DISC FRICTION 



189 



successive layers of the fluid. Under such conditions the resistance 

 would decrease as the clearance was increased, until the latter became so 

 large that sinuous motion was set up. With any clearance large enough 

 to satisfy practical requirements, however, the motion is sinuous, and 

 disc-resistance is modified by the fact that the water nearest the disc is 

 thrown radially outwards by centrifugal force, so that a circulatory 

 current is set up, this passing radially outwards along the face of the 



COMPARATIVE RESISTANCES WITH VARYING SIDE CLEARANCE. 



disc and inwards over the sides of the casing. Due to this current, 

 energy is wasted, firstly from friction at the sides and outer periphery of 

 the casing, and secondly, from the formation of eddies caused by the 

 relative motion of the radial outward and inward currents. 



Considering only the first of these causes, it would appear that 

 the resistance should increase steadily with an increase in the side 

 clearance and surface area of the casing; but with a very small 

 clearance it is probable that the circulatory current is only slightly 

 apparent, and that as the clearance is increased its magnitude, and the 

 loss of energy which it involves, will at first increase rather rapidly. A 

 further increase in the clearance, by giving more room to the currents, 

 diminishes their relative velocity at a point midway between the disc and 

 the casing, and it is conceivable that there will be some definite clearance 



