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 and what this looks like in the real world. This article will demonstrate how to set up a tuft testing session, how to get usable results, and how to start analyzing them.
As I wrote before, tuft testing is cheap and easy; all you need is yarn, tape, and a camera.
Your tufts shouldn’t be too short (they won’t show up well on camera) or too long (long, flapping tufts will give you less resolution in the airflow patterns). I like to hold a length of string between my second and third fingers, stretch it across my hand, and then cut it near the thumb:

This gives me pretty consistent 60-70mm tufts—long enough to be visible to a camera some distance away.
Once you have your tufts cut, tape them to your car. You can use long strips of tape with evenly spaced tufts or tape each one individually. Keep the tufts far enough apart that they won’t get tangled with each other, especially in areas where you suspect the flow will be separated and the tufts might flap around a lot.
I start by laying out a line of tufts in one direction:

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:

Finally, I fill in the grid:

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.
On the Road
Enlist the help of a friend to drive your car (thanks Nathan!) or to use a camera while you drive. You don’t need a high-end camera or fancy setup; a cellphone camera that will take pictures in Burst Mode is perfectly adequate. On Samsung phones, slide the shutter button to the right and hold it to activate Burst Mode; the camera will take up to 30 pictures over just a few seconds. While the car drives by, follow it with the camera and you should end up with a series of clear photos showing the tufts. Download them to your computer and scan through them, looking at any changes in the tufts over time (even in areas of attached flow, there will be some movement of the tufts due to turbulence). Choose the most representative photos for further analysis.
You might be interested in getting a snapshot of the flow over your car as it is, but you could also try changing some things to see what happens to the tufts. In this session, I first took photos of the car as I’ve been driving it (modified from stock); then, I blocked my external air curtain ducts, added the fins I’ve tested before, and threw on a bumper bar light mount since I’ve been considering adding some extra lighting before my road trip through the Southwest later this summer.
What can you learn from a tuft test? Lots.
Test #1: Standard configuration
By “standard” I mean the car as it sits—modifications from stock include 40mm lower ride height, larger front wheel spoilers, external air curtain ducts, external engine air intake, mirrors/rear wiper/antenna removed, small lip spoiler, underbody exhaust tunnel closeout panel, and extended rear diffuser panel with vertical strakes.

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.
That said, there are a few curiosities. Notice the tufts on the front splitter—they don’t point back, toward the cooling air opening, but forward and wrap under the car.

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.
Test #2: Blocked air curtain ducts
In standard configuration, the tufts show air flowing into my air curtain ducts as I intended. The rear duct has attached flow over its outside surface, while the front duct has some separation on its outside middle.

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.
Test #3: Rear fins
Just for fun, I threw on the fins I’ve been testing to improve straight-line stability (I didn’t bother with the crossbar brace since this testing was done at low speed—35 mph—and in light winds).

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 the muffler:

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.
Test #4: Bumper bar light mount
Finally, I attached the light mount. The upper part of this mount is a horizontal flat plate that looks like it should massively disrupt flow across the front of the car.

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.
Don’t Guess, Test
Try it yourself, on your own car!


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