Page 149 - Aerodynamics for Engineering Students

132 Aerodynamics for Engineering Students

By the use of Bernoulli's equation, the pressure p acting on the surface of the cylinder
where the velocity is q can be found. If po is the static pressure of the free stream
where the velocity is U then by Bernoulli's equation:

= p + - p(2 u sin el2
1
2
Therefore

(3.45)

Plotting this expression gives a curve as shown on Fig. 3.23. Important points to
note are:
(1) At the stagnation points (0" and 180") the pressure difference (p -PO) is positive
and equal to 1 U2.
(2) At 30" and 150 where sin 8 = 1, (p -PO) is zero, and at these points the local
ZPO
velocity is the same as that of de free stream.
(3) Between 30" and 15O0C, is negative, showing that p is less thanpo.
(4) The pressure distribution is symmetrical about the vertical axis and therefore
there is no drag force. Comparison of this ideal pressure distribution with that
obtained by experiment shows that the actual pressure distribution is similar to
the theoretical value up to about 70" but departs radically from it thereafter.
Furthermore, it can be seen that the pressure coefficient over the rear portion of
the cylinder remains negative. This destroys the symmetry about the vertical axis
and produces a force in the direction of the flow (see Section 1.5.5).

Fig. 3.23