Wheel well vents

[aerohead]

Originally Posted by serialk11r

So the trailing radii are useless if you don't have air flowing through the wheel wells? Am I reading this correctly?

And I suppose cars that do have the trailing radii or other vents in the wheel well but also dedicated radiator extractor vents, it is there to handle whatever other flow that does make it to the wheel well?

I think that the value of the radius is associated with yaw.
Since the wind is blowing most of the time (Cal Tech, Hucho,'n others tell us this) we end up with a crosswind component.
This crosswind is going to hit at least one side of the car.And since the bulbous nose has the least drag in yaw,the radii on the front quarter panels would help maintain attached flow under really adverse crosswind conditions.
Since streamlining is all about preventing or eliminating separation,the radii are an insurance policy.As long as the air is attacking the radius,the radius is in a favorable pressure regime and the boundary layer is protected.
A flared wheel well might trigger separation under yaw.
In the absence of skirts,it's a good way to go and I'm all for them.
A few dozen cars have used them.I believe that Scaglietti introduced them in the 60s on one of his Ferrari designs.
http://www.performancedirect.co.uk/b...esta-rossa.JPG

[aerohead]

I think that all we can take away from the thesis,is that this model,with this CFD program,developed these results.
Each 'real' car would have to be analyzed on a case-specific basis.
There are just too many variables and complexities to consider a one-size-fits-all solution.

[serialk11r]

Ah that makes sense. One of my headlamps is destroyed so it's an awkward, large cavity at the front of the car. Usually there isn't much in the way of crosswinds as in the Bay Area we have a lot of "sealed off" freeways (walls on either side), and there are tons of buildings and not much wind in the first place, but when I hit the bridges across the Bay, where there are significant sidewinds, that one small change seems to have ruined my crosswind stability as I find myself fighting crosswinds with steering input a lot more. Need to replace the lamp...

So the radiused trailing edge of the fender is generally a good idea, and otherwise you'd want the fender to be flush, not flared?

[whatmaycome14]

Originally Posted by aerohead

There are just too many variables and complexities to consider a one-size-fits-all solution.

Not even front skirts?

[chillsworld]

Originally Posted by aerohead

I think that all we can take away from the thesis,is that this model,with this CFD program,developed these results.
Each 'real' car would have to be analyzed on a case-specific basis.
There are just too many variables and complexities to consider a one-size-fits-all solution.

Oh I agree! Something as simple as the brakes and axle would change the whole model instantly! But the findings of the modeling and the results of the thesis, seem to go along with things that have been said in this thread and others. Different vents and styles solve different problems. The diagram even showed the uneven flow created down the side of the car, something you mentioned as a reason for allowing the air to follow the modified quarter panel before being reintroduced to the airstream.

~C

[aerohead]

Originally Posted by serialk11r

Ah that makes sense. One of my headlamps is destroyed so it's an awkward, large cavity at the front of the car. Usually there isn't much in the way of crosswinds as in the Bay Area we have a lot of "sealed off" freeways (walls on either side), and there are tons of buildings and not much wind in the first place, but when I hit the bridges across the Bay, where there are significant sidewinds, that one small change seems to have ruined my crosswind stability as I find myself fighting crosswinds with steering input a lot more. Need to replace the lamp...

So the radiused trailing edge of the fender is generally a good idea, and otherwise you'd want the fender to be flush, not flared?

I'd recommend the radius if you're not going to later do skirts.
As to the flared vs flush.You don't want want to add frontal area if you can keep from it.
The 1936 Jaray Adler had 'bulged' front skirts which added frontal area and they were only good for a 1% drag reduction.
The flush urethane flexible front skirts on the 1983 Probe-IV concept helped reduce drag 9%.
If you had to camber the skirt to clear the wheel you'd still get a benefit.We just do the best with what we've got.

[aerohead]

Originally Posted by whatmaycome14

Not even front skirts?

I was sharing some data with serial11r:
*The 'bulged',frontal area-adding,front skirts on the 1936 Jaray Adler were only good for a 1% drag reduction.However,with the rear skirts the car saw an overall 13.6% drag reduction.
*The 'flush' front skirts on the 1983 Ford Probe-IV contributed to a 9% reduction.
That's quite a spread.
Since the lowest drag cars have skirts,it kinda infers that we 'want to go there'.
Hucho cites work by Cogotti on wheelhouse drag.
Fiat separated out the wheelhouse drag in a paper they published.
The numbers are scattered,but there's a trend towards drag reduction.
The cleaner the car,the more important they become.Wheel drag constitutes 1/2 the drag of a low-drag car.
Here is the Cd 0.12 Honda Dream-1,with non-enclosed wheels.

Here is the Cd 0.10 Cambridge University CUER solar racer with fully-enclosed wheels.

HOT ROD Magazine's Camaro would have run faster with shielded wheels,but the rulebook forbids it.When you get into the enclosed-wheel streamliner class,you really see some velocity

[aerohead]

Originally Posted by chillsworld

Oh I agree! Something as simple as the brakes and axle would change the whole model instantly! But the findings of the modeling and the results of the thesis, seem to go along with things that have been said in this thread and others. Different vents and styles solve different problems. The diagram even showed the uneven flow created down the side of the car, something you mentioned as a reason for allowing the air to follow the modified quarter panel before being reintroduced to the airstream.

~C

No,you're right.The research is beneficial and we're forced to look for correlations,and we're going to find them.I like to think that more and more info will enter the public domain and we'll see even a richer body of knowledge from which to cherry pick.
The big dogs will try and temper our enthusiasm however,citing that cars won't necessarily behave the same,so we need to be vigilant.
It's very expensive to properly test these body permutations.

[ECONORAM]

Originally Posted by aerohead

I'd recommend the radius if you're not going to later do skirts.
As to the flared vs flush.You don't want want to add frontal area if you can keep from it.
The 1936 Jaray Adler had 'bulged' front skirts which added frontal area and they were only good for a 1% drag reduction.
The flush urethane flexible front skirts on the 1983 Probe-IV concept helped reduce drag 9%.

I was thinking along these lines when I fabricated my front wheel flares. A aerodynamically better shape that did not increase frontal area. They seem to work. I do need to do a coast down test sometime though.

[godscountry]

One of the bike racers at Battle mountain,Nevada was saying how critical the wheel openings are to the overall drag of the vehicle,don't laugh,these bicycles hit 80 plus mph and they're wheel openings are in the mm.So a car or truck or streamlined motorcycle,I would say it would have a big effect as your speed gets higher.Some of these streamlined motorcycles might pick up a few mpg's just by better sealing the wheel openings and getting rid of some of that dirty air.