![]() Interestingly, the viscosity of liquid decreases with increasing temperature, while viscosity of gases increases with increasing temperature. Power requirement to transport (e.g., to pump) a fluid depends strongly on the fluid’s viscosity. Newton’s Law of Viscosity provides a physical definition of viscosity. A high-viscosity fluid has a low tendency to flow, whereas low-viscosity fluids flow easily. He is also an active lecturer at Cradle seminars and training courses and the author of serial articles Basic Course of Thermo-Fluid Analysis.Viscosity, commonly depicted by the symbol μ, is a physical property of a fluid that describes its tendency/resistance to flow. ![]() He is a consulting engineer at Software Cradle and provides technical support to Cradle customers. His doctoral research focused on numerical method for fluid-solid interaction problem. He has a Doctor of Philosophy in Engineering from Osaka University. It may be clearer if you understand that it has the dimension of areal velocity.Ītsushi Ueyama | Born in September 1983, Hyogo, Japan ) ![]() For example, when fanned, the air flow reaches far in contrast, water hardly flows.Īlthough the unit of kinematic viscosity coefficient is quite difficult to memorize, it is. This shows that water is more viscous than air however, air has more ease in traveling. Let us compare air and water at 20☌ and 1 atm (= 101,325 Pa). Magnitude relation between two kinematic viscosity coefficients significantly differ from that between two viscosity coefficients. In contrast, the other fluid with lower density can flow more smoothly because it is lighter. That means the flow has “difficulty in traveling”. A flow propagates to the surroundings through viscous force however, even when two fluids have the same degree of viscosity, in other words, the same degree of ease in traveling, one fluid with higher density has more difficulty to flow because it is heavier. The idea of kinematic viscosity coefficient may be a little bit difficult to grasp. Viscosity coefficient is the degree of shear stress per unit velocity gradient, and its unit is / =. The velocity changes 1.0 over the distance 2.0 therefore, the velocity gradient is expressed as 1.0 / 2.0 = 0.5. Let us see the figure 2.12 as an example. Additionally, the degree of velocity variation against distance is called “ velocity gradient”, and its unit is. This friction is called “ shear stress” and its unit is. Want to know more Units of viscosity and kinematic viscosity coefficientīecause a fluid has viscosity, friction occurs between two fluids or between a fluid and a solid. It is derived by dividing viscosity by density, and its unit is. Here, a measure called “ kinematic viscosity coefficient” is used. Even if brakes at the same power are applied to both, they will stop differently because they have different weights.įigure 2.11 Weight and braking effectivenessĪs we have seen above, to consider how viscosity effects a flow, we need to know the density of the fluid. For example, let us see a bicycle and a truck running at the same speed. Viscosity works as a brake to a flow, and the effectiveness of the brake depends on the density of a fluid. In most cases, liquids have larger viscosity coefficients than gases because links of molecules are tighter in liquids than in gases. When force is applied and relative relation of molecular position is to be broken, molecules try to stay at their fixed positions. We can see the viscosity coefficient of oil is 60 to 80 times as large as that of water. They are approximately 0.001016 for water and 0.06 to 0.08 for oil. Let us compare their viscosity coefficients. We feel that cooking oil is stickier than water at 20☌. Viscosity coefficient is the degree of fluid stickiness and its unit is. 2.6 Viscosity and kinematic viscosity coefficient Fluid
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