Flow visualization Techniques - Vortex Generators Part 2

We know that a typical hardtop will see flow separation a few inches below the roofline.  I wanted to see what the Airtab® vortex generators did to help delay separation.  One simple flow visualization technique is tufts testing.  I did the testing on the rear surface of my car.  My wife really didn't want to help, but she did.  Bless her.

Tufts testing is really easy.  Stick on a bunch of strings onto the surfaces that you want to test at.  Now, a lot of people use yarn.  Yarn might be good enough for these purposes but it really isn't ideal because it is too heavy.  A good choice is a lightweight monafilament string, such as fishing line.  The compromise you have to find is visibility, weight, and cross section of the string.  I think a florescent colored fishing line would work well.

I just used very lightweight nylon strings for this experiment.  The strings should be 4-5 inches, not too long and not too close together to tangle themselves.  I used varying lengths from 4-20 inches just to see what it does.

What you are shooting for is turbulent but attached airflow.  This means a gyration of 20-30 degrees or so is acceptable.  What you don't want to see is strings circling, moving irratically, which is an indicator of detached flow.

Hard to see, but this photo is from a higher quality HD video. The video shows the movement of the tufts pretty well.

Previously seen photo of the VGs

Surprising results.  Flow stays attached all the way to the bottom of the hardtop; at about the bottom 2-3 inches it separates a little bit, then reattaches on the trunklid.  It shows attached flow all the way to the end of the spoiler.  Sucess!  Looks like the Airtab® Vortex Generators are doing a great job.

Another interesting thing to note is that air seems to leak out to the side edges, under the spoiler.  It affects the flow on the quarter panels.  Interesting.  No doubt I'm losing out on some lift there.


Some other interesting, lo-cost flow visualization methods.  Pictures tell a thousand words:


This was done by Jeff at Slick Auto using a high velocity air gun.  The model is Guardair 80LJ.  There is a piece of 5' cotton string taped on.  Thanks for the tips Jeff!

Technical Advisors:
Andrew Brilliant, well-known aerodynamicist in the motorsports industry.
Unnamed engineer who worked on developing the Airtab® VGs. www.airtab.com
Jeff @ Slickauto www.slickauto.com