Wind Tunnel Smoke Testing

[k.h.]

The price of tunnel time is certainly reasonable for sophisticated results.  We are dealing with skin drag comparisons between coatings on pumpkin seed shapes.  Our beloved brethren in the world of NASCAR will buy time to compare differences between paints, waxes, and whatever else they tweak between races.  Better than university settings where poverty stricken students eagerly ask lots of questions and give away your proprietary information to every alumni industrial spy who offers them a free lunch for a chin wag.  Of course, some production vehicles have been given the tunnel treatment and we may dig up that info on which to build.  First, remove side mirrors . . . I'm not a fan of the BMW K1 motorcycle, not a big financial success, but the 100 hp in the brochure gave it a realistic top speed of 150 mph, with significant credit to the aero work the factory did, and ignoring power losses through the shaft drive system.  Fairing's almost big enough for a too-tall like me to duck behind.  Just my $.02.

[joea]

jack, it will be interesting to see how your
changes made from the windtunnel session
work out with respect to actual differences
in real world land speed performance......

tough to know....if any other changes are made over
the interim as well.....

kh...probably more wrong with our last aero effort than
right, alot of things could make it have less drag....our goal
was to be able to ride it as well...therein lies many compromises...

Joe

[PorkPie]

kh...probably more wrong with our last aero effort than
right, alot of things could make it have less drag....our goal
was to be able to ride it as well...therein lies many compromises...

Joe

Joe, thank you... I got a big smile when I read this...

[Blue]

An interesting note: Indy cars and F1 use the vortexes from the mirror fairings to work with the aft wing to make more downforce.  This is a very high drag way of making downforce and I don't recommend it;  it's just one of the few things left open to them in the rules.

Yarn tufts yield far clearer data on the boundary layer than smoke.  While it is interesting to see the smoke flow over the vehicle and dissipate into vortexes and blurry separation at the back of the vehicle, drag for LSR is far more a concern of separation.  It doesn't really matter what the air is doing a foot away from the vehicle, what matters is what the air is doing at the surface that it is acting on.  Every person that has taken up my challenge to put yarn on their vehicle has been shocked at the amount of reversed flow on and under their car.  Fixing it isn't always simple, although some separation is easy to fix.  Seeing it is the first step.

So yes, if you can afford the time, get into the wind tunnel.  With tufts.

And look UNDER the car too.

[HotRodV8]

Blue - -

Thanks for the encouragement about the helpfulness of yarn tuft testing. I have known about yarn tufts method for many years, I thought it was all superseeded now by the new hi-tech windtunnels with all kinds of data printouts.

If my future homemade, two-garage built liner ever makes it to completion, I think I'll do the yarn tuft method. I might even make a home made "windtunnel" for my smallish liner.

I'm game. For the price, it's worth a try.

[SPARKY]

What would work? 
 a 4'x4' "tunnel made of ply wood with a big attic fan mounted in the end to suck air through it?  I have hear that Jack uses leaf blowers-

JUST what would work?

[HotRodV8]

SPARKY - -

You are right, keep it simple with some 4 x 8 plywood and a fan.

If my future liner is say 3' high and about 2' wide the plywood tunnel would work for size. Being from Florida, I have a few friends that have airboats. So the attic fan would be small potatoes. One problem would be since the airboat "pushes" the air, I would prefer to pull the air through as you mentioned. Why, I don't want the turbulence from the prop and wire cage around it. If the air is pulled through, it brings in uniform air flow uninterrupted to give my yarn tufts fresh air flow over my liner body. I might need more plywood to place the airboat in the pull through zone.

I could have small windows in the plywood to take pictures and video at speed. Plus, I thought of a couple of bathroom scales, one under each wheel to view down pressure. I bet I could do it for under $80, not counting the favors to my airboat friends.

I might even put a few yarn tufts on the insides and inner top of the plywood to see if there is any trails out there or a rooster tail?

If it all works out, great. If not, at least I found out something if I can find all that plywood. But if it's good, I might even apply for the "Good Garagekeeping Seal of Approval," 

[SPARKY]

You could use it to board up your windows the next time a HIMicane comes through   

[SPARKY]

Sticking the nose of lakester in there is what I am thinking about---- I need to try to clean up the canopy area 

[Dr Goggles]

alot of things could make it have less drag....our goal
was to be able to ride it as well...therein lies many compromises...

Joe

I'm a Subaru-kicker....but I know that that much is right......there is much time spent on this board arguing about shape and drag.......not many get as far as a refined consideration of drag vs stability.

In our case we maximised our shape, and then compromised in other areas to increase stability, safety and cost effectiveness, others, make their own compromises. Based on results Joe has made a judicious decision on his compromises

[Blue]

Sticking the nose of lakester in there is what I am thinking about---- I need to try to clean up the canopy area 

With all due respect, we need to tuft the areas behind the canopy and tail first.  When we have eliminated all reversed flow, then and only then should we worry about what is going on upstream; i.e. the canopy leading edge.

[interested bystander]

Beware the canopy leading edge.

That's where horseshoe vortices lurk!

[John Burk]

Replica of Wright Brothers tunnel :

http://images.google.com/imgres?imgurl=http://www.wrightflyer.org/WindTunnel/Picts/wright_tunnel.jpg&imgrefurl=http://www.wrightflyer.org/WindTunnel/testing1.html&usg=__VUJ7zA7mSvyj_UJFkxI1DwT0cis=&h=426&w=616&sz=37&hl=en&start=3&um=1&tbnid=7wj-lTQlO1Yf7M:&tbnh=94&tbnw=136&prev=/images%3Fq%3D%2522wright%2Bbrothers%2522%2B%2522wind%2Btunnel%26hl%3Den%26sa%3DX%26um%3D1

[Kato Engineering]

when I built and ran the first new 1985/6 Thunder bird, I hooked up three of my companys giant sweeper vaccumes to one end of the garage and blew the air out the  door. .
we nailed yarn to the  inner wall of garage and viewed various results thru the side window and opposite  kitchen window.

it all can be done if you read the right books.

[Peter Jack]

I can suddenly see small wind tunnels springing up all over ................ not necessarily a bad idea. 

Pete