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National Advisory Committee for Aeronautics, Report - Contributions on the Mechanics of Boundary Layer Transition

SUMMARY

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 derioed 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. (3) 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 difiicult 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 10° 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.