[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.