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National Advisory Committee for Aeronautics, Report - Further Experiments on the Flow and Heat Transfer in a Heated Turbulent Air Jet

Measurements have been made of the mean-total—head and
mean-temperature fields in a round turbulent jet with rarious
initial temperatures. The results show that the jet spreads more
rapidly as its density becomes lower than that of the receiving
medium, even when the diference is not suficientl—y great to cause
measurable deviations from the constant-density, dimensionless,
dynamic-pressure profilefunction. Rough analytical considera-
tions have given the same relatiee spread.

The efectire “turbulent Prandtl number” for a section of the
fully dereloped jet was found to be equal to the true (laminar)
Prandtl number within the accuracy of measurement.

Measurements of turbulence level (u’, o’), temperature fluc-
tuation level d’, and temperature-velocity correlation 1%; permit a
comparison of their relative magnitudes.
Direct measurements hare been made of the double correlations
no and do across a section of the fully dereloped jet, and the
shear-stress and heat-transfer distributions have been computed
therefrom. Finally, these last-mentioned measurements hare
permitted a determination of the distribution of turbulent Prandtl
number across the jet, and these talues agree quite well on the
average with the efiectire value computed from mean relocity and
temperature alone.

The present work is concerned with two particular prob-
lems on the flow of round, low-velocity, turbulent jets: (a)
The effect of mean-density differences upon the rate of spread
of the jet, which can be examined through the average veloc-
ity and density fields; (b) the relative rates of transfer of
heat and momentum in a heat-ed jet, which must involve a
study of fluctuations in velocity and temperature as well as
average fields.

Problem (a) has been investigated experimentally by Pabst.-
(references 1 and 2), Von Bohl (reference 2), and others.
Some British results consist of a few rough measurements
made in the course of a difierent investigation and, unfor-
tuna tely, the German reports are available only in practically
illegible form, so that a thorough reading has not. been feasi-
ble. These references arrived when the present investiga-
tion was in progress and substantiated the result, discussed
in detail in the body of the paper, that reduction in density
of jet relative to receiving medium increases the rate of
spread.