![]() ![]() One square meter per second is equal to ten thousand stokes. A more common unit of kinematic viscosity is the square centimeter per second, which is given the name stokes after the Irish mathematician and physicist George Stokes (1819–1903). This unit is so large that it is rarely used. The SI unit of kinematic viscosity is the square meter per second, which has no special name. Capillary viscometers will be discussed in more detail later in this section. When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, the more viscous fluid takes longer than the less viscous fluid to flow through the tube. ![]() It is frequently measured using a device called a capillary viscometer - basically a graduated can with a narrow tube at the bottom. Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. The other quantity called kinematic viscosity (represented by the Greek letter ν "nu") is the ratio of the viscosity of a fluid to its density. The quantity defined above is sometimes called dynamic viscosity, absolute viscosity, or simple viscosity to distinguish it from the other quantity, but is usually just called viscosity. There are actually two quantities that are called viscosity. ![]() Ten poise equal one pascal second making the centipoise and millipascal second identical. The most common unit of viscosity is the dyne second per square centimeter, which is given the name poise after the French physiologist Jean Poiseuille (1799–1869). The pascal second is more rare than it should be in scientific and technical writing today. Despite its self-proclaimed title as an international system, the International System of Units has had little international impact on viscosity. The SI unit of viscosity is the pascal second, which has no special name. Or if you prefer calculus symbols (and who doesn't)… F The similarity to Newton's second law of motion ( F = ma) should be apparent. The more usual form of this relationship, called Newton's equation, states that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. (dynamic) viscosityįormally, viscosity (represented by the symbol η "eta") is the ratio of the shearing stress ( F/ A) to the velocity gradient ( ∆ v x/∆ y or dv x/ dy) in a fluid. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them. Where `\frac` is also known as velocity gradient in y-direction.Informally, viscosity is the quantity that describes a fluid's resistance to flow. Put the value of dx in the equation of dθ. Thus the distance covered by the upper layer (dx) is given by, Thus the shear strain for this smaller portion of the liquid is given by,įor the smaller value of dθ, tan(dθ) = dθĪs the upper layer is moving at speed of du. The above figure-A shows the position of layers at time t = 0 and figure B shows the position of elemental layers after the time interval of ‘dt’.Īfter a time interval of ‘dt’, the upper layer of fluid gets displaced by distance dx. The lowest layer of fluid that is in contact with the stationary plate has a velocity of u = 0 and the uppermost layer of fluid is moving with the velocity of du.
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