Abstract:
The use of
hot
surface films for
measuring skin
friction
is
examined. It is
shown that all existing theories, which
neglect
heat
conduction within the substrate, are
inadequate
in
predicting the variation of
heat loss from the film
with
skin friction. A
more physically realistic theory is
presented which accounts for the conductive and convective
heat transfer into the flow and also
heat
conduction within
the substrate. This leads to a more general relationship
between
skin friction
and
heat loss from the film.
Experiments
conducted
in flat plate
laminar
and
turbulent
boundary layer flow show that this relationship
is
more
accurate than previous
forms.
The time and temperature dependence of the heat loss
from the hot film are also explored theoretically and experi-
mentally. The
effect of surface misalignment of the film is
shown to
alter significantly the convective heat transfer.
This
effect is
more pronounced in laminar flow than in
turbulent flow.
Using
a glue-on
hot film
probe, calibrations relating
the
heat loss to skin friction
were found to be different in
laminar
and turbulent flow.
An
experimental operating procedure for the use of
hot
surface films for measuring skin friction is proposed which
significantly increases the accuracy of the technique.