Drag Bucket Paradox, sometimes less aero = less drag

[Tesla]

This effect has been mentioned before, but I just read a bit about it in the NASA Dryden truck paper and they called it Drag Minima or the Drag Bucket, this may not be completely proper use of the term, but will use it for now.

As I understand it, every shape has a optimum where it achieves lowest overall drag and to gain improvements fundamental changes need to be made to the highest drag components, hence why some people (in the know) here, keep repeating the "fix the rear" mantra, as this is the worst offender for most vehicles, the cooling system/grill block is possibly in the same category.

If you continue improvements elsewhere, like my self, it is highly likely you may not see any improvements in FE and may actually see losses, I have had improvements in handling and noise levels, but FE remains static and this is most likely because of the Drag Bucket, which says as you improve flow over, under and around the vehicle, this has a reciprocal effect at the rear, which is magnified by some factor.

Put simply, halve the drag at the front and you may well double the drag at the rear resulting in significantly worse FE.

And infact in some cases, by making the front worse at the front, could result in an overall improvement in whole body drag & FE.

This is all related to the golfball dimple effect, boundary layers etc.

A worse front end is acting like a dimpled golf ball, creating more turbulance and a thicker boundary layer although this has an increase in skin/body drag, the thicker layer allows the air to bend tighter around the rear before seperation resulting in a smaller, higher pressure wake and less drag overall.

Smooth out the front, the boundary layer gets thinner, less skin/body drag, bends less, it seperates sooner, the wake is lower pressure and bigger resulting in higher drag overall.

This paradox is most likely a significant factor in the variability of results with some modifications, particularly in the areas of mirrors, full bally pans, wheel caps and skirts. It seems like positive results from grille block, airdam/front undertray, Kamm/boattail are almost guaranteed, everything else is case by case basis.

Now this can be addressed with improvements in the rear, like a boattail, that's fine for those with a sedan/hatch, pick up etc. that have existing vehicle body to modify and/or are prepared to put a long extension on the rear, but there is a class of us here, minority who have a nearly vertical square back vehicle and/or can't or don't want to build a massive overhang, what are the options?

The first is a lot more reading and vehicle analysis to try to identify crucial "low hanging fruit" that will not be detrimental and outside of that, the only option I see is manipulating the boundary layer, hate to say this, but this does take us back to the land of Vortex Generators and their ilk.

There seems to be some solid work using airtabs on large square back vehicles and I have seen a number of different trip mechanisms used on the rear sides of semi trailers, on cars I only really know of the Mitsubishi that came from factory with VG's on the roof, but nothing inbetween, which is where my vehicle lies.

To be generic I will call them BLT's (Boundary Layer Trippers, not the sandwich), of which the application has generally seen zero or negative results, but now I am beginning to question whether they need to be used in conjunction with significant improvements on front, under & sides of vehicle.

Even though it may sound counter intuitive, we may need to smooth out the vehicle to improve flow and reduce thickness of boundary layer, then just before it seperates from the body at the rear use some form of BLT to cause turbulance and thicken/energize it so it can wrap tighter around the corner and produce a smaller wake with all the benefits associated, like lower drag and better FE, a small tighter bend Kamm lip may also be of benefit here.

Without a wind tunnel, there is likely to be quite a bit of hit and miss, but this can be minimised by informed decision making and solid theoretical foundations.

What do you think?

Edit: Just had a thought as a gauge of when benefits could arise from some application of BLT's, when you have completed significant improvements to front of vehicle, with some measurable changes, cabin noise reduction would likely be one of these and no change or even a drop in FE, then this may be a sign that BLT's may be of benefit.

[aerohead]

I've only seen 'bucket' used in the context of lift/drag curves for a limited series of airfoils,based on their angle of attack.
Serious 'flow-turning' has been demonstrated since the 1920s but VGs won't get it done.Typically,blown,or suctioned slots are required.
Georgia Tech continues research in this field.As of last look,the effect still required more energy than it saved.
Based on Cornish's work/report,Alex Tremulis thought we'd have cars of Cd 0.10 by now,using this technology.

[radioranger]

seems to explain my results with modifying the front first, gotta believe though the mirror delete cant hurt. I'm going to build a partial bed cover out of plywood that is pivoted midpoint on a piece of pipe so i can til the back end up for downhill braking when needed, when running free it will sit flush and when braking is needed front edge will dip into the bed area and rear will kick up maybe as much as one foot. tilting mechanism not figured out yet though.

