Tuft testing! 1998 Ford Escort ZX2 (front hood, windshield and rear glass and lid)

[BabyDiesel]

Originally Posted by MetroMPG

I agree - I think you've got a little bit of separation/recirculation right around the center mounted brake light.

Most of the time the tufts are sitting relatively still, but every once in a while you see one going sideways & backward.

To me, it looks pretty good. Recirculation/locked vortex is much more evident on the back window of an '07 Civic or '06 Corolla that I tuft tested:

Civic:



Corolla:

I believe the reason separation is less evident on my car than your Corolla and Civic is because I was using too long of tufts. The ones are the vortex were 6-8"! Your tufts are much shorter and lighter, which would get "sucked up" and around by the vortex easier, making it more evident.

There's an excellent excuse to do more testing

Originally Posted by MetroMPG

Did you make the video of the trunk lid? The few seconds you can see it in your vid, it looked OK. Turbulent, but attached flow.

My brother was not able to focus on just the lid. There is another video that I have not uploaded to YouTube yet that shows it a bit better IIRC. I think that the vortex *might* extend partially down the lid, but once again, tufts are too long and video is not the best to make conclusive assertions. So attached but turbulent flow. That is better than I would have expected! I won't be satisfied until it is all attached though

Retest time

Originally Posted by 2000mc

Without seeing major issues, I think it's hard to read into too much

I though arcosine's tuff test on his spoiler-less sc1 looked pretty hard to beat - http://ecomodder.com/forum/showthrea...tml#post325746

...but my scangauge says my modded spoiler beats no spoiler, even though my spoiler is far from optimal with sawed off ends - http://ecomodder.com/forum/showthrea...tml#post467518

I agree. It is far from optimal, but we can't make concrete statements yet.

Thank you for the links! Arcosine's tufts look very well attached on the entire vehicle. Which is crazy, his car is not that different from my own.

Your modded spoiler makes me want to grab a JY spoiler off a ZX2 and give this a try You should do tuft testing and see the effects between no spoiler vs. modded spoiler. I'm interested the see how the air is effected

Originally Posted by cRiPpLe_rOoStEr

Sounds like a good excuse to make experiences with a Kammback.

I think this will be my next project, before I go full Basjoos.

Originally Posted by aerohead

here's an image hucho shared in his text.

YES! This is nearly exactly what I have seen!

How does remedy these vortices, without doing a boat tail?

Originally Posted by freebeard

i buy into babydiesel's description more than hucho's. I think the vortexes flatten and spread.

Hucho's diagram is a generalization of what happens IMO. Of course, the vortices will be different for every car due to different angles, pillar design, etc.

I'd take my words with a grain of salt. All I have seen is while driving down rainy roads and looking back to make observations, and doing some "pressure washer simulation" in the backyard

[freebeard]

How does remedy these vortices, without doing a boat tail?

This is just a working hypothesis, but a longitudinal fence like NASCAR uses to kill lift when the car goes sideways, grown and rolled or canted in an outward direction opposite the tendency of the vortex to roll inward. As such:


http://blog.caranddriver.com/lemons-detroit-day-1-speedycop-civic-metsho-tempo-dmc-12-leading-classes/

[aerohead]

Vorticity is caused by pressure differentials at flow intersections.
The lowest pressures are on the roof.
If the rear slope is too steep,the flow will want to separate do to too fast a pressure rise.
At separation,everything downstream takes on the low pressure of the separation line.
Higher pressure in the side flow 'see's' the low pressure on the roof and wants to flow there.
Where the two streams comingle,they wrap up into swirling flow as the high pressure side air races into the low pressure top air.
Generous edge rounding helps mitigate vorticity if contours are gentle but can't prevent them if the roof is too steep.
Only lofting the roofline up to the streamline path can kill the vorticity.
Like Kamm and Koenig-Fachsenfeld did.

[2000mc]

Originally Posted by BabyDiesel

Your modded spoiler makes me want to grab a JY spoiler off a ZX2 and give this a try You should do tuft testing and see the effects between no spoiler vs. modded spoiler. I'm interested the see how the air is effected

The spoiler also raises and extends my deck lid, which ends with a much smaller radius than the rounded naked trunk lid. So whether or not I made a recirculating bubble on top of the trunk, the greater effect could be from how the spoiler effects the bubble behind the car. The effect behind the car might be positive enough that if I produced a negative effect on top of the trunk, the net result is still positive... I figure it could be very potentially misleading without tufting the whole back of the car and filming from a chase vehicle, which we can do the next time you're in Kansas

[BabyDiesel]

Originally Posted by freebeard

This is just a working hypothesis, but a longitudinal fence like NASCAR uses to kill lift when the car goes sideways, grown and rolled or canted in an outward direction opposite the tendency of the vortex to roll inward. As such:


http://blog.caranddriver.com/lemons-detroit-day-1-speedycop-civic-metsho-tempo-dmc-12-leading-classes/

Not trying to be a jerk, but that appears that it would promote vortices spinning the opposite way Maybe not if they were straight up...

Originally Posted by aerohead

Vorticity is caused by pressure differentials at flow intersections.
The lowest pressures are on the roof.
If the rear slope is too steep,the flow will want to separate do to too fast a pressure rise.
At separation,everything downstream takes on the low pressure of the separation line.
Higher pressure in the side flow 'see's' the low pressure on the roof and wants to flow there.
Where the two streams comingle,they wrap up into swirling flow as the high pressure side air races into the low pressure top air.
Generous edge rounding helps mitigate vorticity if contours are gentle but can't prevent them if the roof is too steep.
Only lofting the roofline up to the streamline path can kill the vorticity.
Like Kamm and Koenig-Fachsenfeld did.

Aerohead, I believe we have talked about this in my ZX2 Aerodynamic thread. We discussed rounding the boat tail gently from the body shape as it went farther from the car. I'll see if I can find it.

Here: http://ecomodder.com/forum/showthrea...tml#post488018

*From a practical standpoint,I think you're looking at using whatever plan taper Ford put in the Escort and blend from there.
*As to the upper radii,you could do as Jaray did in 1922.Take your more rectangular cross-section,and gently morph it into a more organic half-body cross-section as the body proceeds rearward.HVAC sheetmetal shops do this all the time in commercial/industrial ductwork when the dust must enter a chase or wall for part of the run.You can't do it with plumbing due to the pressure balooning the flat walls,but with air,it's a snap!
*The tumblehome can progressively increase as Honda does with their FIT,and the edge radii can also grow incrementally as she goes further rearwards.I worked this into the flying buttresses on the CRX to pretty good effect.

No idea it could have a huge effect on Cd. This seems to be the way to go in Kammback/boat tail design. I wonder if that is one reason Darin's boat tail did not perform as well as we hoped?

Originally Posted by 2000mc

The spoiler also raises and extends my deck lid, which ends with a much smaller radius than the rounded naked trunk lid. So whether or not I made a recirculating bubble on top of the trunk, the greater effect could be from how the spoiler effects the bubble behind the car. The effect behind the car might be positive enough that if I produced a negative effect on top of the trunk, the net result is still positive... I figure it could be very potentially misleading without tufting the whole back of the car and filming from a chase vehicle, which we can do the next time you're in Kansas

I am thinking about going out to Idaho next year to meet with Pgfpro, that is IF I can get a turbo, injectors, tuning software, etc. I'd stop by to do some tuft testing for sure!

Your spoiler resembles a Civic Hybrid spoiler, in a way. It is one of the things they use to lower Cd, so I can believe it improved Cd by ~0.1 on your Saturn and you caught it at the pump.

[freebeard]

Not trying to be a jerk, but that appears that it would promote vortices spinning the opposite way Maybe not if they were straight up...

This after aerohead posted:

Vorticity is caused by pressure differentials at flow intersections.
The lowest pressures are on the roof.
If the rear slope is too steep,the flow will want to separate do to too fast a pressure rise.
At separation,everything downstream takes on the low pressure of the separation line.
Higher pressure in the side flow 'see's' the low pressure on the roof and wants to flow there.
Where the two streams comingle,they wrap up into swirling flow as the high pressure side air races into the low pressure top air.

Allow me to rebut myself. "You jerk, it's opposite to the Bertone BATs which employ a partial half-body shape."


https://en.wikipedia.org/wiki/Alfa_Romeo_BAT

Still, I think separating the top/side flows could be done, similar to a diverter with fences acting under the rear.

[aerohead]

Originally Posted by 2000mc

The spoiler also raises and extends my deck lid, which ends with a much smaller radius than the rounded naked trunk lid. So whether or not I made a recirculating bubble on top of the trunk, the greater effect could be from how the spoiler effects the bubble behind the car. The effect behind the car might be positive enough that if I produced a negative effect on top of the trunk, the net result is still positive... I figure it could be very potentially misleading without tufting the whole back of the car and filming from a chase vehicle, which we can do the next time you're in Kansas

*I think your intuition was dead on!
*The outboard portions of the spoiler were creating a solution to a problem you didn't have.My guess,a styling concession so as not to scare consumers with 'expectations' of what a spoiler ought to look like.
*Cutting off the ends reduced induced lift (drag).
*The center section guarantees reattachment behind the backlight.
*And it locks in the vortex.
*The air flowing over the vortex decelerates as it travels lower and to the rear,building pressure until the end of the spoiler.
*When the flow does separate at the spoilers edge,it's at a much higher pressure which raises the base pressure of the wake,cutting pressure drag,exactly as Hucho predicts.
You did good! Good thinking!