Description of fluids and fluid flow

Quantity Derived unit Symbol Relationship to primary units 

Dynamic viscosity pascal second, or newton second per square metre Pas N s/m2 

It follows from the above facts that fluids can be treated as continuous media with continuous distributions of properties such as the pressure, density, temperature and velocity. Not only does this imply that it is unnecessary to consider the molecular nature of the fluid but also that meaning can be attached to spatial derivatives, such as the pressure gradient dP/dx, allowing the standard tools of mathematical analysis to be used in solving fluid flow problems. 

Two examples where the continuum hypothesis may be invalid are low pressure gas flow in which the mean free path may be comparable to a linear dimension of the equipment, and high speed gas flow when large changes of properties occur across a (very thin) shock wave. 

Two other simplifications that should be noted are that in most fluid flow problems the fluid is assumed to be homogeneous and isotropic. A 

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