Sunday, November 17, 2013

Computational Fluid Dynamics (CFD) and Handcycle Design

A wind tunnel, I do not own.  So I have been waiting for the day that a Computational Fluid Dynamics (CFD) program would be available at little cost. The idea of a computer application taking the place of a wind tunnel makes sense -- especially for a hobbyist like myself.  AutoDesk Labs is testing a CFD program -- Project Falcon -- and I grabbed a free copy of it as soon as I read about it.  I was able to take my 3D Sketchup model to an STL file and Project Falcon is able to import the STL file. (An STL file is usually used for 3D printing.)  From this, after tweaking a few setting in Project Falcon, Project Falcon was able to calculate the Cd (Coefficient of Drag) of my 3D Sketchup models.  

The lower the Cd the better! The Cd correlates to the wind resistance of the handcycle. The frontal area is multiplied by the Cd.  From the Cd, frontal area, and wind speed, one can determine the Newtons of force and from that the watts required at a set speed in order to overcome air resistance.

F= CdA p [v^2/2]
F = Aerodynamic drag force in Newtons.
p = Air density in kg/m3 (typically 1.225kg in the "standard atmosphere" at sea level)
v = Velocity (metres/second). Let's say 9.33 which is 20.8mph

Since I am hoping for a faster handcycle and since I am getting older but not stronger, my intention is to build a design that significantly reduces drag and thus allows me to move at a higher speed given the same amount of power. Without this software, my option is to design a handcycle, take it out on the road, and see if it is indeed faster.  With Project Falcon, I can tweak a design and check to see if it is indeed has a lower Cd and therefore faster.  

The two-wheel design has a much better (therefore less) drag as compared to the 3-wheel design.  Given my current injuries -- I need stability that the 3-wheel design gives for the next cycling season.  Here is the 3-wheel Delta design that I am hoping reduces air resistance:

An important concept in this design is the shape of the base that migrates into the plane between the rear wheels.BTW, the front fork is from the first bike I designed and built.  

Here is the result of Project Falcon's calculation on my 2-wheeled handcycle design.  The Cd is a mere 0.16:

And here is Project Falcon's CFD result with my newest 3-wheeled handcycle design.  As you can see, the Cd is nearly doubled that of the 2-wheeler.  Of course, the design still has a Cd is that is much less than a racing bicycle (0.88 -- see below).  But my arms are much smaller than most legs -- so I can use any improvement

Here are two pictures from the "smoke" version of Project Falcon's wind tunnel imitation.  One is the representation of a plane that cuts through the shoulder.  The second picture represents a plane through the center of the bike.  As you can see, the design of the rear of the bike around the shoulders, appears pretty good.  The center-plane of the bike gives off a fair amount of turbulence though:

I may increase the size of the headrest behind the helmet in order to see if that improves the Cd.

From the above formula, in order to go about 20 miles per hour with this handcycle (assuming that Project Falcon is correct!), I would need to produce about 45 watts in order to overcome the air resistance.

The frontal area is somewhere around 0.28 square meters:

Cd= 0.32
Area = 0.28 meters square
AirSpeed = 9.33 meter/seconds
AirDensity = 1.225kg/m3

F= CdA p [v^2/2]
F = (0.32 * 0.28) * 1.225 * (9.33 * 9.33) / 2
F = 4.77

Watts required to overcome air resistance = F * AirSpeed
= 4.77 * 9.33
= 45 watts

But all of this makes many assumptions...

Here is an copy of a page showing the Cd's of various forms of cycling: