Semi-trucks: what a drag! (Senior Division).
Semi--trucks consume an enormous amount of fuel in overcoming
aerodynamic drag on the rear end of its trailer. The drag on a
semi-truck's trailer accounts for about one-half of the total drag
force at 55-65 mph. With the increase in fuel cost, the reduction of
drag has become a key factor in keeping semi-trucks profitable. Previous
aerodynamic changes on semi-trucks have primarily involved improvements
to the front profile. Shear stress is dependent on the length of an
object, so if an object is really short then the shear stress is low.
The second is pressure drag, which is the part due directly to the shape
of an object. Pressure drag consists of the direction, magnitude, and
orientation of an object. The philosophy behind reducing drag is to make
an object long enough to reduce the pressure drag, but not so long that
a great deal of shear stress is created. The experiment utilizes a wind
tunnel by taking various pressure readings and condensing the data into
a readable graph by measuring the total, static, and dynamic pressure
upstream and downstream of the object. While the amount of drag shown is
not the actual amount of drag due to the Reylond number, the graphs are
comparable to the baseline graph, which shows the difference in drag
from each object.
Robert Kamber, Laramie High School, Wyoming