Extending previous work on turbulent
diffusion in the wake of a circular-cylinder, a series of measurements have
been made of the turbulent transport of mean stream momentum, turbulent energy,
and heat in the wake of a cylinder of 0.169 cm. diameter, placed in an
air-stream of velocity 1280 cm. sec.-1. It has been possible to
extend the measurements to 960 diameters down-stream from the cylinder, and it
1s found that, at distances in excess of 600 diameters, the requirements of
dynamical similarity are very nearly satisfied. To account for the observed
rates of transport of turbulent energy and heat, it is necessary that only part
of this transport be due to bulk convection by the slow large-scale motion of
the jets of turbulent fluid emitted by the central, fully turbulent core of the
wake, which had been supposed previously to perform most of the transport. The
remainder of the transport is carried out by the small-scale diffusive motion
of the turbulent eddies within the jets, and may be described by assigning
diffusion coefficients to the turbulent fluid. It is found that the diffusion
coefficients for momentum and heat are approximately equal, but that for
turbulent energy is considerably smaller. On the basis of these hypotheses, it
is possible to calculate $he form of the mean velocity distribution in good
agreement with experiment, and to give a qualitative explanation of the
apparently more rapid diffusion of heat.
