The kinematic viscosity of water at 20 °C is about 1 cSt (mm2/s).
Kinematic viscosity is a measure of how hard it is for a fluid to flow when it is under the influence of gravity. It is determined by measuring how long it takes for a certain amount of fluid to move through a capillary in a viscometer that has been calibrated and kept at a certain temperature.
Kinematic viscosity has SI units of m2 s−1. The physical unit for kinematic viscosity is the stokes (St), named after George Stokes.
It is sometimes expressed in terms of centistokes (cS or cSt); 1 stoke = 100 centistokes = 1 cm2 s−1 = 0.0001 m2 s−1.
Honey, oil, toothpaste, and some chemical elements like mercury are examples of viscosity.
The viscosity of water, for example, water is low or “thin,” whereas honey has a “thick” or “high” viscosity. The viscosity law is useful in inkjet printing, protein formulations and injections, and even food and beverage production.
At 20 °C, water has a viscosity of 1.0016 millipascals second. The viscosity of water changes with its temperature, and the greater the temperature, the less viscous the water becomes.
Table of Contents
Kinematic Viscosity Formula
The kinematic viscosity of a fluid is the ratio of its dynamic viscosity to its density.
Kinematic (cSt) = Dynamic (cP)/Density
The SI unit of the kinematic viscosity is m2/s. Other units are: 1 St (Stoke) = 1 cm2/s = 10−4 m2/s. 1 cSt (centiStoke) = 1 mm2/s = 10−6m2/s.
It is calculated by measuring the duration in seconds necessary for a set volume of fluid to flow a known distance by gravity via a capillary within a calibrated viscometer at a well-regulated temperature.
This number is translated into scientific quantities such as centistokes (cSt) or square millimeters per second. Viscosity reporting is only valid if the temperature at which the test was performed is also provided, such as 23 cSt at 40 degrees C.
Key Points
| Definition of kinematic viscosity | Kinematic viscosity is the internal resistance of a fluid to flow under gravitational forces. |
| Kinematic viscosity of water | Water at 20 °C has a kinematic viscosity of about 1 cSt (1mm2/s). |
| Unit of Kinematic viscosity | centistokes (cS or cSt) 1 stokes = 100 centistokes = 1 cm2/sec = 0.0001 m2/sec. |
| Significance of kinematic viscosity | Kinematic viscosity is a key fuel attribute that determines the atomization quality and size of the fuel droplet in the spray. |
The Difference between Kinematic Viscosity and Dynamic Viscosity
| Kinematic viscosity | Dynamic viscosity | |
| Definition | Kinematic viscosity is the measure of a fluid’s resistance to flow in the absence of an external force other than gravity. | Dynamic viscosity is a measure of a fluid’s resistance to flow when an external force is applied. |
| Measurement | The kinematic viscosity of a fluid is measured by determining the time it takes to flow through a capillary tube. The time is converted directly to kinematic viscosity using a calibration constant provided for the specific tube. | Dynamic viscosity is measured using rotational viscometers. These instruments rotate a probe in a liquid sample. The force required to rotate the probe or torque is used to estimate viscosity. |
| Symbol | ν | η (eta) |
| Density | Depends upon the density of a fluid. | Does not depends upon the density of a fluid. |
Newtonian and Non-Newtonian Fluids
Newtonian Fluids
The viscosity of a Newtonian fluid remains constant regardless of the amount of shear applied at a fixed temperature. The viscosity and shear stress of these fluids are linearly related. Water, mineral oil, gasoline, and alcohol are a few examples.
Non-Newtonian Fluids
Non-Newtonian fluids are those that do not obey Newton’s law of viscosity. When subjected to strain or stress, the viscosity of non-Newtonian fluids changes from liquid to solid. Ketchup is an example. When you shake a bottle of ketchup, the sauce becomes more liquid. The force given to the ketchup affects its viscosity/flow.
Some other examples of non-newtonian fluids include custard, starch suspensions, corn starch, paint, blood, toothpaste, melted butter, and shampoo.
Summary
- The resistance to fluid flow, when it is under the influence of gravity, is called kinematic viscosity.
