In-depth understanding of kinematic viscosity and dynamic viscosity

In the field of fluid mechanics and materials science, viscosity is a key physical property that describes the flow properties of liquid or semi-solid substances. The two main concepts of viscosity are kinematic viscosity and dynamic viscosity, which are closely related but slightly different in some ways.

1. Kinematic viscosity: Kinematic viscosity is a measure of the internal resistance of a liquid when it flows under the action of gravity. It is usually expressed by the symbol [mu], in the legal measurement unit system, it is expressed by m ²/s, and in the commonly used non-statutory Measurement Units, it is commonly used in mm ²/s, St (s) or cSt (centis). The value of kinematic viscosity can be determined by instruments such as Capillary Viscometer.

2. Dynamic viscosity: Dynamic viscosity is a measure of the internal friction force of a liquid flowing under a certain shear stress. It is usually represented by the symbol η, and the legal unit of Measurement is Pa · second (Pa · s) or milliPa · second (mPa · s). Non-legal Measurement units are commonly expressed in poise (P) or centipoise (cP). Dynamic viscosity can be measured by devices such as digital Rotational Viscometer.

The relationship between these two viscosities generally applies to newtonian fluids, that is, fluids that satisfy the law of newtonian fluid. The characteristic of newtonian fluid is that the Flow velocity is proportional to the shear rate under different shear stresses. In this case, the relationship between kinematic viscosity and dynamic viscosity can be expressed by the following formula:

Dynamic viscosity = kinematic viscosity × Liquid Density

η t = η t × ρt

Where η t represents the dynamic viscosity (in mPa · s) at temperature t, α t represents the kinematic viscosity (in mm ²/s) at temperature t, and ρt represents the Density of the liquid (in g/cm ³).

This relationship shows that in the case of newtonian fluids, dynamic viscosity and kinematic viscosity are related to each other by the density of the liquid. This relationship is important for many engineering and scientific applications, especially in the analysis of fluid flow properties and fluid engineering design.

In conclusion, viscosity is an important physical property used to describe the flow properties of fluids. Kinematic viscosity and dynamic viscosity are two key viscosity concepts that serve different purposes in different applications, but in the case of newtonian fluids, there is a simple linear relationship between them. In-depth understanding and measurement of viscosity is important for research and engineering applications in multiple fields.