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Old 06-03-2011, 06:49 PM   #24 (permalink)
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Originally Posted by cfg83 View Post
aerohead -

So at air speed X it could shake apart. Does that also mean that at air speed Y it could perform at it's best? Is there an optimum step shape for every MPH?

At some speed the span would reach its natural frequency and begin to resonate with some displacement amplitude.The vibration could work-harden( if you will ) the material causing it to fracture like a coat hanger bent back and forth for a few cycles,or just explode as I have witnessed with some of my prototypes.
At a higher velocities,the span would reach harmonics of its natural frequency and vibrate at these,as in a piano,going up through all the octaves.
Harley Davidson has critical engine rpms which must be avoided or the bottom end will self-destruct.
Changing the velocity will alter the pitch,and move the frequency out of danger perhaps,although if vibration occurs at 'normal' driving speed,we've got to stiffen the part,or maybe shred the major vortex and blast it into smaller,homogeneous eddies of higher frequency using VGs as we see on mirrors for acoustic tuning.
Aerodynamically,the backward facing steps,if following close to the 'Template' profile,will perform the same at any speed above 20 mph.There shouldn't really be a 'sweet-spot'.
If the curvature is hyper-'template',that's okay 'cause there'll be no major stall in the out flow region.Laminar flow,no turbulence.
If the step curvature is hypo-'Template',the the whole device will be embedded within strong turbulence and must be strong enough to survive the constant buffeting.And it WILL get worse with higher speed as a square of the velocity.
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