[RiderofBikes]

Front VG's?! yeah my fiesta mirrors were what i first thought of... But the shape itself, is nice and aero already, I think the main point to them was to reduce wind buffeting for a quieter presence. On the otherhand, IF those small, i mean small strifes are enough to create vortices, then why are all the others we see commercially produced so, huge! If you look at it, even the EVO's arent that big.
Also there is difference to the golf ball theory, because not all model of balls # of dimples/size/depth/dimple pattern are the same.. and the fact its not in ground effect while in flight. make for huge inconsistancies...

If this were the same case with VG theory, then why dont my mirrors have strifes on the bottom(technically there not in ground effect)? as well as on wings/spoilers?

[Tesla]

Originally Posted by aerohead

I've only seen 'bucket' used in the context of lift/drag curves for a limited series of airfoils,based on their angle of attack.
Serious 'flow-turning' has been demonstrated since the 1920s but VGs won't get it done.Typically,blown,or suctioned slots are required.
Georgia Tech continues research in this field.As of last look,the effect still required more energy than it saved.
Based on Cornish's work/report,Alex Tremulis thought we'd have cars of Cd 0.10 by now,using this technology.

The bucket term was used casually by the NASA guy's, but only because they noticed the similarity of effect, probably not the right term though.
It was just after looking at their stuff and then chasing down some other leads I thought it warranted further investigation, more as a last ditch alternative for those difficult situations, I wouldn't expect it to be as effective as body taper, but if you can get 5% improvement by proper application, then it would be worth the effort.

[Tesla]

Originally Posted by radioranger

seems to explain my results with modifying the front first, gotta believe though the mirror delete cant hurt. I'm going to build a partial bed cover out of plywood that is pivoted midpoint on a piece of pipe so i can til the back end up for downhill braking when needed, when running free it will sit flush and when braking is needed front edge will dip into the bed area and rear will kick up maybe as much as one foot. tilting mechanism not figured out yet though.

I'm kind of in the same boat, I have done a number of mod's, car goes better, wind noise is down, handling much better, but FE static, so there must be something sucking up those improvements and it is quite possible that it is the wake gaining more intensity as the flow elswhere improves.
I will continue smoothing things out and may try look at some trip mechanism then to see if it changes things, for better or worse.

[LeanBurn]

What shape would that mirror present if it were folded inward? Flat profile facing the window while smooth curves almost tear-drop profile?

[Tesla]

Originally Posted by RiderofBikes

Front VG's?! yeah my fiesta mirrors were what i first thought of... But the shape itself, is nice and aero already, I think the main point to them was to reduce wind buffeting for a quieter presence. On the otherhand, IF those small, i mean small strifes are enough to create vortices, then why are all the others we see commercially produced so, huge! If you look at it, even the EVO's arent that big.
Also there is difference to the golf ball theory, because not all model of balls # of dimples/size/depth/dimple pattern are the same.. and the fact its not in ground effect while in flight. make for huge inconsistancies...

If this were the same case with VG theory, then why dont my mirrors have strifes on the bottom(technically there not in ground effect)? as well as on wings/spoilers?

The size of the trip is proportional to the object size & length, I believe it is related to the thickness of the boundary layer, which gets thicker the further down the boundary layer you go.
As a guesstimate
A mirror may only require a few 3mm (1/8") of disturbance.
A small car, say 6mm (1/4")
A van maybe 12-25mm (1/2 to 1")
and a large truck 25 - 40mm (1-1 1/2")

The other complication is the boundary layer gets thicker as velocity increases (think that's right), so the scaling is crucial, something that trips the boundary layer at HWY speeds to get a drag reduction may just result in excess drag at lower speeds because it is causing turbulence in a smooth flow, so there is a bit of a juggling act.

[RiderofBikes]

Originally Posted by Tesla

The size of the trip is proportional to the object size & length, I believe it is related to the thickness of the boundary layer, which gets thicker the further down the boundary layer you go.
As a guesstimate
A mirror may only require a few 3mm (1/8") of disturbance.
A small car, say 6mm (1/4")
A van maybe 12-25mm (1/2 to 1")
and a large truck 25 - 40mm (1-1 1/2")

Ok, this makes a bit more sense. so, at a compromise for the most common sized vehicles, would potentially be a 3/8" trip? at least for baseline, makes me want to break out the clay and test... why haven't i done this yet lol..

Regarding the Mirror folded in, well, it only folds in 15-20*, useless, I don't even know why they made it pivot in the first place, only benefit, would be to take it off! but i can't while the fiance is driving it

[RiderofBikes]

On the other hand, i always had a thought, that given the vehicles shape, would eventually reach a "max terminal" layer... once the vehicle starts to taper off(if at all), woulnd't the pressure be the same as it was at its peak?