Keeping the boundary layer on the roof
 

Would it be beneficial from an aerodynamic point of view, to keep the airflow that's going over the windshield and roof, on the windshield and the roof by using some longitudinal fences - say 0.2 to 0.5 inch high to trap only the boundary layer - to prevent it from spilling over to the sides ?

I have a Volvo V50 station wagon with rather squarish corners along the sides of the roofline.

When using the windshield washers, a lot of the fluid spills over the top half of the side windows, obviously by turbulent air - annoying, but also illustrating the aerodynamic effect of the mirrors.

Keeping the washer fluid from the side windows is what sparkled this idea, but why not extend it all the way to the rear ...

Ideas ?

It sounds like a good idea (I think), except for one part.
What happens if there is a crosswind?
Now you are creating extra drag if the air comes from any angle but directly ahead.

If these fins are parallel to the flow, they just add surface drag. If they are not, they thicken the boundary layer as air spills over them. There is probably at least one very peculiar situation they would actually help, but there are easier places to search for improvement.

fences
 

Volvo wagons typically demonstrate enviable drag characteristics when even compare to many coupes and sedans.
And aero engineers have fought for decades to engineer away such 'eruptions',as the 'fence' in order to 'lower' drag.
I agree with the comments of others and would also encourage you to look elsewhere for improvements.

Then I'll just stick with small fences on the windshield to keep washer fluid and rain off the side windows.

I wouldn't worry about wiper spray and rain blowing around A pillars. Air wants to go around. I'd let it. Any attempt to stop it most likely add drag.

Why is spray and rain spilling around the sides such a big problem? I don't understand the motivation very well.

Cheers
KB

It limits my view in rainy conditions - and the A-pilar is already quite thick.

I've attached a rough sketch of where the rain / spray goes (red) with the approximate head position (blue).

Okay. Safety first.

I wonder what effect those stick-on plastic things would have.
http://www.autopartswarehouse.com/pe...rformance.html

I have no experience with them myself.

I hope you find a solution.

KB

When taking a closer look at the windscreen to see where I could put some small fence to keep rain off the side windows, I noticed there already is a fence :
The sides of the A-pilars stick out about 1cm (1/3") from the windshield, and this line even continues with a reduced height along the roof's edges, all the way back to the rear !