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I prefer testing 100Hz+ due to the intense vortex shedding that occurs off of the tubes at yaw above 10 degrees.This was what I was wondering about. Your preference makes sense given the typical Strouhal number for shedding cylinders (of bicycle tube size and at bicycle speeds).
Yes -- with vortex freq. between 10 and 100 (for an entire bike/rider system; 10^3 off of various tube sections), we'd of course want to run sampling faster than that. Chris, you're basing this off of cylinder Strouhal numbers around 0.18-0.20 for low Re cylinders, correct?
Note that testing at these speeds is not entirely imperative as the high freq. vortecies do not often generate measurable drag changes due to the mechanics of how the balance measures drag. At yaw, we're generally measuring a tunnel axis drag component and a tunnel axis side force component (each through 1-2 load cells). The computational error in correcting the drag data to body axis is higher than the load changes due to the higher freq. vortecies. And when a rider is on the bike the load measurement is cyclical based upon cadence and body shifting, not activity in vortex streets.
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Also, so everyone knows, the A2 tunnel doesn't do "sweeps" of yaw angles at this point. They move the balance, let the wind settle to a steady state condition, then begin gathering data.Sorry, I used the term "sweep" loosely...I actually meant what you are referring to above. From the pictures, it looks like the entire setup is on an electronic turntable. Is the typically coarse sampling resolution in yaw (usually every 5 deg) due to practical concerns with tunnel time, or some other technical reason?
Right now, the A2 tunnel is not automated. The model is physically moved and positioned for each yaw test. They are looking to put in an actuator to automate this later this year as Dave has mentioned in his post.
Yes, more yaw points would be ideal but time and money are the constraints. To be honest though, for comparison purposes, the extra yaw data just fills in the graph. In most cases you can clearly see the key stall angles for the bikes. When we test, we'll do our general 5 degree points (+ and - yaw) and then check out the interesting areas (usually 7.5 degrees up to 12.5 degrees -- for the case with the Shiv, 12.5-17.5 degrees were really interesting as flow was still attached on many parts of the bike we haven't been able to achieve with other designs).
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The data does not change based upon the direction you change yaw angles (i.e. negative to positive yaw or positive to negative yaw).This is actually pretty interesting. I wonder if this is due to either 1) nothing has fully/catastrophically separated at the max tested yaw, or 2) the yaw sampling resolution is too low (for an airfoil that stalls at eg; 20 deg AoA, I have seen hysteresis confined to above 15deg AoA on the back sweep). If neither the above is happening, then something else interesting is going on...
If you were to automate the sweep and constantly change the "yaw angle of attack" (if you will), then direction definitely matters. You get some pressure bubbles that form, pop, and reform on the downwind side of each tube as your change yaw. I have yet to hear of or see a wind tunnel that does a constant speed yaw sweep on a bike. I've been looking for a test of this for years but haven't been able to do it anywhere. This is how airplane wings and airfoils are tested at tunnels -- why can't we do it with bikes??
But, we have done point by point sweeps at a bunch of tunnels and there generally is hysteresis, sometimes quite large above 10 deg. This is interesting from an academic sense, but studying this to show data for an aero shootout only opens up more questions. I'll be the first to admit to the aerodynamics community that bicycle aero testing has a long way to go, but we are at a point where we can identify repeatable aerodynamic performance coefficients that line up well with real world data. Yaw is the next big frontier to accurately model what happens in the real world and how we can appropriately test. Gusts? Show me a wind tunnel test that can explain how an 808 handles in gusty 40 mph winds and I'll be a very interested listener.
Chris -- you know this stuff clean. We really need to have a talk in person. Sounds like you've been doing this stuff for years. Thanks for the great contributions to these forums.
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Mark Cote
MITAerobike
Specialized Bicycle Components