Saturday, July 28, 2012

Ground effect experiment: Try this ...

Slide an old or unwanted CD across a desk, table or smooth floor. With the shiny side down (read/write side) the CD will slide a long distance in ground effect.  With the shiny side up, the CD will tend to stick to the surface.


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).

The negative lift coefficient or down-force caused the CD to stick to the surface.  It will make a great race-car in this mode.

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.


1 comment:

  1. Isn't there any contact with the ground surface? Doesn't the friction coefficient affect?