Raising a Trunk?

[JacobLeSann]

Hey all,

I’ve been reading, re-reading, and reading some more of Julian Edgar’s book on aero mods. In a couple different pages he mentions raising the height of the trunk lid on a car. I forget the specifics at the moment, but I believe it made for a gentler transition from the rear windshield down to the trunk on notchback-style cars, overruling the increase in wake size. I haven’t seen many people do this, but I have seen it put to great use on the GM/Chevy Volt. I’m planning aero mods for my 2006 Civic Coupe (FG2), which seems to have a separation bubble at the base of the trunk lid; bottom of the windshield. If figure this could be a good remedy.

Anybody try this? I’ve seen it mentioned in various threads but haven’t seen it done much.

[M_a_t_t]

Does it reconnect before final separation?

[me and my metro]

This has been a common practice on circle track race cars for at least 50 years. It was done to get the rear spoiler higher in the air for more down force. They would limit the spoiler size but not the trunk lid/quarter panel configuration. People discount racing as a waste of time but there have been lots of aero in their designs.

[aerohead]

Originally Posted by JacobLeSann

Hey all,

I’ve been reading, re-reading, and reading some more of Julian Edgar’s book on aero mods. In a couple different pages he mentions raising the height of the trunk lid on a car. I forget the specifics at the moment, but I believe it made for a gentler transition from the rear windshield down to the trunk on notchback-style cars, overruling the increase in wake size. I haven’t seen many people do this, but I have seen it put to great use on the GM/Chevy Volt. I’m planning aero mods for my 2006 Civic Coupe (FG2), which seems to have a separation bubble at the base of the trunk lid; bottom of the windshield. If figure this could be a good remedy.

Anybody try this? I’ve seen it mentioned in various threads but haven’t seen it done much.

1) if you'll compare the original VOLT concept 'notchback', at Cd 0.42, to the production VOLT 'fastback', at Cd 0.281, it provides an insight into GM's 'solution' for the car's aerodynamic performance.
For some unspecified reason, Chevrolet Division modified the original roofline to fit the 3rd- generation aerodynamic streamlining template. Online, you can find a CFD analysis for the production VOLT, indicating for fully-attached flow until the rear of the roof. Yes, the wake IS larger, but the much-higher base pressure creates an overall pressure drag reduction, and total drag reduction.
2) for your CIVIC, you may want to compare it's roofline to that of the 2022 CIVIC, and see if HONDA hasn't followed the VOLT's lead. And if you can locate a 'blueprint' for both CIVICs, you might want to compare both to the AST-III. There's a reason why the lowest drag cars are using rooflines close to it.

[JacobLeSann]

It looks like flow reattaches at the boot and separates cleanly off the top end of the trunk. I’ve posted some tuft-test photos here:
https://ecomodder.com/forum/showthre...tml#post675015

The 2022 civic coupe has a very neat rear end. It’s a very true-to-form fast back and has some sharp separation edges at the rear. My issue being that my rear end isn’t really like that at all. I can’t really get it to take that shape without a lot of modification, hence why I’m looking at raising the lid. The 2022 Civic sedan is a bit of a better model. You can see the progressive raising of the trunk’s height over the different model years.

I think I might go ahead and test out some prototype aero mods. I want to raise the lid toward the base of the windshield, and drop it down towards the car’s rear,, flowing out into a rear bonneville-wing type extension. I think that would work very well as a fastback substitute.

[aerohead]

Originally Posted by JacobLeSann

It looks like flow reattaches at the boot and separates cleanly off the top end of the trunk. I’ve posted some tuft-test photos here:
https://ecomodder.com/forum/showthre...tml#post675015

The 2022 civic coupe has a very neat rear end. It’s a very true-to-form fast back and has some sharp separation edges at the rear. My issue being that my rear end isn’t really like that at all. I can’t really get it to take that shape without a lot of modification, hence why I’m looking at raising the lid. The 2022 Civic sedan is a bit of a better model. You can see the progressive raising of the trunk’s height over the different model years.

I think I might go ahead and test out some prototype aero mods. I want to raise the lid toward the base of the windshield, and drop it down towards the car’s rear,, flowing out into a rear bonneville-wing type extension. I think that would work very well as a fastback substitute.

Theoretically, the CIVIC's roofline will generate too radical of an adverse pressure gradient, jeopardizing the boundary layer.
There may be vorticity that is not revealed by the tufts, and what appears as a 'true' attached flow, is actually 'downwash,' a symptom of separation-induced longitudinal vortices of high drag.
If so, you lose the potential for pressure recovery, as much of the kinetic energy of the separation bubble is lost to heat.
The fastback moderates the rate that the flow can decelerate, avoiding the threshold for separation, allowing all the flow field to gently decelerate, and build back higher static pressure where the flow finally does separate, thereby injecting higher pressure into the wake, W.H. Hucho's near-singular mechanism for streamlining.

[aerohead]

An option to just adding length is to also angle the spoiler up, at no more than a 30-degree angle, 'quitting' the spoiler just as in intersects the AST-III projected contour. It will allow for a shorter spoiler 'length.'
There are dozens of high-performance sports cars that have done this.

[freebeard]

I might be induced to produce a sketch, but a DALL-E prompt might be "2006 Civic Coupe (FG2), tapered hitch mount box that extends over the trunk opening, aluminum construction with rounded edges, photorealistic'.

[JacobLeSann]

A ducktail/lip spoiler is interesting. I see this on newer Chevy Cruze/Malibu models, Chrysler 300 models, teslas, etc. I never really did know how lip spoilers work, but seeing Aerohead’s CRX makes me wonder.

Adding a rear wing that extends the whole length of the rear into a boxed cavity (sort of like a Porsche K35 rear) is the main idea I’ve been looking at. I trialed a quick n’ dirty bonneville style trunk extension on my long distance trip recently, which really did help the mileage.

The fastback is indeed a superior design, it’s just that I’m trying to retain the stock practicality/aesthetics/vision. I plan to give the car to my mom once I fix up my hatchback. That one will get more radical mods. The hitch mounted tailbox is likely gonna go on that one, when traveling.

I think there is relatively stable flow down the top half of the rear glass. If you look at a side view of the FG2 chassis (as freebeard said), you can see that the transition to the rear glass is very gentle, even though it’s a steep angle.

[freebeard]

IRONICK posted about the Ferrarri Purosangue at ecomodder.com/forum/showthread.php/fender-vents-separation-traps-40461-2.html#post675102

While the suspended spoiler helps neutralise the curvature of the roof downstream of the area over the heads of the rear-seat passengers, the nolder, which is barely 7mm high, channels the wake vortices to create a slight recompression at the tail of the car.

https://www.ferrari.com/en-EN/corpor...like-any-other

Ferrari call it a 'nodler'. There are a lot of interesting details on this vehicle