The Physics of Wind Tunnels

At the beginning of Thanksgiving break, I visited my friend at college.  It was a windy day, and as we moved from the quad into a covered underpass between two buildings, we were hit with gusts of wind that almost blew us over!  This made me think of how scientists can use the physical principles behind this (the equation of continuity) to create actual wind tunnels for research.

Wind tunnels can be used to test the aerodynamics of vehicles and airplanes, the effects of environmental factors like temperature and ice buildup, the stability of buildings in response to wind, pressure changes effects, and so much more.  Depending on their purpose, they can create wind speeds that range from subsonic (less than the speed of sound) to hypersonic (5+ times the speed of sound).  The speed of sound itself is 343 m/s (767 mph)!!!  Wind tunnels can either use a closed loop system or be open to the air on the ends.

I decided to look at a specific wind tunnel used for more "everyday" purposes and discovered General Motors' original wind tunnel, built in 1980, in Warren, Michigan.  It is the largest wind tunnel in the world and generates wind speeds up to 138 mph (62 m/s) in order to test its automobiles (this would be an example of only subsonic wind speeds).  The fan that is used to blow air into the system is 13.1 m wide and is powered by a 4500 horsepower DC electric motor.  The tunnel contracts into a space only 5.5 m by 10.4 m.  This contraction of area is what causes the increase in wind speed according to the equation of continuity.  Using this equation, I wanted to figure out what speed the fan would have to generate in order to get to the maximum speed of 62 m/s.

p1A1v1 = p2A2v2 

Densities can cancel because the air does not change.  Therefore we are left with:

(5.5m*10.4m)v1 = (pi*(6.55m^2))(62m/s)

v1 = 26 m/s (58.5 mph)

Therefore, the max speed can be obtained by only generating less than half of it by the fan.  These days, MUCH higher speeds with other wind tunnels can be obtained for varying purposes.  However, if you're interested in trying out GM's, it can be rented for $2,000/hour!

For more information:

http://science.howstuffworks.com/wind-tunnel.htm

http://www.autoblog.com/2010/08/05/gms-wind-tunnel-turns-30/