Wool Tuft Testing - MkIV Jetta

[trebuchet03]

So I had a 200 mile drive today (which is why I didn't do the one tank of gas challenge contest thinger ), and before I left I added some bits of yarn.

For those wondering how to do it..... Here's some tips

* Pick a tuft color for good contrast (black and yellow in my case).
* Try to follow some sort of grid for placement with lines following what you think the air will do.
* Try to avoid tangles. If you expect an area will cause the strands to whip around, make sure there's enough clearance between each strand to prevent entanglement


Front:

I added a few more after this picture - and a couple long ones near the hood trailing edge to watch the hood<-->glass flow interaction/transition.

Rear:

Adjusted Rear:


45mph Hood:

Nice and straight.
45mph Glass:

Also nice and straight.
More coming in a flash, including results from drafting a semi o.0

[trebuchet03]

Rear Glass - 65mph - No Draft

Another Angle


The stills don't tell the entire picture... Some of those tufts did complete circles - wildly. While in the turbulant flow of a semi, this changed dramatically (see next post).

It looks like flow separation occurs rather quickly, something more likely to happen on a sedan with a steeper glass angle compared to a long sloping hatch lid.

Front Glass - 65mph - no Draft


Hood - 65mph - no Draft

Notice the long tufts near the trailing edge of the hood get sucked under - there was no chance of it getting pushed up onto the glass. Room for improvement methinks

[ChrstphrR]

Since I've a Mk IV like you, of course I'm interested in the results -- thanks for the pictures!

Originally Posted by trebuchet03

Rear Glass - 65mph - No Draft
Notice the long tufts near the trailing edge of the hood get sucked under - there was no chance of it getting pushed up onto the glass. Room for improvement methinks

When I was waxing my Jetta a few weeks ago, in preparation for winter (hey, it's Canada, not Florida here!), I was thinking about the big gap between the top of the hood, and the cavity where the wipers reside.

It'd be a nice spot for a fairing, that still allows for the wipers to function.

I don't want to hijack the thread here, but I did want to throw out that idea, since you were pointing to that very spot on your tuft-testing.

[trebuchet03]

I have video... But it's going to take some time to get it online...

Hood - 65mph - behind a semi (104" wide)


A single photo doesn't say what's happening. The video shows that the string stay straight (they don't curl) but go through pulses of left and right motions. Large eddies coming off the rear of the semi truck making it easier for my car to cut through the air


Glass - 65mph -behind a semi (104" wide)

The same applies here.

Rear Glass - 65mph - behind a semi (104" wide)


Here's where the very interesting stuff happens. Not drafting anything, tufts wildly go in circles - occasionally completely detaching for a few moments. While behind the semi, not 1 tuft did a 360. But there was some tuft curling - indicating that the flow was either inconsistent and/or weak (likely both - especially the former). To me, this means the flow is sticking nearby.

Tuft conditions post experiment:


Rear tufts, post test... They are quite frayed - indicated wild changes. The fronts and some of the rear (near the sides and top) were completely fine

[trebuchet03]

This space for rent... Just in case I feel I need to upload more pictures for someone

[ajohnmeyer]

Nice shots so far. So have you come up with any ways to use all of the subjective "data" that you collected during your trip?

[trebuchet03]

Originally Posted by ajohnmeyer

Nice shots so far. So have you come up with any ways to use all of the subjective "data" that you collected during your trip?

Extra emphasis on subjective

Actually, ChrstphrR took the words from my mouth

I was thinking about the big gap between the top of the hood, and the cavity where the wipers reside.

It'd be a nice spot for a fairing, that still allows for the wipers to function.

Which is actually, if I recall correctly, a key point of an old Autospeed article.

Additionally, I want to do some research on methods to trip flow into turbulence.

I don't want to hijack the thread here, but I did want to throw out that idea, since you were pointing to that very spot on your tuft-testing.

Not at all It's perfectly on topic

[ChrstphrR]

Originally Posted by trebuchet03

Which is actually, if I recall correctly, a key point of an old Autospeed article.

Additionally, I want to do some research on methods to trip flow into turbulence.

Well, digging about when you mentioned it... I think this is likely the Autospeed article you had in mind, no?

"Silencing Wiper Wind Noise" - Autospeed, Feb. 26, 2005
http://www.autospeed.com/cms/A_2445/article.html

It looks as though the fellow that did this article managed to quiet down wind noise, but didn't get any appreciable change in fuel economy.

Mind you, the modification he did is quite a bit smaller/shorter than I was envisioning while waxing my car -- the Kamm effect strikes again, I think.
It's also a stealthy change -- the prototype's noticable, but the finished deflector looks relatively stock in appearance.

Little elegant solutions like this make me regret changing away from Mech. Eng. :P

trebuchet03's post here, and Julian Edgar's article are both giving me the urge to go out and buy a ball of wool and some masking tape.

[ajohnmeyer]

I have been thinking about the same thing for my Civic. The obvious solution (at least to me) is a cowl-induction style hood, as one would typically see on a camaro or other such muscle car. Unfortunately, this style doesn't find it's way into the "tuner" (read economy) car market. This is about as good as it gets....IF you have a 6th gen Civic.




looks like you Jetta folks have the same long odds on cowl hoods. Though there seems to be some hope on the adhesive-cowl front.

[2TonJellyBean]

treb, that's just wild seeing those pix!!!

Thanks for posting!