Why?
The first thing to notice is that the label side of the CD is completely (well almost) smooth and flat. While the read/write side has a notch (ring) near the center hole of the CD (where the plastic is transparent) on an otherwise smooth surface.
I performed a 2-D analysis of the CD airfoil (at the center-line) using Hanley Innovations' MultiElement Airfoils software package. The notch was set to a height of 0.0125 inches and the CD was placed 0.025 and 0.0375 inches off the ground. The speed was set to 10 feet/sec and the Euler code was used to model the flow.
Grid generated automatically using MultiElement Airfoils.
The following is the results for the CD with the read/write side up (Cl = -0.105). The bottom line represents the ground plane:
With the read/write side up. The computed lift coefficient was -0.104 with h=0.025 in
Velocity distribution of flow near the notch (bottom line is the ground plane).
Next, the flow, with the CD read/write side facing down, was computed in MultiElement Airfoils. This time, the lift coefficient was +0.105
With the read/write side down, the lift coefficient was +0.105 with h=0.0375 inches.
Velocity distribution of the flow near the notch.
The positive lift coefficient caused the CD to lift free from the friction on the floor and provided a longer journey. This disc models a WIG (wing in ground effect) or Ekranoplan in this mode.
These results suggest that the notch-ring near the center of the CD is the reason for the wing-in-ground effect behavior.
Do your know of other simple ground effect experiment?
Thanks for reading and best wishes.
Patrick
http://www.hanleyinnovations.com
Isn't there any contact with the ground surface? Doesn't the friction coefficient affect?
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