![]() 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. For incompressible liquids, the densities are independent of pressure and hence kinematic viscosities for incompressible liquids only depend on temperature.Informally, viscosity is the quantity that describes a fluid's resistance to flow. EnggCyclopedia's viscosity calculators for Liquids and Vapors can be used to quickly determine the viscosity at a given temperature.įor compressible fluids (gases) the densities depend on pressure of the system and hence kinematic viscosities of gases are also dependent on pressure. Variation of absolute viscosities with fluid temperatureĪbsolute liquid and vapor viscosities are strong functions of liquid temperatures. Hence kinematic viscosity of a fluid is also dependent only in the state of the fluid and not the flowrate. It should be noted that absolute viscosity (µ) and density (ρ) are both fluid properties dependent only of the state of the fluid (pressure and temperature). This ratio is known as kinematic viscosity (ν) of a fluid. Hence the ratio of viscous forces to inertial forces in the fluid is represented by (absolute viscosity / density). Viscous forces are represented by a density of the fluid and viscous forces are represented by the absolute or dynamic viscosity of the fluid. In some cases the ratio of viscous forces and inertial forces in a fluid flow is considered to be important. ![]() Unit of viscous stress are same as units of pressure. Newton's law stands for viscous force per unit area of the fluid, which is represented by viscous stress. Here, the negative sign indicates direction opposite to the fluid flow and µ is the absolute viscosity or dynamic viscosity of the fluid which acts as a proportionality constant. Velocity Gradient = dV/dy ≈ ΔV/Δy (as Δy→0) ![]()
0 Comments
Leave a Reply. |