You were doing so good untill you had to add that last paragraph.

Sorry Bitz

Note : Newton's theroy, or the Laminar shear of fluid between two plates. Is really the most important quality of motor oil for engine use. It is closley related to the bearing clearences in plain bearing engines (Ex 500).

Enjoy your reading

FOG

Viscoelasticity

Fluid mechanics

Fluids

Fluid statics

Fluid dynamics

Navier-Stokes equations

Viscosity

Newtonian fluids

Non-Newtonian fluids

Viscosity is a measure of the resistance of a fluid to deform under shear stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to shear stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid (Symon 1971).

When looking at a value for viscosity the number that one most often sees is the coefficient of viscosity, simply put this is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient). For example, at "room temperature", water has a nominal viscosity of 1.0 x 10-3 Pa∙s and motor oil has a very nominal apparent viscosity of 250 x 10-3 Pa∙s. (Serway 1996, p. 440)

Contents [hide]

1 Etymology

2 Newton's theory

2.1 Measuring viscosity

2.2 Units

2.2.1 Viscosity (dynamic/absolute viscosity): η or μ

2.2.2 Kinematic viscosity: ν

2.2.3 Dynamic versus kinematic viscosity

3 Molecular origins

3.1 Gases

3.2 Liquids

4 Viscosity of materials

4.1 Viscosity of air

4.2 Viscosity of water

4.3 Viscosity of various materials

5 Can solids have a viscosity?

6 Bulk viscosity

7 Eddy viscosity

8 Fluidity

9 The linear viscous stress tensor

10 See also

11 References

12 External links

[edit] Etymology

The word "viscosity" derives from the Latin word "viscum" for mistletoe. A viscous glue was made from mistletoe berries and used for lime-twigs to catch birds.[citation needed]

[edit] Newton's theory

Laminar shear of fluid between two plates. Friction between the fluid and the moving boundaries causes the fluid to shear. The force required for this action is a measure of the fluid's viscosity. This type of flow is known as a Couette flow.

Laminar shear, the non-linear gradient, is a result of the geometry the fluid is flowing through (e.g. a pipe).In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force.

Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u/∂y, in the direction perpendicular to the layers, in other words, the relative motion of the layers.

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Here, the constant η is known as the coefficient of viscosity, the viscosity, or the dynamic viscosity. Many fluids, such as water and most gases, satisfy Newton's criterion and are known as Newtonian fluids. Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity.

The relationship between the shear stress and the velocity gradient can also be obtained by considering two plates closely spaced apart at a distance y, and separated by a homogeneous substance. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. If this force causes the substance between the plates to undergo shear flow (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. Combining these three relations results in the equation F = η(Au/y), where η is the proportionality factor called the absolute viscosity (with units Pa·s = kg/(m·s) or slugs/(ft·s)). The absolute viscosity is also known as the dynamic viscosity, and is often shortened to simply viscosity. The equation can be expressed in terms of shear stress; τ = F/A = η(u/y). The rate of shear deformation is u / y and can be also written as a shear velocity, du/dy. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained.

In many situations, we are concerned with the ratio of the viscous force to the inertial force, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity, defined as follows:

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James Clerk Maxwell called viscosity fugitive elasticity because of the analogy that elastic deformation opposes shear stress in solids, while in viscous fluids, shear stress is opposed by rate of deformation.

[edit] Measuring viscosity

Viscosity is measured with various types of viscometer. Close temperature control of the fluid is essential to accurate.

Now I don't ever expect to have to respond to another OIL Thread.

FOG