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2.2 What is fluid? Shear streass

Fluid in this book is considered as a substance that ``moves'' continously when exposed to a shear stress. The liquid metals are an example of such substance. However, the liquid metals do not have to be in the liquidous phase to be considerd liquid. Aluminum at aproximatly [C] is continously deformed when shear stress are aplied. The whole semi-solid die casting area deals with materials that ``looks'' solid but behaves as liquid.

Consider a liquid that resting on the bottom and the top is moving in a velocity, (see Figure [*]). The force required for this operation is proportional to

    (2.1)

Or in other words, the shear streas is propportional to
    (2.2)

Under steady state condition and a linear veloctiy distribution it can be shown that and therefore
    (2.3)

This assumption leads as to the Newtonian fluid. In this book we only mentioned the fact that there are fluids that that do exhibit non-Newtonian behavior and they are not disscused in this book.

The viscosity is appropety of liquid and it was found that the ratio of the viscosity to the density is importent. This ratio is called kinematic viscosity, and denoted as . For liquid metal this property is function of the temperature and senstive to the pressure.

= 0.4

Figure: A schamatic of shear flow coutee flow



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