The interesting thing about testing is that it can show you that what you were quite confident was working, perhaps isn't.
The rear diffuser on the Insight is angled upwards at about 10 degrees, a figure within the range supported by the tech literature. (But as I always say, all such angles are car-dependent.) The undertray as a whole works very well, giving measurable overall car downforce. (I am prepared to trade off the extra drag for the downforce.) I had always assumed that the diffuser's performance was part of that effective undertray - but perhaps not.
The first clue was that when I put longitudinal strakes in the diffuser, it made no difference the undercar measured pressures.
But today's test was more significant.
I wool tufted the diffuser section of the undertray and took some pics. They're hard to take because you basically need to lie on the ground and snap the shutter as the car drives past. The day was also very dull - I'll try to redo the pics if tomorrow is bright and sunny!
The first thing that can be seen is that the tufts on the lower bumper (ie above the green line) are all in separated flow - not unexpected. But what is startling is the separated flow on either side of the diffuser (red ellipses). The central part of the diffuser seems to have attached flow, but not the sides.
I think someone here (freebeard?) suggested previously that the rear wheel wakes might cause a disturbance to flow on the diffuser, and it certainly looks like that. I also thought that perhaps the lower seats of the air springs might be causing flow problems.
So I added some angled strakes down each side to separate the wheel wakes and lower spring seats from the diffuser flow. (I also deleted the top row of tufts.)
But as can be seen, the pattern is effectively no different.
So tomorrow I might make some mock-ups that reduce the upwards angle of the diffuser and see if I get can attached flow right across its width. Perhaps two lower-angled ramps each side?
