Wool Tuft Testing - MkIV Jetta

[trebuchet03]

Originally Posted by omgwtfbyobbq

Can't we get away w/ it for high drag/low pressure areas like behind a car? I mean, lets say I'm Ernie and I do tuft tests on my Beetle in order to see if there's a difference via the spoiler. Wouldn't less eddies as indicated by the tuft imply there's a decent chance of a drop in Cd? Or for that matter, if the tufts eddy at one speed but not another, wouldn't this imply that the flow hasn't detached at the point and some speed but may detach higher up at some higher speed, leading to greater drag?

I feel it should be like load wrt efficiency in an engine, but I suppose I'm just grossly oversimplifying it.

Lets say it this way.... It's not that there's a change in wake due to a change in cD. It's the other way around - there's a change in cD due to a change in wake. I know it sounds almost exactly the same - I'm just kinda anal about little details like that

But yes, I intuitively agree... See less wild tufts - there's a greater chance for more favorable aero.... With respect to the semi, I think it's a combination of perturbed flow AND slower relative velocity (think corridor effect) That's why I'd like to put an anemometer on my car

[Wyldesoul]

You may not want wild tufts blowing everywhere on your car, but you want all the tufts to get a nice stream of air. The lack of motion on the back window tufts indicates flow separation from the back window, which is undesirable. I'd recommend doing something along the lines of vortex generators on the rear of your roof to pull the flow back down onto the rear window.

[trebuchet03]

Originally Posted by Biffmeistro

You may not want wild tufts blowing everywhere on your car, but you want all the tufts to get a nice stream of air. The lack of motion on the back window tufts indicates flow separation from the back window, which is undesirable. I'd recommend doing something along the lines of vortex generators on the rear of your roof to pull the flow back down onto the rear window.

Lack of motion? That's not a problem, there's too much motion as stated in my original posts - so much so that the tufts do complete 360's in certain places...

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.

I do agree that some form of flow control could be beneficial - I have a non VG concept solution that I haven't seen applied to cars before that I'll be tinkering with within the next month