"flutter" is bad, and why exactly?

[j-c-c]

My question centers here on the "why".
This topic was touched on in the recent Bonneville thread, and I felt best not to clutter that thread up, and sorry if this has been addressed elsewhere, unknown to me.

It seems bad, it looks bad, and "common sense" seems to indicate its absorbing energy, bending the panel that is fluttering, converting some portion of the energy to heat, I suspect.

But the question here is, is there ANY aero benefit (less overall drag?)by allowing the air to seek a path of lower resistance as the fluttering panel reacts to the varying(/) pressure waves, or is just increasing drag, turbulence, etc?

On a slightly related topic, it has always been the goal in high powered speaker cabinet design, to build cabinets as vibration free as practically possible for the situation, because any cabinet vibration has tonal artifacts, and more pertinent to this thread, wastes acoustic energy converting into mechanical energy and the resultant heating.

So is it a fair conclusion that when ever one sees random panel fluctuations in vehicles due to aero loads, the design is lacking in some degree?

[freebeard]

How is the heat being generated? Pressure fluctuations. The heat produced is likely minimal compared to the clutching fingers of turbulent air.

[ksa8907]

First thing that comes to mind is a flag. How much drag is created by those ripples in a flag. Maybe not apples to apples, but it seems a flag being towed by a car at 30mph is similar to a weak body panel at 300mph, no?

[arcosine]

Flutter will cause a plane to crash out of control. It is an oscillation resonance excited by the energy in the air flow, If the cop ( center of pressure) and CG center of gravity do not coincide, if you look closely at the planes control surfaces, you will see weights ahead of the hinge. A flag waving id the wind is flutter.

https://en.wikipedia.org/wiki/Aeroelasticity

[Piotrsko]

It is my understanding that you generally want CG in front of COP for a stabilizing effect. If the converse were true, the surface mass weights would be attached to the rear not the front.

[j-c-c]

Originally Posted by freebeard

How is the heat being generated? Pressure fluctuations. The heat produced is likely minimal compared to the clutching fingers of turbulent air.

My thinking is bending any material, requires energy, and that generates internal heating of the material. Is it likely minor, yes, and I thought obvious, which is why my original question has more of an aero leaning, so does the "flutter" induce additional drag/turbulence/flow separation/etc or is the observed panel flutter simply the result of the above?

This question was first considered by me decades back when Nascar had roof mounted live action cameras and it was apparent the severely lightened hoods would deflect/flutter grossly at speed or when it the wake of another car, and wasn't sure if it mattered little, or they felt it was of no real consequence. It seemed the Bonneville car thread had similar issues.

[freebeard]

But what is causing the material heating? Viscosity transferring energy from the turbulence in the unseparated flow. The turbulence itself robs energy that flows away in the [slightly] heated air.

Parasitic drag is drag that acts on an object when the object is moving through a fluid. In the case of aerodynamic drag, the fluid is the atmosphere. Parasitic drag is a combination of form drag and skin friction drag.[1] Parasitic drag does not result from the generation of lift on the object, and hence it is considered parasitic.

en.wikipedia.org/wiki/Parasitic_drag

[aerohead]

* flutter is cyclic deformation
* cyclic deformation can lead to 'work-hardening'
* work-hardening can lead to embrittlement of the panel
* embrittlement can lead to fracture
* fracture leads to panel failure
* Panel failure alters pressure distribution in the vicinity of the failed panel
* the altered pressure distribution can affect aerodynamic stability.
* Flutter-induced aerodynamic instability has killed a number of people. Aircraft and record cars.

[freebeard]

What could possibly go wrong?


On the return pass, at 427 mph, the lack of good body mounting, and not enough clearance at speed with air pressure, centrifigal force, and kinematics, caused tire deflection which allowed a panel to rub the tire. Which killed the left rear tire.

[kach22i]

Originally Posted by freebeard

How is the heat being generated? Pressure fluctuations. The heat produced is likely minimal compared to the clutching fingers of turbulent air.

In stereo-speak "heat" is the mysterious energy created by vibration and resonance when absorbed into a structure (rack/stand/enclosure).

Interesting incident on the Flashpoint car, thanks for posting.

I think buffeting and flutter is energy at random vectors, and we want all those energies and vectors parallel to the direction our body is moving for maximum efficiency.

Analogy, if I may:

Walking in a crowd with people constantly knocking into you is harder and takes more energy than walking a path unencumbered by rude idiots.