. Air isn't considered "compressed," even though it is, until it's density has increased ~5%. Incompressible objects, such as water and steel, are compressible. It's just that any change in density is extremely small.

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Old 04-18-2008, 03:44 PM   #62 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
I can't make a ram-air inlet to fuel my compressed air car? Dang it


Now that's funny...
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Old 04-18-2008, 03:50 PM   #63 (permalink)
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Quote:
Originally Posted by LostCause View Post
... If the road was static, then I could understand that interference might exist...but that's not reality....LostCause
All of the roads that I drive on are static.

Air trapped under the car will try and escape, but the rougher the underbody is, the more obstructions there are, and more circuitous route to escape.

Reduce the amount of air trapped under the car and/or smooth the path that such air must take. In other words, make it as effortless as possible on the air molecules.

The less disturbance of the air, and/or straighter path that it takes from front to back, the less drag and greater efficiency.

See MacReady et al.

It would be interesting to do tuft tests on a roadway surface, and videotape them as the car drove over at ~70 mph.
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Old 04-18-2008, 04:03 PM   #64 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
I can't make a ram-air inlet to fuel my compressed air car? Dang it
You could build yourself a ramjet...

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Originally Posted by Otto View Post
All of the roads that I drive on are static.
Wrong frame of reference.

Quote:
Air trapped under the car will try and escape, but the rougher the underbody is, the more obstructions there are, and more circuitous route to escape.

Reduce the amount of air trapped under the car and/or smooth the path that such air must take. In other words, make it as effortless as possible on the air molecules.

The less disturbance of the air, and/or straighter path that it takes from front to back, the less drag and greater efficiency.

See MacReady et al.

It would be interesting to do tuft tests on a roadway surface, and videotape them as the car drove over at ~70 mph.
I'm referring to ride height, not underbody smoothness. You posted an analogy of biplane wings that was the focus of my response. In your analogy, the lower wing would have to be moving at a speed equal to freestream velocity relative to the top wing for it to be comparable to a road surface.

The car underbody and road surface are not moving at the same speed, therefore any reference to conventional ducts cannot be applied. I argue that interference drag between the car underbody and road surface does not exist as you imply.

Maybe interference drag does exist, but I can't work out two boundary layers touching each other in this situation. If anyone could clarify, that would help.

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Old 04-18-2008, 10:12 PM   #65 (permalink)
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NACA and its various equivalent research groups studied this in conjunction with biplane wing arrangement, where both wings are smooth and streamlined. What is the optimum gap between wings? Too far apart and they lose the synergistic flap effect, where they produce more lift than the sum of their respective lifts. Too close and the interference drag increases, since the airflow beneath the upper wing conflicts with the flow over the lower wing.
I know Lost Cause already responded to this - but I've got something to add...

Bi planes are a good example of parasitic interference - but not a good example of how that applies to cars. First, the ground isn't static as LostCause said - we're observing a car moving through air - not air moving over a car (ignoring wind). There is no boundary layer at the ground. In order for interference to occur - the boundary layer of the car must all the way down to the ground...

All that, plus interference increases as a function of v^2....

227mph airspeed.


70mph ground speed (max)... 100mph air speed (even that's fast --- I sure as hell won't be driving in constant gale force winds )

Generally, it's a slippery slope to compare the aero features of a streamlined vehicle (plane) to the features of a bluff body vehicle (like most of our cars).

Now, I'm not saying interference drag isn't an issue - I'm saying I don't know.But honestly, I'm leaning more towards the insignificant range -- that is, you'd have to lower your car to impractical levels for it to have a measurable effect. I say this because world records are broken with super low and super high vehicle shapes
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Old 04-19-2008, 03:14 PM   #66 (permalink)
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fineness ratio

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Originally Posted by SteveP View Post
Maybe you mean something different by "fineness ratio" than I understand it to mean, so perhaps you should provide a short definition. Mine is "width/length" (or, for a cylinder, diameter/length). I therefore don't understand how lowering a car can alter its fineness ratio.

Also, would you be so kind as to expand on your comment that "Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag". What does fineness ratio have to do with the ground effect? I could see it if you were *narrowing* the car, but lowering it doesn't do that.

