Best material for aerodynamic tufting

[Dave J]

I'm planning on doing some aero testing on my car. What is the best material to use for tufting to show airflow?

My plan for now is to tape yarn to the car. Is that heavy enough? Should I use a heavier string or other material?

Let me know.

TIA.

[marc sproule]

the one and only time i saw someone use it was on glen biren's crossle and he was using lightweight yarn,

'twas in '74 iirc.

i've seen flo-vis or something similar all the other times i've seen people doing that kind of testing

[Dave J]

I don't want the mess of flow-viz...

[SteveG]

Try reaching out to Art Smith, I believe he worked with Glen Biren on the Crossle.

Steve

[924RACR]

I've used ordinary yarn borrowed from my wife's stash... she could probably say what weight it was...

I've also made up flowviz... yeah, a great mess... and only to find out they just use india ink in the tunnel, water-based so easier cleanup...

I think they each have their place...

[kea]

I have done this a number of times over the years and whatever yarn I can get (and shows well in pictures) is what I've used.

[Hardingfv32]

You might steam iron the yarn straight before using.

I like about 2.5" free length from the tape with a 2" spacing. You can make strips of tufts by wrapping the yarn around a cutting board in 2" spacing and placing tape across the rows of yarn. Them cut with a knife or razor blade.

What about interpretation of the results? How much movement is required to show non-laminar flow? I have heard that small movements of the last 1/3 does not mean anything.

Brian

[R. Pare]

last time I did tufting, i bought some bright pink string from Lowes. Still have most of the roll, but string comes in handier around the shop that yarn.

[Offcamber1]

Ummmm, you didn't knit a hat with the leftover, did you??

[Dave J]

Thanks for the replies. I really appreciate all of the input.

[MascettiTVR]

You might steam iron the yarn straight before using.

I like about 2.5" free length from the tape with a 2" spacing. You can make strips of tufts by wrapping the yarn around a cutting board in 2" spacing and placing tape across the rows of yarn. Them cut with a knife or razor blade.

What about interpretation of the results? How much movement is required to show non-laminar flow? I have heard that small movements of the last 1/3 does not mean anything.

Brian

Interpretation of your results, and your understanding of the term "non-laminar flow" are really important. In aerodynamics research, the terms used have very specific, unambiguous definitions so I wonder are you attempting to identify areas of separated flow or turbulent, attached flow (AKA a tripped boundary layer)? If the former, tufts are going to be your easiest option due to ease of application and identification of characteristic reversed flow. If the latter, which would be useful e.g. if you are checking whether a boundary layer trip is effective, you could have a real job on your hands. This is because aerodynamic measurements can be intrusive, that is, the measurement itself affects the result, and the unintrusive detection of laminar-to-turbulent BL transition is no mean feat, also for those working in wind tunnels. Personally, I"m a high speed (transonic) flow guy but I believe the low speed oil flow or china clay/solvent techniques could show you something useful at the higher car speeds.
With tufts, I would recommend trials with a single tape which you can progressively move forwards/backwards until you see clear changes in the tuft behaviour. I'm not convinced a car bristling with tufts like a porcupine can give true results. A waggling tuft, even 1/3 of its length cannot be sat in a laminar BL. Equally, an unmoving dense, stiff tuft could be sat in a turbulent BL and you'd be none the wiser. If the test is done with super-thick tape and/or tuft it may just indicate that the measurement has itself been successful in causing transition to turbulence. Use the thinnest tape you can with the finest, "limpest" tuft you can see/photo/video.
BTW, great site for me to discover and learn from as just an occasional entry level road car sprinter.

[Neil_Roberts]

I prefer used motor oil. It's free, it's easy to apply, it's not too hard to clean up, and it leaves a distinct streak behind so you can examine it in detail afterward. Just dip a fingertip into the oil and place a dot wherever you want one.

An easy way to apply oil is to follow certain FVs.

[cjsmith]

I used wool yarn, in my case I used white yarn to contrast to the black and red body color. The advantage of using wool is if the air is turbulent or detached in a region the wool will unwind and leave evidence of a disturbed regain. I used small pieces of electrical tape installed vertical to hold the tuffs in place. It’s easy to put on, doesn’t affect the boundary layer and comes off easy. If you can find a way to mount a cheap go pro or other cameras away from the body and capture the tuffs in motion while driving the car it Is a big help. I had video of a number of the runs I had in the wind tunnel and being able to go back to the film can be a help if you think of other options you may want to consider in the future.

[nickal]

Have used yarn tufts doing flow work on aircraft. We use about 2" length high contrast yarn. The subject aircraft flies S&L; we close to photo range and the right seater photographs the area being studied. That 2" length is long enough to see well but short enough to have it's own turbulence problems. These tests are typically 100-150 kts IAS.