﻿On the Instability of Gaseous Jets. 3G9 



be formed, would instantaneously disappear, the transition 

 from the one velocity to the other becoming more and more 

 gradual, until the layer of transition attained a sensible 

 width. When this width is comparable with the wave-length 

 of a sinuous disturbance, the solution for an abrupt transi- 

 tion ceases to be applicable, and we have no reason for 

 supposing that the instability would increase for much shorter 

 wave-lengths. 



"There is, therefore, ample foundation for the suspicion that 

 the phenomena of sensitive jets may be greatly influenced by 

 fluid friction, and deviate materially from the results of 

 calculations based upon the supposition of discontinuous 

 changes of velocity. Under these circumstances it becomes 

 important to investigate the character of the equilibrium of 

 stratified motion. 



" The manner in which viscosity operates is probably as 

 follows. At the root of the jet, just after it issues from 

 the nozzle, there is a near approach to discontinuous motion, 

 and a high degree of instability. If a disturbance of sufficient 

 intensity and of suitable period have access, the regular 

 motion is lost, and cannot afterwards be recovered. But the 

 instability has a very short time in which to produce its 

 effect. Under the influence of viscosity the changes of 

 velocity become more gradual, and the instability decreases 

 rapidly, if it does not disappear altogether. Thus if the dis- 

 turbance be insufficient to cause disintegration during the 

 brief period of instability, the jet may behave very much as 

 though it had not been disturbed at all, and may reach the 

 full development observed in long flames and smoke-jets." 



It would thus appear that in general, an orifice of tubular 

 form would tend to increase the discrepancy between theory 

 and practice, by communicating to the jet a stratified forma- 

 tion, and thus lowering the pitch of the disturbances to 

 which it was sensitive, or reducing the sensitiveness altogether, 

 while a thin-plate orifice might be expected to give, near the 

 root of the jet, an approximation to discontinuous changes 

 of velocity, and consequently a considerable degree of sensi- 

 tiveness for disturbances of high pitch. 



The apparatus used in these experiments consisted of 

 weighted gas-bags ; a special gas-meter reading 1 cu. ft. 

 per rev., the dial being divided into 100 parts ; a water- 

 manometer attached about 2 inches below the orifice: and a 

 number of orifices consisting of holes in thin circular plat o^ 

 of copper and platinum, the plates being secured in an 

 annular screw-head holder, and of tubes, and holes in thick 

 plates. 



Phil. Maq. S. G. Vol. 23, No. 135. March L912. 2 C 



