HYDRAULICS, that branch of engineering science which deals with the practical application of the laws of hydrodynamics.
The chief physical properties of a fluid which affect its mo tion are its density, viscosity and compressibility. For all prac tical purposes the density of fresh water may be taken as 62.4 lb. per cu.ft. The viscosity varies appreciably with tempera ture. The coefficient of viscosity le is given by where f is the resistance per unit area offered to the motion of a flat plate immersed in the fluid at a very small distance y from a parallel surface, and moving in the direction of its own length with velocity v.
Water is so slightly compressible that for all calculations in volving steady motion it may be taken as incompressible. The compressibility becomes an important factor however where sudden changes of motion take place in a confined column, as when the valve at the end of a pipe line is suddenly opened or closed. The value of the modulus of compressibility K may be taken as approximately 310,000 lb. per sq.in.
Transmissibility of Pressure in an Enclosed Mass of Fluid.—If a closed vessel provided with a piston be filled with water, and if such a force be applied to this piston as will pro duce a pressure intensity of p lb. per sq.in. on its face, the pressure intensity at every point of the surface of the vessel will be increased by the same amount.
This property is taken advantage of in many hydraulic ma chines, notably in Bramah's hydraulic press, and in machines of a like type. In the hydraulic press water is forced by means of a small pump, whose plunger has an area a, into the cylinder of the press whose area is A. Neglecting friction, a force P, applied to the plunger of the force pump, will then produce a pressure intensity of P=a in the pump and press cylinder, and hence a force Q=PA-=a on the press plunger.