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.


Tuesday, July 10, 2012

Five Reasons for 3D Aerodynamics

Are you in the process of designing a new aircraft, sailboat, or automobile.  3D analysis can play an important part in the design process.  Please consider 5 reasons (not necessarily definitive) to use 3-D aerodynamics early in your design process.

5.  Your design should not look like your analysis & design tool.  If you are too comfortable with 2-D analysis eventually your real-world designs will all resemble airfoil shapes.  A good 3-D design and analysis tool should render and analyze the exciting concepts that resides within your creative mind.  Feel free to throw caution to the wind and reap the rewards of your imagination.

Stallion 3D analysis of a human in the wind.

4.  An airplane is a 3-D object and is usually designed to be stable in flight.  Good stability in flight requires accurate computations of the location of the aircraft neutral point,  the size of the vertical tail (for lateral stability) and effects of the wing's dihedral angles.

Stallion 3D CFD Analyis of the SeaBee Aircraft from NASA VSP Hangar.

3.  Like humans, air prefers to use the "extra" third dimension to get out of the path of a speeding car.  If you wish to determine the down-force on a car wing in ground effect, two-dimensional analysis can provide much insight into the airfoil shape and angle of attack.  However, in 3D the air has a tendency to flow around wings of low aspect ratio that are too close to the ground.

MultiElement Airfoils analysis of a NACA 2412 wing in ground effect.

The figure below shows the same airfoil shape used on a wing of aspect ratio two.  The downforce on this wing will be smaller than its 2-D counterpart.  Use 3D aerodynamics to develop new and exciting methods to force the air through wings shapes that will capture the maximum downforce.

Stallion 3D analysis of Aspect Ratio 2 Wing in Ground Effect

2.  3D aerodynamics can be used to model the structure and planforms of wings that decrease drag (such as wing with winglets), increase downforce (end plates) or enhance the integrity of the structure (joined wings). 

ESwift Aircraft from VSP Hangar Analyzed in Stallion 3D.

1. Several year ago, 3D analysis was costly and difficult. Now, 3D aerodynamics is easy and cost effective.  My laptop computer is fast enough to analyze the 3-D Euler/Navier-Stokes equations (and so is yours).  Take advantage of modern CFD codes such as OpenFoam, Caedium, grid generation software such as Pointwise or  Stallion 3D for a complete aerodynamics conceptual analysis to enhance your creative process.

More information can be found at

How can you take advantage of 3-D analysis?

Thanks for reading.

References:Stallion 3D: