© 1956 by Oxford University Press
MESUREMENTS OF ATTENUATION OF COMPRESSION WAVES OF FINITE AMPLITUDE IN AIR AND EVALUATION OF THE COEFFICIENT OF FRICTION
( Queen Mary Colleage London )
The purpose of this paper is the investigation of the influence of viscous friction on plane non-steep compression waves generated in air in a tube by means of a pneumatically operated piston.
The tests described include four series of experiments. In every series the shape of the pressure wave (pressure—time curve) was determined by means of a cathode-ray oscillograph using‘ variable capacity’ pressure gauges in four test stations 10 ft. apart. Also the displacement-time curves of the piston generating the waves were recorded for the purpose of determining the piston velocity curves. Each series of tests has a different piston velocity curve.
The acceleration of the piston varied over the range 7,260 to 27,200 ft./sec.2, and the highest pressure of the generated waves varied from 6–5 to 8–7 lb./in.2 gauge.
The measured pressure waves show a decrease in pressure of the generated waves with distance travelled and an increase in the time of duration when compared with isentropic waves.
The equations for calculating the coefficient of friction in non-steady compressive motion of a fluid are presented and the calculated and correlated values of the coefficient of friction plotted.
It has been shown that the coefficient of friction depends on the state of the boundary layer between the sections considered and that it must, therefore, depend on the Reynolds number, Re., based on the distance travelled by the mean particle between these sections. The mean particle, located somewhere in the middle between the sections considered, should be a fair representative of all the particles of fluid in motion between these sections. Thus the assumption prevailing that steady flow coefficients corresponding to Reynolds number based on diameter should be used in the case of non-steady flow has been disproved. A comparison has been made between the coefficients for steady flow (with developed boundary layer), and those for non-steady flow. It has been shown that the acceleration increases the values of the coefficient of friction.