Page 100 - Mechanical Engineers' Handbook (Volume 4)
P. 100
13 Flow Measurements 89
Figure 37 Typical polar diagram showing lift–drag charac-
teristics for an airfoil of finite span.
13.2 Velocity Measurements
A combined pitot tube (Fig. 38) measures or detects the difference between the total or
stagnation pressure p and the static pressure p. For an incompressible fluid the velocity
0
being measured is V 2(p p)/ . For subsonic gas flow the velocity of a stream at a
0
temperature T and pressure p in
p
2kRT (k 1) / k 1
0
V
k 1 p
and the corresponding Mach number is
Figure 38 Combined pitot tubes: (a) Brabbee’s design; (b) Prandtl’s design—accurate over a greater
range of yaw angles.
