![]() Since, water has a viscosity of 1, it is taken as standard substance for relative viscosity. Relative or Specific viscosity is the ratio of dynamic viscosity of any fluid to the dynamic viscosity of water at 20☌. In case of gases, it increases with increase in temperature. In case of liquids, kinematic viscosity decreases with increase in temperature. Likewise kinematic viscosity also involves the magnitudes of length and rime only. The name kinematic viscosity has been given to the ratio (µ/p) because kinematics is defined as the study of motion without regard to the cause of motion and it concerned with length and time only. The kinematic viscosity ( v) viscosity is defined as the ratio of dynamic viscosity to mass density. Dynamic Viscosity (μ):Īs explained earlier, the dynamic viscosity (μ) is defined as the shear stress required causing unit rate of shear deformation. Its unit can be derived asĭu/dy – is the rate of shear deformation or rate of shear strain. Μ (Mu) is the constant of proportionality or co-efficient of dynamic viscosity or Viscosity Unit. A fluid layer at a distance of y from surface moves with a velocity of `u’ and a layer at a distance of dy from y moves with a velocity of u+du.Īccording to Newton’s law of viscosity the shear force, F acting between two layers of fluid is proportional to difference in their velocities du and area A of the plate and inversely proportional to the distance dy between them. The plate moves with a velocity U by a force F as shown in fig 1.2. The space in between is filled with a fluid. Viscosity increases with increase in temperature in case of gases whereas it decreases in case of liquid.Ĭonsider, a plate is placed at a distance of ‘Y’ from the fixed surface. For incompressible liquids, the densities are independent of pressure and hence kinematic viscosities for incompressible liquids only depend on temperature.Viscosity can also be defined as the property of a fluid due to which it offers resistance to the movement of one layer of fluid over another adjacent layer. 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)
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