[aerohead]

Originally Posted by JacobLeSann

Thanks! Work in progress, but still driven everyday. This R18 is an underrated little Honda engine.

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Been raining hard enough to feel European around here lately, and I’ve been watching carefully at how rain is flowing over the car. I’m not quite sure what exactly this is indicating but it’s of interest.



Here’s a driver’s view at 90 km/h. I used a filter to make the water more visible. The water was still and didn’t move 3-5” behind the wipers, in an asymmetrical pattern.



Rear windshield at 90 km/h. No flow. Stray rain drops that landed there would stay there and not move.



Front windshield after parking. A little washed out but the flow pattern was visible.



Rear view of the roof. A clean flow pattern was visible but seemed to break off at the edge between the rear glass and the roof.



Rear end. You can see some streams of water wrapping over the C-pillars. There were no flow lines on the rear glass, and the rear wing had nothing of note.


My theory is that the flow of rain water around the car is a side effect of high pressure zones on the car. The flow seems most intense at the top of the roof, and at the front of the car. Air spiraling out the front wheel casts a big swath of pure water down the sides. There is nothing at the rear. But that’s just my idea.
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I think there is something tricky going on with the boundary layer at the rear of the car. This may match up with photos of tuft lines I have posted in the original thread. If I make a magnehelic gauge and measure pressure zones of the car, maybe that will get me closer to understanding.

It’s very wet out, but if/once it dries I will try some more tuft testing.

1) that's likely a 3"- 5" dead zone in the lee of the wipers, even though they're 'aero' blades. The wiper blades have a Cd of around 1.20, and kicking the flow up just as the flow deflectors do around tires.
2) the backlight is much steeper than the 2011 Audi A7, which had total flow separation at exactly the same place as the CIVIC. The spoiler has likely captured the hoped-for stagnation bubble, but the local streamline is in no position to feed any momentum into this region, as you've lost the boundary layer all the way to the tearing edge / reversal point of the spoiler.
3) the flow rounding the C-pillars triggered the vortice formation in conjunction with low-energy turbulence over the glass.
4) if the length of the spoiler is at the optimum, it's going to be as good as it's going to get for the single mod.