![]() Practice Homework and Test problems now available in the 'Eng Fluids' mobile app The CGS system is cm 2/s and this is referred as a stoke (St). The SI unit for the kinematic viscosity is m 2/s. The kinematic viscosity is the ratio of absolute viscosity and density. For example, crude oil is often heated to a higher temperature to reduce the viscosity for transport. For liquids, the viscosity decreases with temperature, whereas for gases, the viscosity increases with temperature. However, viscosity depends greatly on temperature. Viscosity is not a strong function of pressure, hence the effects of pressure on viscosity can be neglected. Note that only Newtonianįluids will be considered in the future discussion non-NewtonianĮffects are beyond the scope of this eBook. Like a fluid once this yield stress is exceeded. Withstand a finite shear stress without any motion, however it moves The larger theĪpplied shear rate trying to mix water with corn starch, more resistanceĪnother non-Newtonian fluid is Bingham plastic, The less resistance that (viscosity) is encountered.įluids are quicksand and a water-corn starch mixture. Whenīrushing paint on a wall, note that the larger the applied shear rate, An example of a shear thinning fluid is latex paint. Rate, whereas for shear thickening fluids, the apparent viscosity increases The apparent viscosity is the same as the viscosity and is not a functionįor shear thinning fluids, the apparent viscosity decreases with shear The shear stress versus the rate of shear strain. Viscosity is introduced and it represents the slope (not constant) of Non-Newtonian fluids are shear thinning fluids, shear thickeningįluids and Bingham plastic. Note that the shear stress can also be determined by dividing the shearįor non-Newtonian fluids, the shear stress is not a linearįunction of the rate of shear strain. Is also expressed in the CGS system as dyne-s/cm 2 and The absolute viscosity, dynamic viscosity or simply the viscosity. The proportionality constant (μ) is called The above equation is referred to as Newton's law of viscosity. Rate of shear strain in a linear fashion. Most common fluids, such as water, air and oil, areĬalled Newtonian fluids in which the shear stress is related to the Strain) in this case is a constant (du/dy = U/h). Note that the velocity gradient (also known as the rate of shear Of the fluid motion between the plates is assumed to be linear and is The fluid is stuck to the bottom plate, u = 0). Plate moves with the plate velocity U and no-slip condition is applied If the top plate is movedĪt a velocity U while the bottom plate is fixed, the fluid Two parallel plates, as shown in the figure. To illustrate the concept of viscosity, consider a fluid between Viscosity is a fluid property that measures the resistance Kinematic viscosity is defined as the quotient of a fluid’s absolute viscosity in centipoises over its specific gravity, both measured at the same temperature.Another important fluid property will be introduced in The unit of kinematic viscosity is the stoke or the centistoke (1/100 of a stoke). This is the time required by an oil to flow through a capillary tube under the force of gravity. Empirical viscosity, like Saybolt viscosity, is expressed in arbitrary units of time, usually seconds. Absolute viscosity and kinematic viscosity are expressed in fundamental units. This is sometimes also called dynamic viscosity. The absolute viscosity divided by the fluid’s density gives the kinematic viscosity. It is a fluid’s internal flow resistance. This represents the ratio of a fluid’s shear stress to its velocity gradient. The same meaning as the word viscosity used alone, to distinguish from empirical viscosity or kinematic viscosity. In the metric system, absolute viscosity is expressed in poise.
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