Tufting the Prius
When I first started this blog, I wrote a post on tuft testing which turned out to be one of the more popular articles here. I covered the basics of tuft testing but recently thought I could write a little more about how to interpret tuft behavior. This article will demonstrate how to set up a tuft testing session, how to get usable results, and how to analyze them.
This gives me pretty consistent 60-70mm tufts—long enough to
be visible to a camera some distance away.
|The orange and blue is coincidental and not some display of school spirit—that’s what the decals are for.|
Then, I put down another line normal to the first:
Of course, this works better on some panels (flat, rectangular)
than others (curved, irregular outlines) but it’s a good starting point for an
ordered, methodical arrangement of tufts. You don’t have to do it this way;
just ensure that your car has enough tufts to see what you want to see.
How about that—overall, the flow is exactly what we want to
see, and a testament to how well Toyota engineers did their job during the development
of this car. Most cars will have areas of separation, even late-model cars;
common problem areas include the windshield around the wipers, the side windows
behind the A-pillars, the side body behind the wheel openings, and the lower
part of the rear glass, especially on sedans. On this Prius, there is very
little separation behind the wipers, very little disruption at the A-pillars, no
separation behind wheel openings, and attached flow all the way down the rear
glass and across the stock and add-on spoilers. Attached flow from front to
back is exactly what we want on our cars for low drag (that is, the car body is
streamlined—it doesn’t suffer from areas of separation), and this
car looks better than I expected.
At the rear, the tufts taped to the diffuser extension show
that air is blowing backward out of it, but outside of the strakes and behind
the muffler, they’re pointing forward. These stand out
as areas I might be able to improve, perhaps by rerouting the exhaust and
fitting paneling or building an extended or sharper front splitter.
With the duct inlets taped, a recirculation bubble forms in
front of what was the opening. Further, flow now separates completely across
the outside surface of both front and rear ducts. Curiously, the flow on the
body after the wheels doesn’t appear to be affected much (there's a little separation right behind each opening); this compares well with the
pressure measurements I made with the ducts removed completely, which showed no
change in pressure behind the front wheel openings and only a slight rise in
pressure (+10 Pa at 50 mph) behind the rears with ducts fitted.
Interestingly, the fins don’t change the flow pattern on the
upper body as I thought they might. Instead, take a look at the tufts behind
Surprisingly, fitting fins to the upper body looks to be changing
the flow under the body. This is the kind of interaction you will be
very, very unlikely to predict; I had no idea I would find this and it
warrants further investigation.
But it doesn’t. The tuft immediately above the mount shows separation, but above the emblem the flow looks the same as before. This mount might affect flow into the cooling air openings, though, so I’ll plan on measuring pressures across the cooling package before I make any decision about installing it permanently with lights.