- The kinematic viscosity of water is 1 cSt at 20 °C
- The SI unit of viscosity is Pascal seconds (Pa s) which is equal to ten Poise.
- Dynamic viscosity is related to the external force applied to non-Newtonian fluids.
- Kinematic Viscosity is the inherent viscosity of Newtonian fluids, that does not change with a change in applied force.
Frequently Asked Questions
1. What is the kinematic viscosity of air?
The viscosity of air depends mostly on temperature. At 15 °C, the viscosity of air is 1.81 × 10-5 kg/(m·s), 18.1 μPa·s or 1.81 × 10-5 Pa·s. The kinematic viscosity of air at 15 °C is 1.48 × 10-5 m2 /s or 14.8 CST.
2. What is viscous flow?
A viscous fluid is defined as a fluid with high flow resistance. The degree of resistance between the fluid layers is measured by viscosity, which is a fluid factor. Fluids that have no or minimal internal friction resistance are categorized as non-viscous fluids.
3. Laminar flow definition
Laminar flow is a form of fluid (gas or liquid) flow in which the fluid flows smoothly and in predictable patterns. In contrast, turbulent flow is a type of fluid in which the fluid fluctuates and mixes irregularly.
4. What is the viscosity of water and what are SI units of viscosity?
At 20 °C, water has a viscosity of 1.0016 millipascals per second. The viscosity of water changes with its temperature, and the greater the temperature, the less viscous the water becomes.
The viscosity of a fluid is used to describe how difficult it is for a fluid to flow and how difficult it is for an object to move through a fluid. The Pascal second (Pa s) is the SI unit of viscosity, and the Poiseuille is the name for it. The dyne sec/cm2 is used more often and is called “Poise.” One Pa is worth ten Poise. It takes about 0.01 poise, or 0.001 Pa.s, for water to be very viscous at 20°C (Pascal seconds).
The unit of viscosity is newton-second per square meter, which is usually expressed as pascal-second in SI units.
5. Why does oil float on water?
Oil floats on water because its density is lower than that of water. Density in liquids is defined as the amount of mass that may be filled in a cubic meter of volume. Water has a density of roughly 1000 kg/cubic meter, while oil has a density ranging from 800 to 960 kg/cubic meter.
6. What is the kinematic viscosity of water?
The kinematic viscosity of water at 20 °C is about 1 cSt.
The physical unit for kinematic viscosity is the stokes (St), named after George Stokes. It is sometimes expressed in terms of centistokes (cS or cSt); 1 stokes = 100 centistokes = 1 cm2 s−1 = 0.0001 m2 s−1.
7. What is the value of surface tension of water?
Surface tension refers to the ability of a liquid’s surface to resist an external force because of its molecules’ cohesion. At 20 °C (68 °F), water has a surface tension of 0.07275 joules per square meter.
8. Is water vapor a greenhouse gas?
Water vapour is a major greenhouse gas because it absorbs longwave radiation and radiates it back to the surface, contributing to global warming. Furthermore, by weight and volume, it is the most abundant greenhouse gas in the atmosphere.
9. Is water a pure substance?
Pure water is also known as distilled water or deionized water. In distilled water, evaporation removes all of the dissolved substances. As it evaporates, water either distill or leaves the salt behind. Pure water is collected and condensed to produce distilled water.
10. What is portable water?
Water fit for human consumption is defined as potable water (i.e., water that can be used for drinking or cooking). The term implies that the water is both safe and drinkable.
Non-potable water is water that is unfit for drinking but, depending on its quality can be used for a variety of other purposes.
11. What is heavy water?
Heavy water (D2O), also known as deuterium, is water made up of deuterium, a hydrogen isotope with twice the mass of ordinary hydrogen, and oxygen. Heavy water is used in nuclear power plants as a neutron moderator.
12. How many cups are in a gallon?
A US liquid gallon is equal to 16 cups, and a US dry gallon is equal to 18.61 cups. In the United States, one cup equals half a pint (236.6 ml).
To get a more detailed answer, click “how many cups in a gallon.”
More Interesting Topics
Joke of the day
Why did the Kinematic Viscosity of Water break up with the Dynamic Viscosity of Oil?
Because they had different flow rates!
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