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National Advisory Committee for Aeronautics, Technical Notes - Investigation of Effects of Distributed Surface Roughness on a Turbulent Boundary Layer Over a Body of Revolution at a Mach Number of 2.01

An investigation has been made of the effects of distributed sur-
face roughness, consisting of lathe—tool marks, on the skin friction
of a turbulent boundary layer over a body of revolution at a Mach num—
ber of 2.01. The investigation was made on three ogive-cylinders at
zero angle of attack over a surface-roughness range from 25 to #80
microinches root mean sguare and for a Reynolds number range based on
body length from h x 10 to 30 X 10.

The results indicate that the effects of distributed surface rough-
ness on a turbulent boundary layer at a Mach number of 2.01 are generally
similar to those found at a Mach number of 1.61 and at subsonic speeds.
That is, for a given roughness height, some critical Reynolds number
exists at which the skin friction begins to depart from the classical
turbulent skin-friction law because of the form drag of the individual
roughness particles. The results further indicate that (in the Reynolds
number range of these tests) increasing the Mach number from 1.61 to 2.01
increases the allowable roughness for a turbulent boundary layer by about
to percent. This increase is in good agreement with that predicted on
the basis of a constant ratio of allowable roughness height to laminar-
sublayer thickness or to a constant value of the Reynolds number based
on allowable roughness height, shearing-stress velocity, and local con-
ditions at the surface.

As maximum airplane and missile speeds increase from subsonic to
supersonic and hypersonic regimes, the effects of surface roughness on
boundary—layer skin friction and heat transfer become of greater impor—
tance. Consequently, an investigation (ref. 1) was made in the
Langley #- by h-foot supersonic pressure tunnel to study the effects
of uniformly distributed roughness on the skin friction of a turbulent
boundary layer over a body of revolution at a Mach number of 1.61. The
results of reference 1 indicated that the effects of surface roughness
(for a turbulent boundary layer) at supersonic speeds were generally
the same as those predicted by subsonic-speed theory. The most exten-
sive experimental data available on this subject were Nikuradse’s
incompressible-flow data (ref. 2 or 5).