fine water deposit on roof

[lunarhighway]

a while ago after driving in wet conditions (it had just rained so there was a lot of watter spray from traffic, but no actual rain anymore) i noticed a watter spray pattern on my roof. it seemed as if the first part of the roof was covered in a fine layer of water but halfway my front door sill there was an abrubt schange in the water pattern and after that seemed to be a thick wet coat wich near the end exhibited streaks parallel to the driving direction. note that this was a very thin coat of water and not separate doplets as would be visible after direct rain.

it remined my of something i saw in an article on turbulators where on a gliders wing theres and area where the boundry layer separates. oil streaks where used to find the right place where turbulators should be placed to reduce this effect.

on the other hand what i've gatered is that cars opperate in a more turbulent aerodynamic enviroment and that the boundry layer on a car is much thicker than on an airplane.

i can think of only two things wich might cause this effect.

either the airflow "sepparates" at the windshield-roof transition and "reattaches" later on
or it's the other way around where the air flows close to the roof and than sunnenly becomes more turbulent.

if it's the later it might be beneficial to place turbulators at the point where the flow sepparates, but if it's the first than perhaps the window-roof transition is the problem and turbulators might make a difference there.

how should i interpret what i've seen?

[thatguitarguy]

Have you checked out the information here? Index of Phil Knox Aerodynamics Seminars & Mod-data lists It look like he's studied this a lot.

[MetroMPG]

Modern cars don't have flow separation at the top of the windshield where the roof begins. (OK, possibly Jeeps with the flat windshields do, but they're not cars.)

I can't answer your question about why there's a difference in water pattern, but a contributing factor could be the dramatic difference in pressure / local airspeed at the location you noticed.

Here it is in pictures, showing relative lift forces on an RX7. Note the very low pressure / high airspeed in the area you described.



Source: Browser Warning

[Piwoslaw]

For those who don't remember, here's a reminder:
higher airspeed --> lower pressure --> more condensation.
Just a little physics...