ME 3320 Lecture Notes - Lecture 99: Viscosity, Shear Rate, Lubrication
14 Nov 2020
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Larger viscosity is useful for generating the pressure that will push the surfaces away. If viscosity is too great, excess frictional resistance will take place, like hydrodynamic frictional resistance, which is not good. Need to weigh out speed and applied load. Dynamic viscosity = shear stress/shear rate eta = tau/shear. Kinematic viscosity = dynamic viscosity/density of fluid. 3 main ways viscosity can vary: temperature, pressure, shear rate: temperature, viscosity decreases with increase in temperature. Viscosity index (vi) empirical parameter describing an oil"s viscosity sensitivity to temperature. High vi oils little sensitivity to oil temperature. Low vi oils large sensitivity to temperature. Vi is empirical in the sense that vi=0 corresponds to viscosity-temp relation of a very standard oil. Gulf coast and pennsylvania crude oil are reference oils. Vi = 0 gulf coast crude oil. Vi = 100 pennsylvania crude oil. To calculate the vt of a given oil: Measure the oil"s kinematic viscosity at t=40 c.
