Viscosity is independent of pressure (except at very high pressure) and.This leads to an additional contribution to the shear stress though the exact mechanics of this are still controversial. In liquids, the additional forces between molecules become important. Viscosity is independent of pressure and *Viscosity increases as temperature increases.The kinetic theory of gases allows accurate prediction of the behaviour of gaseous viscosity, in particular that, within the regime where the theory is applicable: Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow.
s −1.Ĭonversion between kinematic and dynamic viscosity, then, is given by, and so if ν=1 St then.usage, stoke is sometimes used as the singular form.ġ stokes = 100 centistokes = 1 cm 2 It is sometimes expressed in terms of centistokes (cS or cSt). The cgs physical unit for kinematic viscosity is the stokes (abbreviated S or St), named after George Gabriel Stokes. Kinematic viscosity (Greek symbol: ) has SI units (m 2 The centipoise is commonly used because water has a viscosity of 1.0020 cP (at 20 ☌ the closeness to one is a convenient coincidence). It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). The cgs physical unit for dynamic viscosity is the poise (P) named after Jean Louis Marie Poiseuille. Care must be taken in not confusing the poiseuille with the poise named after the same person! In France there have been some attempts to establish the poiseuille (Pl) as a name for the Pa The SI physical unit of dynamic viscosity (greek symbol: ) is the pascal- second (Pa The fluids without a constant viscosity are called Non-Newtonian fluids. For some fluids, it is a constant over a wide range of shear rates. Viscosity is measured with various types of viscometer, typically at 25 ☌ ( standard state). Viscosity is the principal means by which energy is dissipated in fluid motion, typically as heat.įor more details see Measurement of viscosity This ratio is characterised by the kinematic viscosity, defined as follows: In many situations, we are concerned with the ratio of the viscous force to the inertial force, the latter characterised by the fluid density ρ.
Thus, water is "thin", having a low viscosity, while vegetable oil is "thick" having a high viscosity. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. It is commonly perceived as "thickness", or resistance to pouring.
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