![]() Many non-Newtonian fluids do not have a yield stress of σ 0 because the flow will occur even at very low shear stresses. Shear-thickening fluids have a flow behavior index of above one. For shear-thinning fluids, the flow behavior index is greater than zero but less than one. However, the Herschel-Bulkley equation cannot calculate rheopectic (time-dependent shear-thickening) or thixotropic (time-dependent shear-thinning) fluids.įor the Herschel-Bulkley equation, the flow behavior index cannot be infinite. In other words, the Herschel-Bulkley equation can determine viscosities for pseudoplastic and dilatant fluids. The Herschel-Bulkley equation below can calculate apparent viscosities only for fluids whose viscosity is not time-dependent. In contrast, high molecular weight fluids (such. Most fluids of low molecular weight are Newtonian fluids. Its unit is Stoke or m2/s (1 stoke 0.0001 m 2 /s). (2) Kinematic Viscosity: The ratio of dynamic viscosity to density appears frequently and this ratio is given by the name kinematic viscosity. A common unit of dynamic viscosity is poise. This article details calculations for the absolute viscosity of Newtonian fluids and the apparent viscosity of non-Newtonian fluids. There are two ways to write viscosities: (1) Dynamic viscosity: This is also termed as absolute viscosity. Suspensions or emulsions containing macromolecular components do not obey Newton’s law and require consideration of the entire Herschel-Bulkley equation. A Newtonian fluid is a fluid where the stresses from fluid flow are linearly correlated to the strain rate. Newton’s law defines the relationship between applied stress, σ, and the resultant fluid flow with a shear rate (velocity gradient) of ̇γ. While Newtonian fluid viscosity varies depending on temperature alone, non-Newtonian fluids have different viscosities depending on shear rate, shear stress, and temperature. The amount of flow resistance a fluid has is based on the internal molecular interacts of the fluid and the magnitude of the external stress applied to the liquid. Viscosity is the measure of a fluid’s resistance to flow.
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