Air friction behavior in spinning objects

[tadietz]

In simulating a rapidly rotating flywheel. I find that Algodoo seems, unfortunately, to use just the diameter of the rotating body as the factor which determines the rate at which air friction slows different rotating objects fixed to the background in the same scene, ignoring the other physics considerations if air friction is turned on. Consequently, I get pretty much the same duration of spin for rotating solid disk flywheels with the same diameter and overall mass as my rim-weighted flywheel, which is an O or ring-shaped object where all mass is in a thin outer circle. Once the motors used to drive these two objects to the same velocity are toggled off, they spin to a stop in exactly the same amount of time with air friction on.

The mass distribution makes no difference, apparently, in this case. With air friction on, as long as the diameters and overall masses are the same, the spin down times are, too. Flywheels do spin longer with their mass concentrated in the rim in the physical world, as the accepted equations for flywheel moment of inertia (I) are: I=mr^2 for ring-shaped and I=1/2mr^2 for solid disk flywheels, and with H = Iw (H = angular momentum and w = angular speed) for identical initial speed and mass flywheels, a ring-shaped flywheel has 2x the H value and would should take longer to shed this energy in spinning down in Algodoo, as is true in the physical world.

Also, intuitively it seems to me that the rotating ring-shaped flywheel would be less effected by wind resistance than a solid flywheel of the same thickness and diameter, unless there are odd turbulence effects with the ring shape that come into play; anyone know about that? I guess that I would suggest that Algodoo incorporate some sort of 'surface area factor' into its air friction calculations rather than just using diameter along with the linear and quadratic factors as is stated in the air friction panel.

Other situations, like the ring rolling down a ramp, seem to work more realistically. A solid disk with uniform mass distribution rolls more quickly downhill than does a ring of the same diameter and mass - which is consistent with what physics predicts. I would still suspect that Algodoo - with air friction on - probably imposes an undue penalty on the ring vs. the much larger surface area disk in these situations, but I am not sure how to prove that without a lot of extra effort that is not pertinent to my simulation.