Rear Wheel arch ducting to rear to minimise base drag??

[aerohead]

Auto makers have attempted to make the wheel cavities air tight,other than for brake cooling and engine bay hot air extraction.
If air can essentially 'pool' inside the wells,the outer airflow will skip over the stagnant air as a flat pebble on water.Some rear deck spoilers are designed on this principle.Holden aero-modded a Commodore for racing,using such a 'birdbath' rear boot.
In addition,if you rob air from the 'outside',you're taking energy away from air which already has a direct path to the back,jeopardizing the boundary layer and setting yourself up for even more separation than you already have.Which can only increase drag.
Remember,streamlining is a lot like algebra.Whatever you do to one side of the equal sign,you've got to do to the other.

[Tesla]

Aerohead,
Thanks for the response.

Got the algebra thing, just trying to work out what I am actually subtracting from.
If we want to seal up the wheel arches as much as possible, why did the honda taper out the rear of the front wheel arch on the insight, is this just for the air turbulance from the ground contact point so it can rejoin the side air more smoothly?
If this is an appropriate mod for the front, is it also an appropriate mod for the rear?
The UK study indicated that there was a pressure build up at the top of the rear wheel archs, although they did not test this in a moving ground wind tunnel.
I was planning to put full rear wheel skirts on, was just concerned whether I should address the wheel arch pressure/ducting issue.
Oh, so many questions.

[Sven7]

He's saying that when the wheel arch is closed like stock the air will form a sort of bubble and under-body airflow will treat it just like a "phantom" belly pan. Letting air out of this bubble somewhere else will just direct air into this high drag area, increasing your overall drag.

What comes out, has to go in. And "in", in this case, is a very turbulent area.

Honda's Insight and more notably Ferrari's 250 Testa Rossa have rounded the rear of the front arch to facilitate better flow in this turbulent area. My advice is to install flat wheel covers, block in the front and top of the wheel gap, and leave it alone. Using a rear wheel skirt will help you a bit but rounding out the edge of the arch is probably a waste of time afaik. You could attempt some front wheel skirts but I'd be more concerned about a boat tail at this point. Low hanging fruit.

I hope that helps.

[Tesla]

Originally Posted by Sven7

He's saying that when the wheel arch is closed like stock the air will form a sort of bubble and under-body airflow will treat it just like a "phantom" belly pan. Letting air out of this bubble somewhere else will just direct air into this high drag area, increasing your overall drag.

What comes out, has to go in. And "in", in this case, is a very turbulent area.

Honda's Insight and more notably Ferrari's 250 Testa Rossa have rounded the rear of the front arch to facilitate better flow in this turbulent area. My advice is to install flat wheel covers, block in the front and top of the wheel gap, and leave it alone. Using a rear wheel skirt will help you a bit but rounding out the edge of the arch is probably a waste of time afaik. You could attempt some front wheel skirts but I'd be more concerned about a boat tail at this point. Low hanging fruit.

I hope that helps.

Thanks mate,
Yeah it does help,
I did understand where Aerohead was coming from, my confused comment was more of a generalisation about how many different concepts are out there and how to choose whats best.
Thanks for your opinion on the wheels as well.
I have temp partial front wheel skirts on currently, as per a previous thread, am happy with the basic idea, but longer term I think the front wheel area could actually benefit from a vent which can be done, in my case the actual plastic guards stick out about 2" from the body and with a bit of redesign a vent could be put at the rear of that guard, which would allow the vent air to run along the channel formed between the lower body and top of sidesteps as it merges with the rest of the side air. As said I plan full rear skirts and am letting go of the venting idea, too many unresolved questions there.
I appreciate your comment on the low hanging fruit, but at the moment I am investigating a wide range of ideas to get a picture of the final product, then work towards putting it all together.
Funny you should mention the boat tail, I have been looking at how to do that as well, but starting another thread for that one.

Thanks to everyone for their comments, I really appreciate all the help and I feel like I am starting to get a much better handle on all this aero stuff, but still lots of study and work to do.

[Frank Lee]

Some or maybe even a lot of aero sure seems counter-intuitive at first hence the general mystery and confusion.

[Tesla]

Originally Posted by Frank Lee

Some or maybe even a lot of aero sure seems counter-intuitive at first hence the general mystery and confusion.

That's a fact Jack,
Like anything, you need to imerse yourself in it for a while and then it start's to come together, it's all logical, you just need to find the right perspective.
Slowly getting there.

[Sven7]

"Doing" helps too, as you start to really understand it in three dimensions.

It might be worth setting up a build thread to consolidate stuff into one place

[Tesla]

Yep, will do as soon as I can take a breath & if I run out of duct tape.

[aerohead]

Originally Posted by Tesla

Aerohead,
Thanks for the response.

Got the algebra thing, just trying to work out what I am actually subtracting from.
If we want to seal up the wheel arches as much as possible, why did the honda taper out the rear of the front wheel arch on the insight, is this just for the air turbulance from the ground contact point so it can rejoin the side air more smoothly?
If this is an appropriate mod for the front, is it also an appropriate mod for the rear?
The UK study indicated that there was a pressure build up at the top of the rear wheel archs, although they did not test this in a moving ground wind tunnel.
I was planning to put full rear wheel skirts on, was just concerned whether I should address the wheel arch pressure/ducting issue.
Oh, so many questions.

On the 1st-gen Insight,engine bay air may be escaping through the front wells,and that beautiful radius at the back will help any transverse flow to wrap around the trailing edge and blend with the side flow.So it's a good idea.I have done this in the T-100.
On the rear,they're not really 'aspirated' as up front so,say on a rainy day,you won't see a 'jet' blast from the rear as you do see up front,and the radius would be overkill.With wheel skirts,they would be superfluous.
If you'll do the skirts the pressure issue will probably be moot.I do think they should have done the test with a moving ground plane.
The volume of the house affects drag although that would be a very challenging modification.
And yes,the more answers we get,the more questions arise.

[Tesla]

Originally Posted by aerohead

The volume of the house affects drag although that would be a very challenging modification.

You can get an echo in mine.
Nevermind, i suppose that will just mean a bigger gain, particularly when I cover the rear wheel arches.

I will do the vent on the rear edge of front wheel arches, as there is an opportunity, and it would look much beter that just being curved back to the body.

Thanks again for your input.