Wind Tunnel Smoke Testing

[A2WindTunnel]

Another Smoke video of a Bagger getting ready for Bonneville.  The customer was kind enough to let us use some video footage from the test. 

http://www.youtube.com/watch?v=G7eSiIOQ02g

Dave

[fredvance]

Thanks Dave, always interested in the bike tests.He needs to do something about those flapping gloves. Also it looked like he had the vents open on his helmet and it didn't look nice where the air was exiting the helmet.

  Fred

[1212FBGS]

id like to see a vid of smoke around the riders legs and if the air had a chance to attach and stableize on the bags.... my guess the bags are to short to be effective
kent

[V8Pinto]

I have never understood why helmet companies put that duckbill on the back.  Ok I take that back, it is probably there to create a low pressure and help pull air through the vents.  Now as to the aero aspect, how can that possibly help with the dirty air behind the helmet? 

[hotrod]

I have never understood why helmet companies put that duckbill on the back.  Ok I take that back, it is probably there to create a low pressure and help pull air through the vents.  Now as to the aero aspect, how can that possibly help with the dirty air behind the helmet? 

It is to reduce buffeting as I recall, but not for a motorcycle rider who is hunkered over the gas tank, but someone who is more upright. The ones I have seen are intended for use in open cockpit race cars --- see the GTX helmet in the link below.

http://www.cscracing.com/helmets.html

Larry

[1212FBGS]

actually... as air travels over and around a helmet it picks up speed and pressure from compressing air... this creates lift... just as seen in the back of a car, a spoiler allows the air to snap off the back and not to stumble and curl down behind it, a trip lip on the back of a helmet spoils (spoiler) the airflow and reduces lift.... the helmet is way to short of an aerodynamic device to attach and "fill" the disturbed air it caused... the helmet is usually not the worst aero component on a bike.... the fairings on that Aprilia are horrible
Kent

[V8Pinto]

the duckbill on my helmet is roughly at the 2 o'clock position when viewed from the side with the helmet on and looking straight ahead.  When I am making a pass and tucked in, that duckbill is straight up at 12 o'clock and it is higher than the windscreen, full on in the wind.

Following Ken's example of a spoiler on a car, shouldn't the duckbill be at the 2 or 3 o'clock position when I'm tucked in and going down the track?  That would be more "rear decklid" like.  The position the thing is in now is like if you were driving down the road and stuck your hand straight up out the sunroof.

[A2WindTunnel]

id like to see a vid of smoke around the riders legs and if the air had a chance to attach and stableize on the bags.... my guess the bags are to short to be effective
kent

Kent, the rider disturbed the air pretty bad so it never really reattached behind him.  I don't have any video but some pictures (not really what you are asking to see) but in the region to kind of give you an idea.

[Blue]

Tufts, tufts, tufts. 

Smoke only works on the front half of flow, while the majority of drag is on the back half.

Don't ever take my word on what works; tuft it and see for yourself.

[A2WindTunnel]

And for those who tuft, or want to tuft check out the picture below.  This is a good example of how to tuft.  Short (0.75"-1.5" lengths) and use plenty of them to see the entire picture.  People that only put a few strings on here and there are not seeing the picture.  A tuft board is a good way to make them quick and have consistant spacing.  I will try to get a picture up of a tuft board up when I get some time.

[fredvance]

Isn't that one of Larry Forstalls Bikes?

[A2WindTunnel]

Isn't that one of Larry Forstalls Bikes?

Yes it is.

[1212FBGS]

Dave
that's what i've been telling people for 25 years... the first 1/3 of the fairing is the only effective control of air... After the air snaps off of the trailing edge of the fairing and rider, the back tail is far to short to reattach and stabilize any air... even the +10 allowed in SCTA rules still isn't enough.. if they gave us 3' it might but that would be 1 heck of a ride...

[racer x]

Do you have any videos of tuft tests like that?
  Are the ares that have the tufts turned down bad OR is it good that a line of them are turned down. I want to understand what I am looking at.

[hotrod]

It looks to me like the air speed in the tunnel is not sufficient to fully extend the tufts on the side of the bike.
I have seen that on my car at air speeds of around 20 mph. When air speeds are high enough the tuft follows the local flow direction.

If the flow is attached (good smooth airflow over the surface) the tuft will be pinned to the surface and arrow straight. If there is turbulent flow but still mostly attached the tuft will flutter over about a 15 degree arc centered on the average direction of air flow.

I set up my camera on rapid fire mode and shoot 5 - 10 shots in rapid succession so I can see the range of motion of the tufts. If you pick the right shutter speed so the fluttering tuft blurs a bit it is easy to see the pie shaped wedge it is fluttering in.

If the flow is totally detached and turbulent the tufts can point in almost any direction even back toward the direction of the air flow due to recirculation as the air tries to fill a low pressure area.

At low air speeds the tufts on the sides of the vehicle tend to droop below the true air flow direction and if you look at that bike all the tufts on the vertical surfaces are drooping downward.

I would suspect that picture was taken at low air speeds so as to demonstrate the method without giving away much useful info on the true airflow at speed.


See my posts on tuft testing on the other aero thread about aero stability

http://www.landracing.com/forum/index.php/topic,8252.210.html


Larry