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Advances in Cryogenic Wind Tunnel Technology

In any attempt to justify the expenditure of considerable
manpower and effort on a project such as that forming the
subject of this Course it is appropriate to reflect for a
moment on the underlying reasons for the work, which I
will first attempt to do. The root cause of us being here is
the fundamental weakness ofclassical mathematics: despite
the undoubted brilliance of mathematicians past and present
they have not been able to give us the means to forecast by
calculation, and with certainty, the behaviour of real life
devices such as the products of aerospace industries. This
failure reveals inadequacy in the discipline and not in the
practitioners. A quotation specifically about our business of
aerodynamics is as follows: "The disparity between the
designer’s need for aerodynamic prediction and the power
of his analytic methods seems to be so vast as almost to
defy description"(Ref 1). This statement was published by
a very experienced aircraft designer in September 1971,
close to the time ofthe beginning of construction ofthe first
cryogenic wind tunnel (Ref 2). Since then the two avenues
of endeavour, empirical and theoretical, have advanced in
healthy competition with improvement in each, which is a
recognition that the former was not without weakness.
The birth of the cryogenic wind tunnel was preceded by a
20 year period spawning almost all of the transonic wind
tunnels now in use. During this period the need to provide
for the needs of experimental aerodynamics in a reasonably
economic way followed the pattern already set, that of
matching the required Mach number but in most cases not
the required Reynolds number. The reason for this is
because Mach number effects were known to be strong,
particularly at speeds near the speed of sound, while it was
felt that the effects of Reynolds number on performance
were rather weak and perhaps systematic and predictable. If
the same circumstances existed now and we had to choose
between the two parameters there is no doubt that we would
still pick Mach number for proper matching. It is perhaps
fortunate that background research in Japan and the USA.
in the 1930’s led to the development of the ventilated test
section for transonic testing, allowing the immediately most
pressing needs to be satisfied at reasonable cost. Had
Reynolds number effects seemed more important there is no
knowing what solutions might have emerged, but possibly
the cryogenic wind tunnel because the necessary information
and most of the technology was around and the route to full
scale Reynolds number by more conventional means is
inordinately expensive.