| dc.description.abstract | A theoretical and experimental study is made of the
closed 'bubbles` of separated flow formed when a laminar
boundary layer separates from an aerofoil surface and,
after undergoing transition to turbulence, subsequently
re-attaches. Attention is mainly confined to the
so-called 'short' type of bubble, which is distinguished
from the 'long' type by its relatively slight overall
effect upon the pressure distribution.
In Part I, a semi-empirical theory for the
prediction of the growth and bursting of two-dimensional
short bubbles is developed. The existing data concerning
short bubbles are re-examined, with particular
emphasis upon the conditions governing re-attachment.
A criterion for the determination of turbulent re-attachment
is proposed, and approximate quadrature methods
developed for the calculation of the momentum thickness
in the separated region. These results, together with
am empirical formula for the determination of the
position of transition, are combined with a simplified
model of the pressure distritbution in the bubble region
to predict the re-attachment position. It is found that,
for a given imposed pressure distribution, there exists
a Reynolds number at separation below which re-attachment
is impossible. This is associated with the phenomenon
of short bubble bursting. The predictions of the theory
are in reasonable quantitative agreement with experiment.
Part II deals with bubbles in three-dimensional
flow. Experiments are described in which separation
bubbles were produced using an apparatus closely
simulating conditions near the leading-edge of a swept
wing of infinite span. Measurements of surface pressure,
mean velocity and turbulence level are presented, from
which it is deduced that the bubble structure is similar
to that of two-dimensional bubbles, apart from the
existence of cross-flows in the shear-layer and a strong
spanwise flow in the reverse-flow vortex. An extension
of the two-dimensional bursting theory by means of the
independence principle is in reasonable agreement with measured bursting parameters. | en_US |
