Using air viscosity (μ) of 185 × 10- 5 N s/m 2 at 25 °C, an entrance velocity ( v0) of 5.3 cm/s, a PP web with a thickness of 0.3 mm and basis weight of 30 gsm, a pressure drop of the air across the web of 2.8 mmH 2 O, and a PP having a polymer density of 0.91 g/cm 3, Eq. 2.2 can be used to calculate the porosity of the web as 89%.
- The kinematic viscosity is the dynamic viscosity divided by the density of the fluid. In cgs units the unit is called the centistoke. In cgs units the unit is called the centistoke. The viscosity of water decreases smoothly from the freezing point while the density remains essentially constant except for the small maximum in density near 4°C.
- The value of the dynamic viscosity coefficient is found to be a constant with pressure but the value depends on the temperature of the gas. Sutherland provides an equation for the dependence on temperature T: mu = mu0. ((T / T0)^1.5). ((T0 + 198.72) / (T + 198.72)).
We all have a feel for viscosity. More viscous a fluid moredifficult for it to flow. Oils flow at a slower rate than water.We understand viscosity as a property that tends to retard fluidmotion.But we do have a more rigorous definition of viscosity,which can be developed from the thought experiment describedbefore. It was seen that when a shear force is applied to thetop plate the fluid undergoes a continuous deformation( What is a Fluid? Fig.1.4). As a result the block of fluidabcd deforms to ab'c'd after a time t. Letthe speed of the top plate be U. It is an important property offluids that the layer of fluid adjacent to a solid surface moveswith the same velocity as the solid surface. This is called the'No Slip' condition. Accordingly fluid layer closer to the topplate moves with a speed U while that closer to the lower plate isat rest. Thus the velocity of the fluid varies continuously fromzero on the lower plate to U at the upper plate. In other words avelocity gradient develops in the fluid. In the simple case of theflow between parallel plates this is a linear profile. Thevelocity gradient is given by
where h is the distance between the two plates. In a small instant of time we find that the upper platehas moved by a distance bb' which is equal to . Now Noting that for solids the shear stress isproportional tostrain while for fluids it isproportional to rateof strain, , which in turn is defined as
.Substituting for we have Since is proportional to , wehave or It is found that for common fluids such as air, waterand oil therelationship between shear stress and velocitygradient can beexpressed as,
The constant of proportionality is an important property offluids in determining the flow behaviour and is calledDynamic Viscosity or AbsoluteViscosity. It is usual to refer to it as justViscosity. It has the dimensions andunits of in the SI system. Fluids for which the shear stress varies linearly as rate ofstrain are called Newtonian Fluids. Many of the commonfluids belong to this category- air, water. When the relationshipbetween shear stress and rate of strain is not linear, the fluidis designated Non-Newtonian. Examples of thiscategory are some of the industrial fluids such as plastics,sludge and biological fluids such as blood. Typical plots of shearstress vs rate of strain are shown in Fig.1.5. Rheologyis the branch of fluid mechanics which specialises in thesefluids. We consider primarily common fluids such as water and airand hence restrict ourselves to Newtonian fluids. Viscosity of a fluid is strongly dependent on temperature and is aweak function of pressure. For example, when the pressure of airis increased from 1 atm to 50 atm, its viscosity increases only byabout 10 percent allowing one to ignore its dependence onpressure. Fig.1.6 shows the manner of dependence ofviscosity on temperature for some of the common fluids. It is seenthat the viscosity of liquids deceases with temperature while thatfor the gases increases with temperature. This difference inbehaviour is explained by the cohesive and intermolecular forceswithin the fluid. Liquids are characterized by strong cohesiveforces and close packing of molecules. When temperature increasescohesive forces are weakened and there is less resistance tomotion. Hence viscosity decreases. With gases, the cohesive forcesare very weak and the molecules are spaced apart. Viscosity is dueto the exchange of momentum between molecules as a result ofrandom motion. As the temperature increases the molecular activityincreases giving rise to an increased resistance to motion or inother words viscosity increases.
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Online calculator, figures and tables showing dynamic (absolute) and kinematic viscosity of air at temperatures ranging from -100 to 1600°C (-150 to 2900°F) at pressure ranging from 1 to 10 000 bara (14.5 - 145000 psia) - SI and Imperial Units
The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.
For further definitions, go to Absolute (dynamic) and kinematic viscosity. Absolute or dynamic viscosity is used to calculate Reynold's Number to determine if a fluid flow is laminar, transient or turbulent.
Tabulated values and viscosity units conversion are given below the figures.
Online Air Viscosity Calculator
Viscosity Of Air
The calculator below can be used to calculate air dynamic or kinematic viscosity at given temperatures and atmospheric pressure.
The output dynamic viscosity is given as Pa*s, N*s/m2, cP, mPa*s, lbf*s/ft2 and lbm/(ft*h),
while the kinematic viscosity is given as cSt, m2/s, and ft2/s
Note! Temperature must be within the ranges -100 - 1600 °C, -150 - 2900 °F, 175 - 1900 K and 310-3400 °R to get valid values.
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