• Version
• 165 Downloads
• 3.16 MB File Size
• 1 File Count
• December 9, 2016 Create Date
• December 9, 2016 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - An Analysis of Base Pressure at Supersonic Velocities and Comparison with Experiment

In the first part of the investigation an analysis is made of base
pressure in an inviscid fluid, both for two—dimensional and axially—
symmetric flow. It is shown that for two—dimensional flow, and also for
the flow over a body of revolution with a cylindrical sting attached to
the base, there are an infinite number of possible solutions satisfying
all necessary boundary conditions at any given free—stream Mach number.
For the particular case of a body having no sting attached only one
solution is possible in an inviscid flaw, but-it corresponds to zero
base drag. Accordingly, it is concluded that a strictly inviscid—fluid
theory cannot be satisfactory for practical applications.

Since the exact inviscid—fluid theory does not adequately describe
the conditions of a real fluid flow, an approximate semi—empirical theory
for base pressure in a viscous fluid is developed in a second part of the
investigation. The semi—empirical theory is based partly on inviscid—
flow calculations, and is restricted to airfoils and bodies without boat—
tailing. In this theory an attempt is made to allow for the effects of
Mach number, Reynolds number, profile shape, and type of bOImdary—Zlayer
flow. The results of some recent experimental measurements of base
pressure in two—dimensional and axially-symmetric flow are presented for
purposes of comparison. Some experimental results also are presented
concerning the support interference effect of a cylindrical sting, and
the interference effect of a. reflected bow wave on measurements of base
pressure in a supersonic Wind tunnel.

The present investigation is concerned with the pressure acting on
the base of an object moving at a supersonic velocity. This problem is
of considerable practical importance since in certain cases the base drag
can amount to as much as two-thirds of the total drag of a body of
revolution, and as much as 80 percent of the total drag of an airfoil.

In the past, numerous measurements of base pressure on bodies of revolu-
tion have been made both in supersonic wind tunnels and in free flight,
but these experimental investigations have had no adequate theory to .
guide them. As a result, the present—day knowledge of base pressure is
very limited and many inconsistencies appear in the existing experimental
data.