<DOC>
<DOCNO>
244
</DOCNO>
<TITLE>
an improved smoke generator for use in the visualisation
of airflow, particularly boundary layer flow at high
reynolds numbers .
</TITLE>
<AUTHOR>
preston,j.h. and sweeting,n.e.
</AUTHOR>
<BIBLIO>
arc r + m 2023, 1943.
</BIBLIO>
<TEXT>
and rapid method by which boundary
layer flow was rendered visible has been
previously described in the journal of the royal
aeronautical society .  it gave promise of
being useful at the highest tunnel speeds provided
a denser smoke could be obtained, which at
the same time was free from the troublesome deposits
associated with the wood smoke .
of the aerodynamics division attempts were made
by the fuel research station to improve the
density of the wood smoke and to reduce the
deposits .  these they showed were conflicting
requirements, and whilst some improvement
was effected, it was not sufficient for observation
in the new tunnels at high speeds .
  the staff of the director-general of scientific
research and development, ministry of supply,
was then approached and it was decided to
develop an oil smoke generator from a simple
generator of this type which was demonstrated
to us .  this has been done successfully .
the final apparatus in contrast to the wood smoke
generator is light and compact .  it takes
only a few minutes to start and can be run as long as desired .
improvement on the wood smoke both as regards
density and freedom from deposits, which cause
premature transition .  the density and quality
of the smoke are now under control .  smokes
ranging from a light smoke of bluish white colour
to a heavy smoke dense white in appearance
can be obtained .  the oil smoke retains the
advantages of the wood smoke in that it is noncorrosive
and non-irritant, and the smell can be
tolerated even when it is present in a considerable
concentration .  a certain amount of condensation
is inevitable with oil smokes, but with suitable
precautions troubles arising from this can be
avoided .  a dry solid smoke made by melting a
hard wax was successfully generated with the
same apparatus .  unfortunately because of its
flocculent nature this smoke gave rise to solid
deposits when passed through bore tubing,
leading eventually to complete blockage .  this
seems to be a feature of solid smokes .
  the apparatus has been used to determine
transition and laminar separation points on model
wings in a number of the national physical
laboratory tunnels .  smoke filaments have been
maintained in the laminar state up to wind
speeds of 180 ft. sec. in the new tunnels .
  there is much to be said for making a standard
practice of visualising boundary layer flow on
models, particularly as the technique is simple and
rapid .  it would greatly assist the interpretation
of force measurements and the more detailed
explorations of the boundary layer by total
head tubes and hot wires .
  the use of oil smoke is not limited to
boundary layer flow visualisation .  the apparatus
described in this report would seem to be
particularly suited for educational work in small
demonstration tunnels .
</TEXT>
</DOC>
