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Old 06-26-2008, 12:15 PM   #51
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Yes it doesn't look so sharp on your pic as it did on the one I found labelled as an '02. However, it's also not well curved enough to indicate that they intended the air to wrap round the back any. If the rear air deflectors aren't known to do anything, then sharpening that edge up sound like the better plan.

The void between the axle/tank and the rear bumper could be handled in either of two ways, either by boxing it in, or by taking advantage of the space there to make something of a diffuser. Boxing it in helps because it kills a lot of drag, making a diffuser helps more because it kills that drag and lifts air behind the car, killing some induced drag. The decider would be in how easy it is to mod the rear bumper, if the bottom half looks "useless" then making big holes in it, or cutting it away entirely is doable for the diffuser plan, but if it looks solider, has a metal box section behind it etc, that is part of the structural integrity or crash protection, then you should just settle for boxing it in.

Airflow detaches after about 11 degrees deflection, but there's also a minimum surface area it will attach to, at our typical speeds this seems to work out that it will stay attached to a curvature of around 3-4 inches radius, so curves of that radius it will tuck quite far round, curves of lesser radius it will detach messily at 11* deflection and greater. But... what you're doing in turbulation is pre-empting the "messy" formulation of large vortices, by forming small controlled vortices, that stay semi-attached and roll along the surface, rather than being large and shedding unsteadily making chaotic flow above them or stagnating in place and causing airflow deviation as bad as if they were a chunk of protruding bodywork. Behind the vehicle instead of shedding smoothly they may also suck back at the rear edge causing vacuum drag. So, what you're doing is kind of making loads of little energetic "sticky balls" instead, that because they're rolling can sort of disobey the 11* rule and manage to follow a sharper curve... however, you have to get them attached to the surface before it deviates, otherwise they're just jumping off into space... shoving an air stream upwards that will have a large vortex form behind it that does the same as what you were trying to avoid in the first place. So, you need to make these sticky balls just before the surface deviates, with enough surface left for them to "cling" to to roll themselves round the edge more tightly, to help the bulk flow take that path instead of getting pushed higher by the larger vortices.

The change of Cd thing due to flow transition, it means that your aerodynamic drag going up with the square of speed type graph is still starting to head sharply upwards at 55, but when the flow transitions if it was plotted as actual drag force against speed, you'd see a sudden dip in the graph where the flow transitioned from one regime to the other, and then it would take off straight up again. I think that probably older cars with very little aerodynamic refinement maybe get nothing at all when the flow transitions, and that newer cars probably find that point quite high. So it probably doesn't change the "in general, stay at 55" advice, since some cars might need to go to illegal speeds to find the dip, and draggier cars might not have a dip to find.
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Old 06-27-2008, 02:11 AM   #52
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RoadWarrior, thanks.

It looks like my avenues are pretty clear: Test a lip and fill the undercarriage somehow (there is metal structure behind the plastic bumper).

For the lip, should I try to just angle it down or flat, just following the roofline



Or just stick a wedge to kick the airflow upwards a little



And why VGs would not achieve the same?

I'd expect the vortices to provide support for the airflow above it, and shape a virtual spoiler, creating a clean cut of the air away from the body instead of a messy decompression near the curve of the rear hatchdoor when the air cannot follow the curve > 11%.

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Old 06-27-2008, 10:31 AM   #53
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VGs on top risk turning the airflow through too great an angle and creating more induced drag (due to shape and lift) than the base (Pressure/vacuum) drag they reduce. I'd guesstimate that you want the airflow coming off the top of there at no more than about a 20 degree angle from the horizontal. Therefore picture number 1 would be my choice, maybe with the angle tweaked downward just a touch, or made adjustable so you can find the sweet spot (Shim washers under the fittings???)

The other way to do it, would be to bring air completely down the back screen then seperate it from the body at about halfway down the back. For this you might need a large "whale tail" hatch spoiler at the bottom of the window, or to make a "trunkette" type shape. However, the angle the roof lip turns and the angle of the back screen don't look particularly helpful for this, VGs or turbulators might manage about a 45 degree deflection, but to "catch" that and turn it horizontal, you'd have to stick something way out of the back there, like a 2 foot wide plank strapped on top of your tire carrier. So then you'd be left with the rear screen air deflector method, but again need to turn the air. However, since this should be bringing the air round a sharper turn than the VGs will, you don't need a large surface to turn it horizontal, maybe just a 3 or 4 inch "shelf" type spoiler at the base of the rear screen.

A third option might be to put a large air dam on the front, and use the VGs on top anyway. This may also get gains in conjunction with 1 on it's own or the deflector on it's own. The reason would be that the airdam will lower the point that the airstream separates at the leading edge of the vehicle, thus effectively "tilting the airfoil" forwards, such that greater departure angles of air from the rear of the vehicle will not cause extra induced drag, but you get the gain from the reduced base drag. Adding the airdam should improve things anyway. but say you got 1 mpg from the airdam on it's own and 2 mpg from the "better" options on the back on their own, and 0 mpg for the the others. What should happen is that they are complementary, such that you might see 5mpg with airdam and better options, and 2 or 3 mpg with the airdam and the other options.
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Old 06-27-2008, 12:36 PM   #54
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I don't plan to do monster aero mods to the 'V (I'm not hateful LOL!) so I'll start by the spoiler.

