CHAPTER VII. 



Fluid Friction Froude's Experiments Disc Friction. 



ART. 61. FLUID FRICTION. 



WHENEVER a liquid flows over a solid surface, or when a submerged 

 plane moves in the direction of its length through a liquid, a resistance 

 to motion is experienced. This is commonly termed fluid friction, and 

 should not be confused with the wave-making resistance which is experi- 

 enced owing to the formation of surface waves, when a partially submerged 

 body is in motion. 



Though initially due to viscosity, the laws governing fluid friction are 

 usually very different from those of simple viscous resistance, because of 

 the fact that except at extremely low speeds the motion of the fluid 

 becomes unsteady, eddies are formed, and the energy absorbed in fluid 

 friction now chiefly consists of the energy of formation of these eddies. 

 This energy is finally absorbed in overcoming the viscous resistance of 

 the fluid at points remote from the surface at which the eddies are 

 generated. 



As previously indicated, the laws of fluid friction for a liquid for steady 

 and unsteady motion are widely different. 



With Steady Motion Stream-line Motion : 



(1) The frictional resistance is directly proportional to the velocity. 



(2) Is sensibly independent of the pressure in the fluid. 



(3) Is directly proportional to the area of the wetted surface if this is 

 large, i.e., if the resistance is not sensibly affected by the acceleration of 

 the fluid at the leading edges of the surface. 



(4) Is independent of the nature of the wetted surface. 



(5) Is directly proportional to the viscosity of the fluid, and so varies 

 greatly with temperature. 



These laws may be most easily verified by experiments on the flow of 

 water through capillary tubes. 

 With Unsteady or Eddy Motion : 



(1) The frictional resistance varies with a higher power of the velocity 

 than the first, and is usually approximately proportional to F 2 . 



(2) Is independent (within wide limits) of the pressure in the fluid. 



