The **u** velocity profile in figure 6.18 has nearly the same shape as the
velocity difference **U-u** for the impulsively moved plane surface (see
figure 6.14). From this velocity profile, we can calculate the value of the wall
shear stress to be:

where the numerical coefficient is obtained from the numerical solution to the Blasius
equation 6.87. It is customary to define a dimensionless wall shear stress, called
the * skin friction coefficient* , by dividing by the dynamic pressure
of the oncoming flow:

The flat plate skin friction coefficient can be found by substituting equation 6.88 into 6.89:

Engineers may be interested in the total drag force on a flat plate. For a plate
of length **L** in the streamwise direction and width **W** perpendicular to the flow, the drag
would be:

where the factor **2WL** is the total plate area of the upper and lower surfaces exposed to the
flow. The dimensionless form of the drag force,

called the * drag coefficient*
, is the ratio of the drag force to the product of the exposed area
**A** times the dynamic pressure :

For the flat plate, the drag coefficient is calculated by substituting equation 6.91 into 6.92 to obtain:

where is the plate length Reynolds number.

Thu Feb 16 17:47:33 EST 1995