However, it is the oil’s resistance to flow and shear due to internal friction that is being measured in this example, so it is more correct to say that the gear oil has a higher absolute viscosity than the turbine oil because more force is required to stir the gear oil.įor Newtonian fluids, absolute and kinematic viscosity are related by the oil’s specific gravity. The force required to stir the gear oil will be greater than the force required to stir the turbine oil.īased on this observation, it might be tempting to say that the gear oil requires more force to stir because it has a higher viscosity than the turbine oil. Use the rod to stir the oil, and then measure the force required to stir each oil at the same rate. To measure absolute viscosity, insert a metal rod into the same two beakers. Which one will flow faster from the beaker if it is tipped on its side? The turbine oil will flow faster because the relative flow rates are governed by the oil’s kinematic viscosity.

Imagine filling a beaker with turbine oil and another with a thick gear oil. This leads us to the fact that dynamic viscosity is a constant property, but kinematic viscosity is a derived property.An oil’s kinematic viscosity is defined as its resistance to flow and shear due to gravity. Kinematic viscosity has units of diffusivity, (length) 2/s, which means that kinematic viscosity is sometimes known as diffusivity of momentum, dependent on thermal and mass diffusivity.

In Newton’s Law of Viscosity equation, the proportionality constant is μ or dynamic viscosity. The ratio of shear stress to shear rate is defined as the viscosity. Newton’s Law of Viscosity states that shear stresses between parallel layers of fluid are proportional to the corresponding velocity gradients. Kinematic viscosity allows you to work out the speed at which the fluid moves when a certain force is applied.Its units come out as mass/(distance*time). Dynamic viscosity lets you work out the force needed to make a given fluid flow at a certain rate.Kinematic and dynamic viscosity measurements serve different purposes in calculations. tomato ketchup.ĭifference between kinematic viscosity and dynamic viscosity Thixotropic fluids – decrease in viscosity when shaken, e.g.Rheopectic fluids – these increase in viscosity when shaken, e.g.corn starch combined with water – it gets thicker when stirred faster fluid. Shear-thickening fluids – the viscosity increases when the shear rate increases.wall paint – as you stir wall paint, it becomes increasingly fluid and liquidy. Shear-thinning fluids – the viscosity decreases when the shear rate increases.They get this moniker because they follow the formula set out by Isaac Newton in his law of fluid mechanics.įluids that don’t exhibit constant viscosity across all shear rates are known as non-Newtonian fluids.Įxamples of some Newtonian fluids are gases, oil, water, alcohol, petroleum, etc.Įxamples of some non-Newtonian fluids include: Kinematic viscosity in Newtonian and non-Newtonian fluidsĪ Newtonian fluid is one that keeps a constant value of viscosity across all shear rates. The table below contains kinematic viscosity values for some common fluids.

When used in this equation, the viscosity μ is often referred to as dynamic viscosity or absolute viscosity.

Kinematic viscosity is measured in units of (length) 2/time – most commonly using centiStokes (cSt), where 1 Stoke = 1 cm 2/s.

Kinematic viscosity definition