This viscometer can adjust the turning speed of the probe as it moves in the liquid, and the viscometer detects the variation in the sample’s viscosity as the speed. By measuring the force or torque, viscosity is determined. To measure dynamic viscosity, the rotational viscometers are one of the more popular instruments used, and in the liquid sample, these instruments rotate a probe. The dynamic viscosity = shearing stress/shearing rate change. The measure of fluid’s resistance to shear flow when some external force is applied is known as dynamic viscosity.įor describing the behavior of fluids under stress, it is useful, and also in describing non-Newtonian fluids by observing how viscosity changes as the shear velocity changes, it is mostly used. The tangential force required to move one horizontal plane of a fluid to another is called dynamic viscosity. In SI units, dynamic viscosity units are well established as mPa-s, and the most common kinematic viscosity units are cm2/s. The kinematic viscosity tells how fast the fluid moves when a certain force is applied, while dynamic viscosity gives you information on the force needed to make the fluid flow at a certain rate. The ratio of dynamic fluid viscosity to its density is called kinematic viscosity, and the unit of measurement is m2/s.Īny two different fluids will never have the same kinematic viscosity because of density difference but can have the same dynamic viscosity. This value must be converted back to 1.004 x 10^-6 m²/s for use in calculations.The ratio of shear stress to shear strain is called dynamic viscosity, and usually, it is measured in centipoise (cP), and Pa-s is the unit of measurement. Water at a temperature of 20☌ has a viscosity of 1.004 x 10^-6 m²/s When the equations are rationalized the fluid head term is eliminated leaving the units of Kinematic viscosity as area / time.ġ.0 m²/s = 10000 Stokes = 1000000 Centistokes The motive force driving the fluid out of the cup is the head of fluid, which is also contained within the equation that makes up the volume of the fluid. Kinematic viscosity value of the fluid in Centistokes (cSt). The timings can be used in a formula to estimate the Viscosity can be measured by timing the flow of a known volume of fluid from a This value must be converted back to 1.002 x 10^-3 Pa Temperature of 20☌ has a viscosity of 1.002 Centipoise. Pascals can be combined with time (seconds) to define dynamic viscosity.Ĭentipoise (cP) is commonly used to describe dynamic viscosity because water at a The Pascal unit (Pa) is specifies pressure, or stress = force per area Laminar flow will occur where the Reynold's number is less than 2300.įrom the above equation it can be determined that the dimensions of viscosity are force multiplied by time divided by length squared or FT/L² The units of viscosity in the English system and the SI system are: To determine whether the flow is laminar or turbulent requires that you calculate the Reynold's number of the flowing fluid. However, the above equation is not applicable for turbulent flow where a large amount of the shear stress is due to the exchange of momentum between adjacent layers of the fluid. This results in the important equation for fluid shear for viscous or laminar flow: The derivative dv/dy is called the velocity gradient. Viscosity is usually denoted by the Greek symbol μ (mu) and is defined as the ratio of shearing stress τ (Greek letter tau) to the rate of change of velocity, v, which in mathematical terms can be expressed as dv/dy (where this is the derivative of the of the velocity with respect to the distance y). Fluid Viscosity Properties Fluid Viscosityįluid Viscosity, sometimes referred to as dynamic viscosity or absolute viscosity, is the fluid's resistance to flow, which is caused by a shearing stress within a flowing fluid and between a flowing fluid and its container.
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