Page 54 - Aerodynamics for Engineering Students

Basic concepts and definitions 37

Fig. 1.14 Pressure on a normal flat plate

Parallel flat plate (Fig. 1.15)
In this case, the drag is entirely skin-friction drag. Whatever the distribution of
pressure may be, it can have no rearward component, and therefore the form drag
must be zero.
Circular cylinder (Fig. 1.16)
Figure 1.16 is a sketch of the distribution of pressure round a circular cylinder in
inviscid flow (solid lines) (see Section 3.3.9 below) and in a viscous fluid (dotted
lines). The perfect symmetry in the inviscid case shows that there is no resultant force
on the cylinder. The drastic moditication of the pressure distribution due to viscosity
is apparent, the result being a large form drag. In this case, only some 5% of the drag
is skin-friction drag, the remaining 95% being form drag, although these proportions
depend on the Reynolds number.
Aerofoil or streamlined strut
The pressure distributions for this case are given in Fig. 1.13. The effect of viscosity
on the pressure distribution is much less than for the circular cylinder, and the form
drag is much lower as a result. The percentage of the total drag represented by skin-
friction drag depends on the Reynolds number, the thickness/chord ratio, and
a number of other factors, but between 40% and 80% is fairly typical.

+*--.)
Fig. 1.15 Viscous tractions on a tangential flat plate

- Inviscid
flow
---- Real fluid,
Re >lo6

Fig. 1.16 Pressure on a circular cylinder with its axis normal to the stream (see also Fig. 3.23)