Contributions on the Mechanics of Boundary-Layer Transition Page: 1 of 12
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REPORT 1289
CONTRIBUTIONS ON THE MECHANICS OF BOUNDARY-LAYER TRANSITION1
By G. B. SCHBAUER and P. S. KLEBASOFFSUMMARY
The manner in which flow in a boundary layer becomes
turbulent was investigated on a flat plate at wind speeds
generally below 100 feet per second. Hot-wire techniques
were used, and many of the results are derived from oscillo-
grams of velocity fluctuations in the transition region. Fol-
lowing a presentation of the more familiar aspects of transition,
there are presented the very revealing facts discovered while
studying the characteristics of artificially produced turbulent
spots. These are: (1) Oscillograms of natural transition are
identical to oscillograms of spot passage. (2) Transition
starts from perturbations in the laminar flow as spots which
then grow in accordance with the general concept proposed
by Emmons. (8) Turbulence always moves downstream
followed by laminar flow. (4) The following flow is in a
state of calm for a period during which transition will not
occur.
INTRODUCTION
The present paper presents the principal results of an
experimental investigation performed in the boundary layer
of a flat plate in an attempt to supply much-needed informa-
tion about the process of transition from laminar to turbulent
flow. In spite of the fact that a great deal was known
about stability and the general circumstances surrounding
transition, little was known about the actual mechanics of
transition and its immediate cause. Consequently, it has
been difficult to explain why flows known to be unstable do
not necessarily become turbulent and flows calculated to
be stable do not always remain laminar. The engineer
has had disappointing results in his attempts to maintain
laminar flow, and he has often been unable to locate the
source of the trouble. The theorist has not been able to
come to grips with the problem for want of a physical
model.
Experiments have failed to agree on a consistent picture
of transition. The water-table experiment of Emmons
showed isolated patches of turbulence which suggested to
him the theory of transition by formation and growth of
turbulent spots, as described in references 1 and 2. Hot-
wire probes used in boundary layers in air generally could
not confirm this picture. The same was generally true of
short-exposure schlieren and shadowgraph observations.
For example, in the recent experiments of Evvard, Tucker,
and Burgess on a 100 cone at supersonic speeds (ref. 3),evidence for the growth of turbulent spots was seldom seen.
While their results did not preclude a mechanism of transi-
tion involving turbulent spots, they indicated transition to
be abrupt and fluctuating and followed by flow that was
predominantly turbulent. One might even suspect that
nature has confused the issue by providing more than one
transition pattern.
The present investigation employed the same flat plate
and wind tunnel as were used for the investigation of lam-
inar-boundary-layer oscillations described in reference 4.
The plate, which was a Y-inch rolled aluminum sheet with
a sharpened leading edge, was 12 feet long and completely
spanned the 4%-foot distance across the test section of the
tunnel. The top speed of the tunnel was around 140 feet
per second, but most of the work was done below 100 feet
per second in order to place the transition region at a
convenient location on the plate. The pressure gradient
was adjustable between moderate limits and, unless other-
wise specified, was zero.
Except for the measurement of pressures and mean-
velocity profiles, for which static- and total-head tubes were
used, all measurements were made with hot-wire probes and
the associated amplifying and recording equipment. Only
hot-wire arrangements sensitive to the longitudinal com-
ponent of the fluctuations were employed, but in many
cases the signals from two wires or two probes were observed
simultaneously. While certain mean quantities, such as
the root-mean-square value of the fluctuations, were useful,
records of the actual wave form of the fluctuations turned
out to be by far the more meaningful; hence film recording
from a cathode-ray oscilloscope yielded most of the significant
information.
The cases studied included free transition with various
amounts of free-stream turbulence, transition induced by
the so-called trip wire, the turbulence wedge behind a three-
dimensional roughness element, and spark-initiated transi-
tion and subsequent growth of the turbulent spot. Only
after the last of these was it possible to appreciate the real
significance of what had been observed in all previous cases.
In short, a transition region was found to be a region of
spot growth as had been concluded by Emmons from his
water-table observations. The seemingly futile attempts
to reach and study phenomena at a transition point, all
of which turned out to be steps toward a final picture, to-
gether with the confirming experiments are described in
the following sections.I Supersedes NACA TN 3480, "Contributions on the Mechanles of Boundary-Layer Transition," by G. B. Schubauer and P.. . Klbanoff, 195.
853
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Schubauer, G. B. & Klebanoff, P. S. Contributions on the Mechanics of Boundary-Layer Transition, report, February 28, 1955; (https://digital.library.unt.edu/ark:/67531/metadc60685/m1/1/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.