No two fluids can have the same kinematic viscosity, but they can have the same dynamic viscosity.
When external forces are applied to a fluid, dynamic viscosity measurements are the best choice.
Whenever a fluid is not subjected to physical forces other than gravitational force, kinematic viscosity is the right measurement.
In fluids, the resistance to flow or to deform is known as viscosity
In fluids, the resistance to flow or to deform is known as viscosity. Fluids differ in the internal resistance that is exerted, causing differences in viscosity as well. The viscosity of fluids can be expressed in terms of either dynamic viscosity or kinematic viscosity. To distinguish dynamic viscosity vs. kinematic viscosity, it is important to note that no two fluids can have the same kinematic viscosity, but they can have the same dynamic viscosity. In this article, we will compare the dynamic vs. kinematic viscosity of fluids.
Dynamic vs. Kinematic Viscosity
The absolute or dynamic viscosity is the ratio of shear stress to shear strain. The dynamic viscosity divided by the density of the fluid gives kinematic viscosity. The dynamic viscosity measures the resistance offered by the fluid to the flow when an external force is applied.
When the resistance to flow is measured under the weight of gravity, then the viscosity is called kinematic viscosity. The kinematic viscosity gives the inherent resistance of the fluid to flow when there is no external force applied to it except gravity.
The dynamic viscosity of two different fluids can be the same. However, there is no way that the kinematic viscosity of two different fluids can be the same due to the differences in their density. The table above compares the properties of dynamic viscosity vs. kinematic viscosity.
The Dynamic and Kinematic Viscosity of Water
There are several engineering properties for water, including thermal expansion, isothermal compression, thermal conductivity, specific heat, specific entropy, specific enthalpy, saturation vapor pressure, dynamic viscosity, and kinematic viscosity. The viscosity of liquid water decreases with an increase in temperature. The table below compares the dynamic viscosity and kinematic viscosity of liquid water from 0℃ to its normal boiling point.
Choosing the Proper Measurement
When a fluid changes its characteristics upon the application of force or pressure, it is a non-Newtonian fluid. For such fluids, determining the internal resistance is important to calculate the force required to move the fluid. In such a fluid application, dynamic viscosity should be measured.
The other important application requiring dynamic viscosity measurements is the design of pumping systems. As the speed of fluid movement varies with pump velocity and pressure, dynamic viscosity measurements are necessary for such fluid systems. When external forces are applied to the fluid, dynamic viscosity measurements are the best choice.
In Newtonian fluids, the viscosity does not change with applied force or shear rate. The changes in viscosity under different temperatures or environmental conditions are evaluated and are useful for the effectiveness of fluids such as gasoline, lubricating oil, and glycerine. Whenever the fluid used is not subjected to physical forces other than gravitational force, kinematic viscosity is the best measurement.
Measurement Methods for Dynamic and Kinematic Viscosity
Dynamic Viscosity Measurements
The most commonly used instrument to measure dynamic viscosity is a rotational viscometer. Rotational viscometers are commonly used to measure the viscosity of non-Newtonian liquids. The peculiarity of non-Newtonian fluids is that there is a change in the viscosity depending on the conditions to which they are exposed. A probe is present in the rotational viscometer, which is rotated in the liquid sample. The dynamic viscosity is determined by measuring the torque or force
needed to turn the probe. The speed of turning the probe can be adjusted in the rotational viscometer as it moves in the liquid. With the changing speed, which is proportional to the shear rate, the rotational viscometer detects variations in viscosity.
Kinematic Viscosity Measurements
To measure kinematic viscosity, the capillary tube method is used. This method converts the time taken by the fluid to flow through the tube into kinematic viscosity. The calibration constant provided for the capillary tube is used for the time-to-kinematic viscosity conversion. Distinguishing between the dynamic vs. kinematic viscosity is important when the fluid application is subjected to only external forces or gravitational forces, respectively.
When solving complex fluid flow problems, a clear understanding of the viscosity of a fluid is necessary. You can solve fluid flow problems using Cadence’s suite of CFD software. With these tools, it is easier to run CFD simulations in complex fluid-dependent systems that facilitate fluid flow modeling.