Phy Chapter 4 Fluid Flow Caltec S.3

FLUID FLOW Fluid Friction Fluid Friction is a retarding force which acts on a body moving through a fluid. It is due to the internal friction existing between layers of fluids of a liquid in motion. It is caused by the attraction between the molecules of one layer and the molecules of another layer. The frictional resistance of a fluid (liquid or gas) opposing motion of a body moving through it is called viscosity. Factors which determine the magnitude of fluid friction 1. Unlike friction between solids’ viscosity is proportional to the surface area and the velocity of the object moving through the fluid. 2. Nature of fluid. Viscosity depends on the type of fluid for example glycerin, engine oil, and natural honey flow much less easily from a vessel than liquids such as petrol, paraffin and water. 3. Temperature: Viscosity of liquids decreases as a temperature of liquid increases while that of gases increases with temperature. NB: Liquids which have high viscosity e.g. glycerin, engine oil are called vicious liquids and are used to make lubricants. Bernoulli’s principle Bernoulli’s principle states that when the speed of a fluid increases, the pressure in the fluid decreases and vice versa. For an incompressible non viscous liquid, the kinetic energy per unit volume plus the potential energy per unit volume plus the pressure is constant assuming stream line flow. K.E + P.E + Pressure = constant LIQUID FLOW IN PIPES The speed of a liquid e.g. water is greater at the narrow part of the pipe (constriction) and lower at the wider part. This is because since liquids are incompressible the same volume of liquid flows at a wider part as that at a narrower part at the same time. Hence it has to flow faster at a narrower part to cover …