- I'll probably make a proto with an L corner to shove it in the door, and use spacers to manage angles. SInce it has to be 20ish degrees downwards, it's going to be interresting to keep it in place there.

- As for the air dam, I'm not sure it would help: The 'V is high up so that's quite a lot of frontal surface added... I'll try to make sketches to see if I understand what you say about the dam on the final airflow.
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Old 06-28-2008, 01:38 AM   #55
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Ok, with some ABS plastic, that was a quick prototype... I did a quick drive... I seem to be at 30MPG at 72MPH with cruise control over a short drive, that's pretty good, the # looks higher rather than lower than normal (no A-B-A test).EDIT Did another 2x 25miles trip and got 27MPG this time at 73MPH cruise ctrl... That's not very good.

I put some pics here of the spoiler and shots of the geometry/shape of the car.

Aero

Here's 3 figures hand drawn. I suspect I probably did not quite understand all the car aero so maybe my description is wrong and so plz correct if you see stupidities.

1- Ideal shape, the car pushes the air out evenly and it regroups behind cleanly. Symmetry avoids lift drag.

2- CR-V SUV shape. The desing creates lift, the air travels more on top than bottom. Increasing lift also increases drag (?). Furthermore sharp tailgate angle forces undercarriage and roof air to rejoin at sharp angle, colliding while decompressing, causing turbulence drag.

3- Spoiler extends roof, detaches air cleanly forming a virtual boat tail with less angle so less lift and less associated drag. Undercarriage and roof airflows meet without colliding as much, less turbulent flow means less energy spent in eddies, less drag. This does create an air hole depression behind the SUV.



Airdam
If I were to make one, I'd increase the surface area by 15% or more:
Ground to bumper 15", ground to bottom of car 9", width of car 67" (tires 7" wide each), height 60". That's about 3100sqin frontal surface vs3600sqin with a full air dam.

Spoiler
I made the spoiler to follow the roofline, slopes down a little, and the ABS is flexible so it should sump by 1" or so with wind pressure. Is it still riding too high?



underbody diffuser
Behind the rear left wheel I've got a air hole, that should be fixed. Notice the tape job I did to seal holes around the bumper. Tape joins metal parts to plastic parts. Near the mud flap I didn't finish the tape job.

Incidentaly a friend took me to the BMW dealer and I looked at SUV undertrays. It's all covered & flat, surfaces just have some ridges parallele to airflow.
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Old 06-28-2008, 06:47 AM   #56
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Quote:
Originally Posted by RoadWarrior View Post
Actually it's the combination of the lift and drag vectors, due to free body diagrams of aeroplane aerodynamics being rigidly fixed in the up/down left/right frame of reference rather than in terms of direction of incident airflow.
What text is that? I studied this stuff in school and lift and drag are defined by the direction of the freestream velocity. Lift is always perpendicular to the freestream and drag is always in the direction of the freestream. What doesn't change is the direction of gravity and the weight vector.

Quote:
So if a wing makes 100lb of lift going forwards, at a certain airspeed, if it can maintain that airspeed through thrust, flying straight up, it's still got 100lb of "lift" pulling at right angles to the plane of the wing, but in the way that aero textbooks describe it, it's now 100lb of "drag" because up is always up...
The lift you describe is still lift, but what matters is resolving it when compared to gravity.

The amount of lift generated is a function of the angle of attack, the angle between the zero lift line of the body and the freestram velocity.

Quote:
Now, all this seems a bit odd, because we'd think that the average angle of airflow would be horizontal at all times.
Here is where I agree. It would be nice to know the zero lift line of a car. Perhaps some manufacturers or grad students might have that info.

Quote:
the air is prevented from fully flowing underneath it and compensating.
The air is flowing under the car, it is just a mess. The other complicating factor is the boundary layer under the car. Recall that the car is moving over the pavement and through the air. The air is stationary at the ground and the car is dragging the air as it moves. You may notice this when driving in the rain and looking at the car ahead of you. You may also see this during auto races in the wet. This is an issue with wind tunnel testing, the most accurate results are generated in tunnels with floors that move at the same speed as the freestream.

Not sure what you mean by "compensating."
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Old 11-11-2010, 10:27 AM   #57
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Re: Tubulence - Friend or Foe? | Here's a lot of Aero info, not spread in several thr

so i understand what they are doing, but how are they helping exactly? Are they breaking up the low pressure area behind the window and after the trunk area? if so... couldnt you just cover the entire surface of the car in "dots" to break up the surface tension? like domes the size of quarters or so?
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Old 11-11-2010, 10:28 PM   #58
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Re: Tubulence - Friend or Foe? | Here's a lot of Aero info, not spread in several thr

They did that on Mythbusters. They covered a car with bumps and compared fuel mileage and found the bumps helped. Yes, it helps but it also looks rediculous and isn't needed.

VG's help the air curve around the back of a vehicle and fill in the wake so there isn't as much low pressure sucking on the back of the vehicle.
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Old 06-03-2012, 02:19 PM   #59
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Re: Turbulence - Friend or Foe? |Here's a lot of Aero info, not spread in several thr

try a chase car to observe and/or record wind effects on tufts.
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