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Old 06-24-2008, 10:23 AM   #41
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Originally Posted by RoadWarrior View Post
That's interesting, I mentioned on mpgresearch a while back that it might be possible to use closed cell foam "draft insulating" tape strip to get the same effect.
If you have it handy, link it... We gotta be careful planes use those to attach flow and avoid bubbles that disrupt plane stability, and not to improve MPG. I dunno if one implies the other? BTW, added http://aerodyn.org/Drag link to my previous post.
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Old 06-24-2008, 01:45 PM   #42
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Diffuser:

In one of the links, it mentions for diffusers the use of a hollow space under the rear of the car to allow airflow to expand and slow down before exiting the underbody, to avoid further turbulences in the wake. If anything then the holes behind the rear bumper should not ne covered up. Maybe poke holes in the bumper at best. Maybe just put vertical separators to direct the flow and reduce turbulences in that gap?
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Old 06-25-2008, 07:04 AM   #43
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If you have it handy, link it... We gotta be careful planes use those to attach flow and avoid bubbles that disrupt plane stability, and not to improve MPG. I dunno if one implies the other? BTW, added http://aerodyn.org/Drag link to my previous post.
Yes, sometimes you've gotta think upside down and backwards on what applies to planes to apply it to cars, because you don't really want high lift.

Unfortunately what I thought I posted over there seems to have been eaten by one of the hacks or server crashes.

Anyway, I think it was the mentioning of use of turbulation devices to pre-empt flow detachment and formation of large vortices, and to cause small vortices that would entrain the bulk airflow round sharper angles than it would normally follow without separation. I determined that a minimum 1/4 inch feature was needed at the typical Reynolds numbers we see at highway speed in a typical size car, for it to have significant effect on airflow. The ideal spot for such turbulation devices is supposed to be in the rear 3rd of a body. I did some rough back of envelope figuring that seemed to suggest that about 2 inches of surface is the minimum that flow will reattach to at "our" reynolds numbers, so placing of turbulation strips should be a minimum two inches from changes in curvature, unless you're using them to separate flow.

Bear in mind that trucks by virtue of length are in a different calss of Reynolds numbers than cars, so that devices which work on trucks may need to be scaled differently to be effective on cars and vice verse i.e. quarter inch strips won't do crap on a semi-trailer, they may be getting towards marginal on full size extended vans. Also they would have to be placed further than 2 inch from the trailing edge.

The most effect is seen apparently with groups of 3 strips spaced their own height apart.

However, I have come to consider a slightly different application approach more theoretically desirable. Because the angle at which the air departs the vehicle body determines the "angle of attack" as it were of the body shape as an airfoil, you want to get that angle as shallow as possible, near horizontal. There is of course though a balance between doing this and adding too much base drag. One needs to keep the angle of attack as neutral or negative as possible otherwise the lift forces are actually pulling backwards on the body, giving induced drag. Keeping the "trailing edge" higher than the "leading edge" is one reason why it's beneficial to have a clean edge separation off a trunk or stub-trunk at about half the height of the vehicle at the rear. However, this doesn't fix everything because of a outflow from the roof centerline that will happen due to air at higher pressure trying to take the easiest escape route rather than going all the way over the top. This leads to similar phenomena as tip vortices on aircraft wings. This flow will attempt to wrap over the sides of the vehicle, and will cause a net downflow effect on air departing the vehicle at the rear. This is actually not good, Kammbacks get talked up, but to be truly efficient they should kick the air back up before separation. There is a balance however between where base drag reduction is good and where induced drag becomes a problem.

Anyway..... turbulation strips could be doing 3 things at once... Placed at a shallow angle to the airflow, 15-30 degrees in groups of 3 they would i) act to keep flow attached as turbulators, ii) enhanced lower drag action by virtue of delta wing swirl effect exploited by other turbulation devices, and iii) direct airflow away from the vehicle with an upward component at the rear, helping to kill downwash from the "tip vortex" effect and reduce induced drag. These would be most effective in this manner on the lower half of the vehicle on the sides. With vehicles like vans I think you'd be better off having them straight on the top half of the side and angled towards the centerline on the roof.
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Old 06-25-2008, 08:12 AM   #44
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Quote:
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Diffuser:

In one of the links, it mentions for diffusers the use of a hollow space under the rear of the car to allow airflow to expand and slow down before exiting the underbody, to avoid further turbulences in the wake. If anything then the holes behind the rear bumper should not ne covered up. Maybe poke holes in the bumper at best. Maybe just put vertical separators to direct the flow and reduce turbulences in that gap?
I wouldn't spend too much time worrying about it. A diffuser is mostly applicable to race cars for generating downforce. They were developed to recover underbody-generated downforce that was lost when cars moved from "airfoil tunnel" to "flat-bottom" underbodies.

