What is a Newtonian fluid?

A Newtonian fluid is a fluid in which the viscous stresses arising from its flow are directly proportional to the local strain rate at every point. Although no real fluid fits this definition perfectly, common substances like water and air act as Newtonian fluids under ordinary conditions. They serve as the simplest mathematical models in fluid mechanics for accounting for viscosity.

What we know

• In a Newtonian fluid, viscous stresses are linearly proportional to the local strain rate, which is the rate of change of its deformation over time. [1, 2]
• A Newtonian fluid's rate of flow cannot be altered by shaking, pumping, or stirring. [2]
• While no real fluid fits the definition perfectly, common liquids and gases like water and air can be assumed to be Newtonian for practical calculations under ordinary conditions. [2]
• Newtonian fluids are named after Isaac Newton, who first used a differential equation to postulate the relation between shear strain rate and shear stress. [2]

Where to go next

• compared withWhat are examples of non-Newtonian fluids?
• the bigger pictureHow does fluid mechanics model matter?
• whoWho was Isaac Newton?
• a case in pointWhat is a Herschel-Bulkley fluid?
• how it workedHow does continuous shear deformation work in fluids?
• an unexpected connectionHow do mathematical fluid models apply to meteorology?The basic mathematical models used to describe simple fluid viscosity also form the foundational framework applied in complex atmospheric and meteorological research.

Sources

• [1]Newtonian fluid (Wikidata Q366968) · CC0 1.0
• [2]Newtonian fluid · CC BY-SA 4.0
• [3]Fluid mechanics · CC BY-SA 4.0
• [4]Fluid · CC BY-SA 4.0
• [5]Herschel–Bulkley fluid · CC BY-SA 4.0