A fluid with zero viscosity is called a nonviscous, or inviscid fluid. Laminar flow can be described as a well-ordered pattern whereby fluid layers are assumed to slide over one another. In this case while fluid has irregular molecular motion, is macroscopically, well ordered flow. For a laminar flow, whereby fluid particles move in straight, parallel lines, called Newtonian fluids, the shear stress on an interface tangent to direction of flow is proportional to the distance rate of change of velocity, wherein differential is taken in a direction normal to the interface, Txy a dy or Txy = idy, Where i is called the coefficient of viscosity with the dimension of (F/L2)t (figure 2.6).

This unit in a system of centimeter (cm)-gram (g)-second(sec)is g — cm-1 — sec-1and is called the poise. At room temperature, i is about 1 centi-poise for water and about 0.002 centi-pois for air. This is well-known Newton's viscosity law. All gases and simple liquids are described by the later formula. For gases at low density viscosity decrease as temperature increases. Whereas for liquids the viscosity usually decreases with increasing temperature. The reason may be that, in gases, molecules travel long distance between collisions and the momentum is transported by the molecules in free flight, which as in liquids the molecules travel only very short distances between collisions, the principal mechanism for momentum transfer is the actual colliding of the molecules.

In Newtonian law of viscosity for a given temperature and pressure i is constant. Experiments show that in some fluids, Txyis not proportional to the change of rate of velocity in normal direction of it. These few industrial

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