For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. In other materials, stresses are present which can be attributed to the deformation rate over time. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. The relative strength of this force is a measure of the fluid's viscosity. Since the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. Definitions Dynamic viscosity Illustration of a planar Couette flow. Viscum also referred to a viscous glue derived from mistletoe berries. The word "viscosity" is derived from the Latin viscum (" mistletoe"). A fluid that has zero viscosity (non-viscous) is called ideal or inviscid. Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. However, the dependence on some of these properties is negligible in certain cases. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. Thus its SI units are newton-seconds per square metre, or pascal-seconds. Viscosity is defined scientifically as a force multiplied by a time divided by an area. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Its units: stokes.The viscosity of a fluid is a measure of its resistance to deformation at a given rate. the ratio of the absolute viscosity ( μ) to the density of the fluid ρ. It can be determined through kinematic viscosity (also called momentum diffusivity), i.e. (2) In fluid dynamics, the term absolute viscosity (or dynamic viscosity) refers to the force per unit area applied tangentially to a fluid, causing the unit rate of displacement of parallel planes separated by a unit distance. Therefore, a decreasing ambient temperature will increase the viscosity of a fluid. For instance, the viscosity of a fluid is lower when the temperature is higher. The ambient conditions also affect viscosity. External forces such as shear forces or tensile stress act upon the fluid and therefore influence the flow of the fluid. The molecular structure of a fluid influences viscosity in a way that when the molecules are tightly linked, the higher is the resistance to deformation, and therefore it will have less tendency to flow. (1) The viscosity of a fluid is influenced by the following factors: molecular structure, external forces, and ambient conditions. (1) Pitch, which is a dark viscous waxy material derived from tar, could be the most viscous fluid. A fluid’s viscosity is measured by determining its internal resistance to gradual deformation by shear forces or tensile stress. As a physical property, it determines the internal resistance of the fluid to flow. It is measured as a physical property of a fluid. Viscosity refers to the state or condition of being viscous.
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