> Is Chris Krok reading?
Yo!
> I was thinking all the way home about temporarily
> pinning some dowells on the bike at the handlebars, fairing, front fender,
> behind the rider and at the tail light. I could secure yarn to them in 2
> inch increments and take front and side pictures at 25, 35, 45, 55, 65, 75
> mph and see what the Kawi original, Kawi tall and Rifle windshelds look > like with me on it. Make-shift wind tunnel. Is this feasible? Asking you
> because your signature has "wind tunnel" in it...
What you're describing is actually common practice in wind tunnels, and
the pieces of string/yarn are called "tell tales." They are also
commonly used on sailboat sails to watch for detached flow. Mind you,
I'm just building the wind tunnel, I've never done tests myself. But,
we do learn about testing in basic AE training, so here goes: The
easiest thing to do would be to just tape tell-tales around your helmet
and down your sleeves. If the yarns hang limp, you're either in the
windshield's protection zone, or there's separation from something
else. If the strings flow back smoothly, you're in the air, but it's
"clean" (can be turbulent, but the main flow is steady). Finally, if
the strings are swinging around wildly, you're in the buffet zone. The
only problem with attaching the yarns to dowels is that any circular
object creates shedding vortices (swirling regions of flow), so that
might upset your test. If you can use pieces with a more rectangular
cross-section, that will greatly reduce the effect of the standoff.
(Also, the amplitude of oscillation of the strings due to the dowels
should be pretty small, on the order of the dowel diameter. Oscillation
from windshield buffet will be large, on the order of the windshield
width, so that may be enough to differentiate the two. To check, mount
a dowel in the clean air out in front of the bike on a standoff or
something, and see how the yarn reacts there to get a baseline.)
If you put the tell tales too far back on your arms or helmet, you'll
pick up the separated flow there, too, so watch out for that. Actually,
if you want to get snazzy, try to mount some kind of sting coming off
the front of the bike, and pump smoke through it. then, you'll get full
flow visualization. You can't do it from a vehicle in front of you,
because it will screw up the flow.
Finally, since this is really a dynamic phenomenon, it would be best if
you could have a friend riding chase, with a passenger carrying a
camcorder. Do it on a calm day on open interstate, early on a Sunday
morning or something when few people are out. Make sure the chase bike
is far enough away from you to avoid affecting the flow. We actually
did some tests in the old 10' wind tunnel on a Honda
(
http://www.galcit.caltech.edu/~stingray/awt/mtrcycle.jpg)... the guy I
work with is the tunnel operating engineer, and he's been running our
tunnels for years. I'll ask him if there's any more documentation on
the bike test. By the setup, it looks like they were doing drag
measurements, rather than flow viz.
Things are kinda busy today, but drop me a line if you have any more
questions, and I can post more later in the week. There's also a book
you could check, "Low-Speed Wind Tunnel Testing," by Pope and Rae. If I
get a chance, I'll take another look at it myself.
D.J. (Re: jet bike):
> If you need a test pilot/crash dummy give me a call. I'd do it for
> free. Hell, I might even pay YOU! My job just doesn't excite me
> anymore.
Hey, if I'm building it, I'm riding it!
(My construction creedo
also applies here: "I stand behind and ON everything I build." So, I'd
want to test run it before risking damage to someone else.) But, y'all
can print and cut out the cupon below:
***************************************
* *
* ADMIT ONE *
* This cupon good for one free ride *
* on Chris' jet bike, if he ever *
* manages to build one. Good for *
* KLR listers only (past and/or *
* present). Cupon expires when *
* Chris does. *
* P.S. Don't crash it! *
* *
***************************************
--
Dr. J. Christopher Krok Project Engineer, Adaptive Wall Wind Tunnel
Graduate Aeronautical Laboratories, California Institute of Technology
MS 205-45 Phone: 626.395.4794
Pasadena, CA 91125 Fax: 626.449.2677
