Prius Aerodynamics tuft test vids (no rain on rear glass = flow separation?)

[MetroMPG]

The tufts on your rear glass indicate attached flow. They're lying flat and relatively still. That's exactly what you want to see.

Separated flow is characterized by reversal of flow direction and/or tufts lifting clear off the surface while swirling randomly / violently.

For a clear example of flow separation revealed by tuft testing, watch the bottom row of tufts on the rear glass of this Toyota Corolla at around the 0:50 to 1:30 point in the video (the part of the video where there are NO vortex generators on the roof).

www.youtube.com/watch?v=We8LWkqjdX4

It is also possible to see a fair amount of turbulence, but still have attached flow (tufts move around with some side to side motion, but still continue to point in the expected direction of flow).

[ryannoe]

Separated flow is characterized by reversal of flow direction and tufts lifting clear off the surface while swirling randomly / violently.

That's reversed flow. You can have separated flow without it reversing... but I'm not the leading expert in flow separation... I only have a bachelors in the subject. I'll consult my partner (Master's on Vortex generation)


-Ryan

[MetroMPG]

Willing to be educated here...

Are you interpreting from the video that the "still" tufts near the center of the glass are laying in a separated region of zero velocity/dead air that stretches all the way down the glass?

[AeroModder]

My guess would be that the boundary layer is thick enough to not move the tufts much at all, but not quite enough to cause separation with reversal. For example, if you look at the smoke testing for the Insight, you'll see the gap between the surface and the smoke widen.

There's also the spoiler to consider. It's creating an area of slightly increased pressure around the bottom of the window, but it's not enough of an angle to cause a locked vortex. I'd be interested to see a tuft test without it in place, if that was possible.

[ryannoe]

Are you interpreting from the video that the "still" tufts near the center of the glass are laying in a separated region of zero velocity/dead air that stretches all the way down the glass?

Yes, that is what initial results appear to show.

I have additional footage with the camera mounted to the vehicle to show this. However, the goal at the time was to characterize the C-pillar vortex and which was not successful. I'll update the videos once I leave work so you guys can see the stagnation of the tufts.

AeroModder - I can't see the video at the moment.


-Ryan

[ryannoe]

Happy New Year!

I completed testing on a 2010 Civic for comparison to the 2012 Prius.

The airflow acted completely differently.

I know the camera angles are not ideal for a good comparison for you guys, but maybe you can make do.

2012 Prius Rear Window Tuft

2010 Civic Rear Window Tuft

Let me know what you guys think...

-Ryan

[MetroMPG]

Happy new year to you too. And thanks for posting all these videos - very instructive!

Re the non-fluttering tufts on the Prius glass and the "dead air" hypothesis...

Yes, that is what initial results appear to show.

I still think what we're looking at is the well-designed Prius shape, leading to "clean" flow with little turbulence right to the end of the car (on the top, anyway). I'd bet a dollar if there were some way to "release" those tufts from the glass at speed, they'd be instantly swept off.

How could this be tested? One of our members recently drove his boat-tailed Ford Probe hatchback after it got covered by a dump of snow and found some interesting patterns (note: car has a steeper hatch angle and more pronouced roof/hatch transition):

Before:



After a 100 mph run:



From: http://ecomodder.com/forum/showthrea...tml#post346077

Lucky for you, that particular test approach isn't an option! It's food for thought though.

The Civic rear glass shows what we'd expect to see: the boundary layer getting progressively more turbulent (presumably thicker) as flow descends the glass, with separation imminent at the base (but not established as a sustained trapped separation "bubble" as we saw at the base of the steeper glass on the Corolla). Some of the Civic's bottom window tufts flick back ~90 degrees occasionally indicating some reverse flow.

One last thought: testing for vortex formation off the C-pillar, you might try attaching much longer tufts. If placed in the right spot, they'd have a better chance of being lifted clear off the vehicle and drawn away by the vortex.

[ryannoe]

I was hoping to complete more testing today but its raining... If I get a break, I'm going to rush out and gather data. I'd like to think you are right and that it is attached but initial observations led both myself and my friend to believe it isn't.

[ryannoe]

Originally Posted by ryannoe

I was hoping to complete more testing today but its raining... If I get a break, I'm going to rush out and gather data. I'd like to think you are right and that it is attached but initial observations led both myself and my friend to believe it isn't.

No testing with video or yarn was completed today. I did drive the Prius in the rain and during stop lights, rain gathered on the back glass. After accelerating to 60 mph (power mode because I didn't want them to collect into jumbo drops and let gravity do all of the work) the droplets were unscathed and BARELY pushing their way to the rear. After my 6 mile run, there were still roughly half of the droplets attached to the window. I shouldn't say this, but I was pretty much watching those droplets the entire drive

You guys had me doubting my initial thoughts of the separated flow, but now I'm more confident that the flow has separated. Hope to have something for you soon.

-Ryan

[F8L]

I'm really curious to see how all of this pans out, particularly anything related to the wheels and the high offset (sunken in) effect. So many Prius owners opt for wider wheels with a lower offset. I am concerned about the effect the increased width and decreased offset will have on aerodynamics. If you push the wheel out so it is flush with the body line is this going to increase turbulence?