Consider steady flow of a fluid through a circular pipe attached to a large
tank. The fluid velocity everywhere on the pipe surface is zero because of
the no-slip condition, and the flow is two-dimensional in the entrance region
of the pipe since the velocity changes in both the r- and z-directions. The
velocity profile develops fully and remains unchanged after some distance
from the inlet (about 10 pipe diameters in turbulent flow, and less in laminar
pipe flow, as in Fig. 1–20), and the flow in this region is said to be fully
developed. The fully developed flow in a circular pipe is one-dimensional
since the velocity varies in the radial r-direction but not in the angular u- or
axial z-directions, as shown in Fig. 1–20. That is, the velocity profile is the
same at any axial z-location, and it is symmetric about the axis of the pipe.
Note that the dimensionality of the flow also depends on the choice of
coordinate system and its orientation. The pipe flow discussed, for example,
is one-dimensional in cylindrical coordinates, but two-dimensional in Cartesian
coordinates—illustrating the importance of choosing the most appropriate
coordinate system. Also note that even in this simple flow, the velocity
cannot be uniform across the cross section of the pipe because of the no-slip
condition. However, at a well-rounded entrance to the pipe, the velocity profile
may be approximated as being nearly uniform across the pipe, since the
velocity is nearly constant at all radii except very close to the pipe wall.
A flow may be approximated as two-dimensional when the aspect ratio is
large and the flow does not change appreciably along the longer dimension.
For example, the flow of air over a car antenna can be considered two-dimensional
except near its ends since the antenna’s length is much greater than its
diameter, and the airflow hitting the antenna is fairly uniform