As the discussion in Sect. 2.1.1, and Eq. (2.A.12) show, in order for a
rotating fluid element to maintain its equilibrium (static position in the rdirection),
the pressure on its surface at higher r must exceed that on its
surface a lower r. Thus the static pressure must increase monotonically with
increasing radius. This, in fact, is borne out by experiment—a classic example
of which is the data of Ter Linden (1953), a sample of which is presented in
Fig. 3.1.2. Here the lower curves contained within each set of curves represents
the variation in static pressure, p, with radial position; the upper curves, the
total pressure, p+ (1/2)ρv2 (static plus dynamic). Comparing with Eq. (2.1.3)
and realizing, as before, that the second term in Bernoulli’s trinomial is small,
we see from the profiles of total pressure in Fig. 3.1.2 that Bernoulli’s trinomial
is almost constant in the outer, nearly loss-free part of the vortex, while it
decreases significantly in the center. This is as we would have expected.