Since cohesion decreases with temperature, the liquid viscosity does likewise.Īs the random molecular motion of fluid particles increases with increases in temperature, the viscosity increases accordingly. The viscosity of a fluid is caused mainly by two factors:ĭue to strong cohesive forces between the molecules, any layer in a moving fluid tries to drag the adjacent layer to move with equal speed, and thus produces the effect of viscosity. (But note that kinematic viscosity is often measured in stokes (St) 104 St = 1 m 2s -1.) Units: square meters per second (m 2 s -1).
The kinematic viscosity ( v) is defined as the ratio of dynamic viscosity to mass density. (But note that the coefficient of viscosity is often measured in poise (P) 10 P = 1 kg m -1 s -1.) Units: newton seconds per square meter (N s m -2) or kilograms per meter per second (kg m -1 s -1). The coefficient of dynamic viscosity (μ) can be defined as the shear force per unit area (or shear stress τ) required to drag one layer of fluid with unit velocity passed another layer a unit distance away from it in the fluid. For fluids obeying Newton’s law of viscosity, taking the direction of motion as the ‘x’ direction and v x as the velocity of the fluid in the ‘x’ direction at a distance ‘y’ from the boundary, the shear stress (τ x) in the ‘x’ direction is given by the formula Shear stress opposing the relative motion of these layers are set up, their magnitude depending on the velocity gradient from layer to layer. A fluid at rest cannot resist shearing forces and if such forces act on a fluid in contact with a solid boundary as shown in below figure, the fluid will flow over the boundary in such a way that the particles immediately in contact with the boundary have the same velocity as the boundary, while successive layers of fluid parallel to the boundary move with increasing velocities.
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