Viscosity (also known as toughness) is a fluid characteristic due to molecular interaction by which an internal friction opposes deformation.

Newtonian fluids, at laminar flow, create shear tension and pressure super-imposed normal tensions which are proportional to the deformation velocity, the proportionality factor being the absolute viscosity.

The kinematic viscosity is defined as:

The viscosity depends on temperature and pressure, whereby pressure dependency of fluids is of negligibly small characteristics.

Viscosity of non-Newtonian fluids can moreover be time-related (thixotropic or rheopexic flow characteristics); its characteristics then becoming undefined.

The viscosity of a medium has an influence on both the system curve and the pump performance curve. For centrifugal pumps, the pump curves are converted in practice at a kinematic viscosity of more than 10 mm²/s.

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The density is an important fluid property that depends on the temperature. The density of the pumped medium has a direct influence on the power requirement of the pump.

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Newtonian Fluid

A fluid is classified as being Newtonian if it conforms to NEWTON’s friction law, i.e. if viscosity remains constant with agitation or varying shear rate and its shear rate being proportional to the velocity gradient vertical to the direction of flow. That means, that the viscosity is only affected by temperature changes.

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