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SteveP, tried to post a reply a while back,guess it got lost in the cyber netherworld.Here's second attempt.The fineness ratio is defined by the vehicles length,divided by it's height.It's very much like the aspect ratio of a wing,if you take the vehicle height to be the "thickness" and length of the vehicle to be the "chord".5.5-1 is considered the ultimate for a vehicle in ground-effect,as it respects the relationship between reduced profile drag,and skin friction.With respect to the "mirroring",its the phenomena which effects bodies close to the ground,which effectively doubles the height of the vehicle,as if it was placed on a mirror,and the air was striking the real AND the reflected image.If you look at very early windtunnel photographs for car testing,you will see model cars joined at the wheels,one upright,one inverted,in the test-section of the tunnel.Cars that look very "swoopy" on the ground look very "blunt" to the air,and to get the efficiency of a 2.75:1 fuselage form in free air,a car body must have a 5.5:1 L/H ratio.
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Old 06-24-2008, 01:26 PM   #67 (permalink)
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Quote:
Originally Posted by LostCause View Post
The car underbody and road surface are not moving at the same speed, therefore any reference to conventional ducts cannot be applied. I argue that interference drag between the car underbody and road surface does not exist as you imply.

Maybe interference drag does exist, but I can't work out two boundary layers touching each other in this situation. If anyone could clarify, that would help.

- LostCause
This will probably be way to simplistic, and I may be missing the implication, but driving on gravel and/or leaf-covered roads suggests to me that cars and roads interact with each other aerodynamically. I guess that must be an insignificant effect.

I am interested in this topic because I'll be lifting my truck slightly to improve its truck functions. It sounds like I may be able to at least be more aero than stock if I cover the underbody (quick release). That would put a flat undertray 7-9" off the ground.

Might a truck be a candidate for the catamaran shape listed above? I think I've seen some concept cars with that design.
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Old 06-24-2008, 01:42 PM   #68 (permalink)
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See Hucho, page 183, Fig. 4.74 for a picture of an Opel Calibra (said to be about the most aero-efficient production car of recent times) and note the ~60 degree outboard deflection angle of the smoke trail, which then impacts the front tire for even more drag. If this is not interference drag, I dunno what is.

Also see the various cites and illustrations in Hoerner's "Fluid Dynamic Drag," the aerodynamicist's Bible. When one body is too close to another body, the flow from each tangles with the other, for total drag greater than the sum of the parts. Hoerner cites studies of biplane wing placement and interference, bombs/drop tanks hanging from fuselages and wings, etc.. Basically, this says you need to keep such bodies adequately separated, so the airflow has a smooth and sufficiently wide gap to go.

Bruce Carmichael is one of the best modern aircraft aerodynamicists, and has published in Sport Aviation and Soaring magazines for decades, in addition to his own books and papers. He tells of the Douglas DC-3, which had a streamlined fuse and a streamlined wing. Putting the two together made a ~63% increase in drag, due to interference.
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Old 06-24-2008, 02:38 PM   #69 (permalink)
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Quote:
Originally Posted by Otto View Post
See Hucho, page 183, Fig. 4.74 for a picture of an Opel Calibra (said to be about the most aero-efficient production car of recent times) and note the ~60 degree outboard deflection angle of the smoke trail, which then impacts the front tire for even more drag. If this is not interference drag, I dunno what is.
Yes! This may be the next area of attack for me, to shield the inside sides of the front tires. Maybe mount some coroplast to the front suspension components so the shield can turn and move vertically with the wheel. That, plus adding some HD (Home Depot lawn edging) side fairing to keep what air does make it under the car from blowing out the sides.

In any case, yes, there most definitely is aerodynamic interaction between the vehicle underbody and the road.
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Old 06-24-2008, 03:16 PM   #70 (permalink)
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Originally Posted by garys_1k View Post
Yes! This may be the next area of attack for me, to shield the inside sides of the front tires. Maybe mount some coroplast to the front suspension components so the shield can turn and move vertically with the wheel. That, plus adding some HD (Home Depot lawn edging) side fairing to keep what air does make it under the car from blowing out the sides.

In any case, yes, there most definitely is aerodynamic interaction between the vehicle underbody and the road.
Note that Mercedes and perhaps others have fairing/deflectors canted inboard in front of the front wheels. This because the relative wind is not straight front-to-back, but rather deflected ~60 degrees outboard. Best to do tuft testing to see what's up with your particular vehicle, whose dynamics may differ from Calibra.

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