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National Advisory Committee for Aeronautics, Technical Notes - An Investigation of the Phenomenon of Separation in the Air Flow Around Simple Quadric Cylinders

The tests, conducted at the Guggenheim Aeronautic Labora—
tory of Stanford University, to investigate the phenomenon of
separation in the air flow past geometric bodies are described
in this report.

The experimental work consisted of wind—tunnel pressure—
distribution tests and determinations of the line of separation
on one circular and two elliptical cylinders. All three models
had the same perimeter, and were mounted in the tunnel so as to
give two—dimensional flow symmetrical about the major axis.

The speeds of test employed were approximately 40, 57, 73, and
91 feet per second. Theoretical pressure—distributions for the
models were computed for purposes of comparison.

The tests show a recession of the line of separation and
an improvement of the agreement between the experimental and
theoretical pressure—distributions with an increase in either
fineness ratio or velocity. A given increment of velocity pro—
duces a constant recession of the line of separation regardless
of fineness ratio. For each model, irrespective of scale, sep—
aration occurs after adverse pressures act through a constant
distance. This distance, however, increases with fineness ratio.

The most important result of the investigation is the unique
relation which was found to exist between the pressure at the
point of separation and the minimum pressure. The ratio of
these pressures, the pressures being reckoned from stagnation
pressure as a datum, is shown to approximate 91% and is inde—
pendent of change of scale or fineness ratio.

An ideal fluid flowing past a disturbing body divides at
the nose of the body, streams along the sides, with perfect
slip, to the rear where it unites and continues downstream with—
out turbulence or loss of energy to the system. Real fluid mo—
tion, however, separates from the after portion of the disturb—
ing body where a wake is formed, and energy is expended in drag—
ging this region along behind the body. This turbulent area
originates in the boundary layer where, under the influence of
an unknown sequence of events, the flow departs from the solid
generating vortex sheets, which bound the 'dead water" region.
Where the wake is small, viscous forces outside the boundary
layer are negligible (Reference 1) and are disregarded in the
outer region/which the fluid, except for the presence of the
wake, behaves essentially as an ideal fluid.