100 miles in the rain, Back window dry? Why?

[pred]

Last evening I drove from Phila to Pittsburgh in the usual fashion of the turn pike the entire way, I had the cruise set at 75 and drove thru mild to hard rain for over 100 miles and not a drop of water on the rear window, Clear as a bell the entire time,
I have had vehicles which exhibited similar things including a 88 Toyota 4 runner which when the rear top was removed and you kept it over 60 the rear passenger and stowage area stayed dry.
My Ti is a 97, Active bone stock.
Just thought I would share.
Peter

[Bobtail_Bimmer]

Aerodynamics. Ever see commmercials of cars in a wind tunnel and you see the air flowing over the car? That figured in with the direction the rain is blowing would be why.

[teetime4one]

because racecar?

[KSKarl]

Here in Kansas, I'd be overjoyed to drive through a hundred miles of rain.

[pred]

Originally Posted by teetime4one

because racecar?

Not a race car.
Thinking about this I realize that the faster it goes the more rear lift it must have making the rear end unload at speed,, No rear down force,
I bet a race car would be wet all over from rain because when engineered down force is a major "Want" for a race car...
Oh well, At least the rear window stayed clear.
Peter

[adamud]

pred,

The air is not flowing down over the back windshield. It is leaving the roof line and swirling around above the rear WS. This is not good for any car, regardless if it is a racecar or not. In an optimal condition, you would want to "trip" the air down the WS to keep the air profile along the car as far as possible. By not "tripping" it, you create a pressure that slows (drag) the car down.

I wish I could show you pictures of good and bad air flow from our wind tunnel testing (on racecars).

[cooljess76]

Originally Posted by adamud

pred,

The air is not flowing down over the back windshield. It is leaving the roof line and swirling around above the rear WS. This is not good for any car, regardless if it is a racecar or not. In an optimal condition, you would want to "trip" the air down the WS to keep the air profile along the car as far as possible. By not "tripping" it, you create a pressure that slows (drag) the car down.

I wish I could show you pictures of good and bad air flow from our wind tunnel testing (on racecars).

+1 Turbulance = not good.

OP, it's not just a ti thing. Most car's rear windshields remain dry when the car is in constant motion/driven at highway speeds.

[adamud]

The image I found below is of a simple aero profile at a tilted angle.
Imagine it as the slope of the rear hatch on our cars. There is separation from the top profile (think roof) due to the boundary layer.


Since the angle from our roof to the hatch is so steep, the air doesn't stay planted. If you were to "mess up" the air before then, you could allow it to stick to the back of the window better. Look up Vortex Generators. I think Mitsubishi is the only car I have seen with them stock. You can make your own or buy some offline that probably wont work. We played around with these on a friends car one time. Taped a piece of string afterwards to see if it would lay flat or stick up in the air.

[pdxmotorhead]

Wicks Aircraft supply sell vortex generators if I recall....

Dave

[Mr.Squeelerz]

It's getting a bit serious up in here...

[pred]

Yea kinda funny, But it does show a flaw in the design and how your rear end will lighten the faster you go, So at 70 Mph your rear may unload 70 LBS off the rear tires or more,
Several years back I read an article about how the Datsun 280Z unloaded a LOT at high speeds and how a rear wing really helped above 60 Mph.
I wonder if one of those vortex things would make a difference I could feel while scooting along at 80 Mph?
Peter

[KSKarl]

OK time for a little aero from the engineer --
The wing where you show the smoke streams separating from the wing is stalled. It no longer generates lift. It is generating lots of drag.
To parallel our hatch glass, the turbulence that keeps the rear window dry is reducing rear lift while generating drag. It also means any spoiler on the lip of the lid buried in a turbulent layer that kills its effectiveness. See how the rear wing on a racer is raised into clear air.
As adamud said, you can tape a string on it to see how the air flows. Much easier than smoke streams for us without a wind tunnel.

Sorry to get so heavy into the tech discussion, but balancing lift and drag is a real challenge for all the race cars.

[pred]

Well now this is getting interesting,
Is there something we can do to reduce this effect on the rear glass and get the air flowing/ Or do something to tweak more Mpg?
Peter