For a normally used road car, just putting something on the bottom of the car to "clean it up" will help reduce drag, thereby benifitting FE.
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Old 06-25-2008, 08:57 AM   #45
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However, killing "upforce" would be beneficial or at least translating it from up and 15 degrees backwards, to up and 5 degrees forward.
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Old 06-26-2008, 03:05 AM   #46
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RoadWarrior,

i'm trying to decypher your last message in my brain into something applicable to my CR-V... It's a 2-box style SUV, with garbage sticking out in the back (spare).

I still think I should want a clean airflow in the back.

From what you say, it'd be best to detach air like kammbacks (read like honda CRX) in the middle height of the vehicle, with a spoiler kicking air back up 15 degrees to clean the virtual boattail airflow.

For my CR-V, I'd want a clean break.

From what you say, on the roof I could use 3 rows of V-strips (or VGs since you seem to say they do the same... maybe at a larger dimension), say 4~6 inches away from the rear hatch door (farther than for cars) and angled in a V shape with the point towards the front of the car. Also I don't have to worry about the roof airflow spilling out to the sides, as i have a built-in raised roof rails that helps direct the flow.

If I do this, what I understand is the air would reattach and go around the rear hatch bend better, instead of detaching into turbulences.

However doesn't that mean I'd still need a spoiler to get the air kicked out clean of the roof to form a cleaner virtual boattail to eliminate turbulences?

Or is it better to have the air attach the whole length of the trunk to eliminate the dead air bubble in the back, at the risk of creating turbulences from the roof airflow crashing with the underbody airflow?

I noticed that many 2-box suvs have a rear spoiler with a hole. I assume some of the air is accelerated to attach to the rear window and some of it is kicked out and cleanly separated from the car. Maybe that helps fill the stagnant air pocket with some low pressure air turbulences.

A few questions:

- What's the reynold number for highway speeds (car/small SUV)? I suppose it's small.

- Wouldn't the best spot for the V-tape be above the windshield, to reattach cleanly the roof flow, and maybe influence how the flow happens above the hood?

- How about the leading edge of the hood itself?

... or maybe that would just make the flow more uniform and attached, aka more stable ride, but increase surface drag?
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Old 06-26-2008, 07:28 AM   #47
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I guess I was unclear, the V should point rearwards when on a topside surface, pointing it forwards enhances the downwash vortices. If using angled strips on the lower half of the side, they should be low in the front high at the back.

Looking at the best photos I could find of an '02 CRV, it looks like you've got a semi-sharp lip on the back of the roof. Since the CRV is fairly tall for it's length, causing too much air to come down the back probably makes for a large increase in induced drag. So I'd say to avoid using VGs on top. You might try something to crispen the lip up a bit and allow cleaner separation. Just a strip of something protruding half an inch over the edge might do it. The downwash vortices look like they will be taken care of with the roof rails. Also around the rear corners it looks like things have been taken care of with the shapes in the bumper and ahead of the light clusters, the way the air trips and falls into those kind of scallop shapes should attach it further round the back. If anything the tire carrier helps a little, allowing small vortex cushions to form either side of it and round the back of the vehicle. The only place I see where you might consider turbulators on the back, is in the area of the rear side window. If that has a black border on it, you might consider just two strips of 1/4 high closed cell insulating strip, top to bottom there.

Now, any turbulation device can't magically pull down flow in an area that is already turbulent, so to have any effect on the windshield area, it would actually have to be on the windshield before the flow separates. If you've got a darkened windshield at the top and your wipers don't go up that far, then you might get two or three straight across strips there, spaced their own height apart. Likewise on the front, they would have to be on a surface that had attached flow. There doesn't look like anywhere where you could put any, apart from maybe curving a single strip around between the headlamp area and the signal light, making sure you don't block either. If you did an upper grille block then maybe just below the lip of the hood could take a set. Mirrors, standing out from the body somewhat, might be acting in a different class of reynolds number, and the 1/4 inch strip would be too big. If you could find something sticky and 1/8th of an inch high, then putting those in sets at 1/3 and 2/3 of the mirror height might marginally improve it, however, these might also tend to get the mirror dirtier with spray.

