Airflow past ordinary size objects is only laminar at low speed. As the
airspeed increases, a boundary layer forms. This boundary layer fills the region
behind the object with a turbulent wake.
The figure above shows the airflow past a cylinder as the airspeed and
therefore the Reynolds number increase. Remember
In pictures 1-3 the Reynolds number is
below 2000, in picture 4 it is approximately 10000, and in picture 5 it is above
- The first two pictures show laminar flow at low speed. The air
directly before and behind the cylinder comes to a stop. The pressure is
highest here, but the net force on the cylinder due to pressure differences is
approximately zero. There is no pressure drag
due to pressure differences between the front and back side of the cylinder.
The cylinder experiences only viscous drag due to air friction.
- In the fourth picture a turbulent wake has
formed. The air behind the cylinder no longer slows down and the pressure no
longer rises behind the cylinder. Due to the high pressure in front of the
cylinder it now experiences a large pressure drag. This happens for Reynolds
numbers of approximately 2000 to 100000. The pressure drag is much larger than
the viscous drag. It can decrease the forward component of velocity of an
object moving through a fluid or gas very rapidly. A thrown object can seem to
stop in midair and drop straight to the ground. You can easily observe this by
throwing an air-filled balloon.
- As the airspeed increases and the Reynolds
number becomes larger than 100000, the turbulent region works itself forward as
shown in the fifth picture.
We have what is called a turbulent boundary layer.
The flow lines now separate from the cylinder and follow the turbulent boundary
layer, as shown in the fifth picture. We have something similar to laminar flow
around an object of a different shape. The pressure behind the object rises
again and the pressure drag is drastically reduced.