MOTION OF VISCOUS FLUIDS 49 



ppear in the wake as a spiral (Fig. 23 C). This type of vortex formation 

 usually found in the rear of a plate, normal or slightly inclined to the 

 current. If the plate be inclined to the current at an angle exceeding 

 about 30 the formation becomes discontinuous, as indicated in Fig. 23 D, 1 

 which shows the eddies formed behind a square plate inclined at 40 to 

 he stream. - In this type of motion two spirals are formed from the sides 

 f the plate and are linked together to form a chain of eddies by a sheet 

 f fluid flowing from the trailing edge of the plate. 



From the manner of eddy formation it is evident that the pressure on 

 he rear surface of the body in a current of fluid will be less than normal, 

 ind subject to a periodic variation, the period depending on the time of 

 brmation of an eddy and so on the shape and dimensions of the body 

 ind on the velocity of flow. 



If the body is supported so as to be capable of vibration, and the period 

 eddy formation synchronises, or nearly so, with that of its natural 

 bration, oscillations are set up which may become very large. This 

 feet may often be noted where a flexible bough dips into a steady stream. 

 n account of this action, current meters, in which the pressure on a 

 ationary flat plate is taken as a measure of the velocity, are not 

 tisfactory. 



ART. 16. CONDITIONS REGULATING THE Two MANNERS OF MOTIOX. 



Several conditions combine to determine whether in any particular 

 stance the motion of water shall be steady or unsteady. Osborne 

 eynolds 2 came to the conclusion that the conditions tending to stability 

 nd steadiness of motion are : 



(1) An increase in viscosity. 



(2) Converging solid boundaries. 



(3) Free (exposed to air) surfaces. 



(4) Curvature of the path, with the greatest velocity at the outside of 

 le curve. 



And that the conditions tending to instability and unsteadiness of 

 otion are : 



(1) A decrease in viscosity. 



(2) Solid (rigid tangentially) boundaries in general and particularly 

 verging solid boundaries. 



(3) A stream of fluid flowing through fluid at rest. 



1 Tech. Report, Aviation Committee, 191112. 



2 " Phil. Trans. Royal Society," 1883. 



H.A. I? 