With newer vehicles there's not a heck of a lot to fix, the major comprimises are in the general body shape, and all the details to work around that have usually been taken care of.

Reynolds number calculation for a typical car @ 55mph...
http://galileo.phys.virginia.edu/cla...ds2/node4.html

Wikipedia Reynolds number...
http://en.wikipedia.org/wiki/Reynold%27s_number
NOTE, this mentions the flow transition at about 3x10^6 for a cylinder resulting in a DROP in Cd... with cars being in the region of 2.4 @ 55mph, transitioning to a lower drag regime by going faster is possible. It's probable that the laminar to turbulent transition for a boxy 1970s car didn't gain anything over 55mph, but with cleaner aero in later cars it's probable that immediately after transition, DRAG IS LOWER, then climbs again. Hence 55mph is probably not such a "saver" for anything with a Cd under .45 or so, that sheds turbulence with any semblance of aero design competence.

Pretty pictures...
http://www.featflow.de/album/catalog...w_2d/data.html
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Old 06-26-2008, 09:21 AM   #48
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However, killing "upforce" would be beneficial or at least translating it from up and 15 degrees backwards, to up and 5 degrees forward.
Sure, but how much lift are we talking about? For most of us, the answer is "not much."

Out of curiousity, I've seen you mention the "15 degrees backwards" idea a few times. Where did you come up with that?
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Old 06-26-2008, 09:49 AM   #49
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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. Soooo aerodynamics texts will talk of lift and drag separately there, when the actual physical force of lift might be pulling at an angle some degrees off from the vertical. 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...

So, what I'm meaning is that in considering the average angle of airflow over the vehicle, and the angle at which it meets the "leading edge" and departs the "trailing edge" you might get "lift" pulling forwards if the angle is inclined forwards, or you might get "lift" pulling you backwards and thus becoming drag, what's known as induced drag if the angle is inclined rearwards.

By expanding air out from under the car, you effectively kick up the angle of the air departing the car inclining it forwards, thus altering the average angle of airflow over the body for the better.

Now, all this seems a bit odd, because we'd think that the average angle of airflow would be horizontal at all times. Well it would be if the car was flying in free air, 1000 ft above the earth, but it's not, the air is prevented from fully flowing underneath it and compensating. It all sorts itself out a hundred feet behind you, but by that time it's not having dynamic effects on the vehicle.

Best way to think of it, is imagine you've got a flat block moving through the air, average angle over that is horizontal, now a wedge moving point first, average angle of flow over that is going to be inclined forward from horizontal, now the wedge going backwards, inclined backwards from horizontal, Stick a flat bottom airfoil on the block, lift is straight up (for an airfoil designed to have lift at zero incidence) stick it on the wedge going forward, the lift force is inclined forward from the vertical, stick it on the edge going backwards, it's inclined backwards from the vertical.

So, if you took a chassis cab, and gave it a smooth sweeping kammback down to the rear bumper, you'd be approximating an airfoil at a high angle of attack, which in conventional terms would make little lift and lots of drag, but the actual force of lift is a component of the drag, because it's pulling the body backwards. This is analogous to our backwards wedge. However, if there were more expanding air coming out from under the back bumper then the air would not depart the body at such a downward angle, it would effectivly "kick up" the back edge of the airflow, such that it moved the lift force vector forwards WRT to the body and stopped the lift making induced drag. In other words you'd tilt the average airflow angle back to the horizontal.

Hope that's not as muddy.... it's clear in my mind, but it's hard to get around the "broken" method of describing forces on a body that's favored by aircraft aerodynamicists.
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Old 06-26-2008, 10:53 AM   #50
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There's 3 generations of CRVs, you may have looked at the most recent, with better aero and no outside spare?

Mine doesn't have a lip on top.


Unless you're talking about this aftermarket air deflector


which people seem to say is just decoration... I don't recall seeing anyone claiming gains from it.

You also mention a change of Cd. Could that explain that -I believe- I get same or better MPG at 75 than at 55?

So from reading you, I also have the feel I should not try to fill the void behind the rear wheel to flatten the underbody. What I did so far is tape holes under there to closed internal voids between metal and plastic parts like the rear bumper and side runners.

The airflow seems from what you say to detach way before a curvature point... So a VG shouls always be a couple of inches before it.
Note: I get lots of bugs and some pebbles impacts on the windshied. It must indicate rough air direction changes that could benefit cleaning?
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