343 Whilst this may be so it is also true that this in itself is not
sufficient to deter- mine it completely. In fact the extent of the dead
air region and the behaviour of the shear layer are also of prime
importance and in short a unified treatment comprising external flow,
boundary layer, shear layer and dead air region becomes necessary to
complete the investigation. This would take us outside the scope of the
present article and for the substantial progress that has been made
towards such a treatment the reader is referred to a paper by HOLDER and
GADD 1 and its comprehensive list of references. v. Heat transfer in
incompressible boundary layers. 25. Introduction. The term fluid
includes gases and liquids. Both gases and liquids are to some extent
compressible but in many problems of fluid flow the density changes
occurring are small. When they are small enough to be negligible we can
regard the flow as incompressible. In Chap. IV we have established the
equations for compressible flow of gases and these can of course be used
to deter- mine when density changes in a gas flow are in fact
negligible. Broadly speaking this will be so when the temperature
changes as determined by the energy equation are small enough.