Viscosity refers to the internal friction between molecules when the fluid is subjected to an external force. Dynamic viscosity is also referred to as absolute viscosity, simple viscosity, and dynamic viscosity. It refers to the ratio of stress to strain rate. When two plates with an area of 1 square meters and a distance of 1 m are immersed in a liquid, they move relative to each other at a speed of 1 m/s. The internal friction force generated by the interaction between the fluids, the unit is Newton second per square, and Pa·s Pa·s.
What is kinematic viscosity? Kinematic viscosity is also called kinematic viscosity, which refers to the ratio of hydrodynamic viscosity to fluid density ρ, in square meters/s. There are multiple unit expressions for kinematic viscosity, and the conversion unit is converted to: 1㎡/s=10^4St=10^6cSt. cSt is centistokes. The instrument for measuring kinematic viscosity is called a kinematic viscometer.
Kinematic viscosity calculation formula:
The force exerted on the unit area of the liquid layer is called the shear stress τ (N/m2).
Shear rate (D) D=d v /d x (S-1)
Shear stress and shear rate are two basic parameters that characterize the rheological properties of the system
Two parallel fluids with different planes have the same area "A", separated by a distance "dx", and flow in the same direction at different flow rates "V1" and "V2". Newton assumed that the force to maintain these different flow rates is proportional to the fluid's Relative velocity or velocity gradient, namely:
τ= ηdv/dx = ηD (Newton’s formula) where η is the dynamic viscosity, which is related to the material properties.
Newtonian fluid: a fluid that conforms to Newton's formula. Viscosity is only related to temperature and has nothing to do with shear rate. τ is proportional to D.
Non-Newtonian fluid: does not conform to the Newtonian formula τ/D=f(D), and ηa represents the viscosity under a certain (τ/D), which is called the apparent viscosity.
Viscosity is the property that generates internal friction between molecules inside the fluid. Dynamic viscosity refers to the ratio of the internal friction force per unit contact area of the fluid to the rate of change of the flow velocity perpendicular to the direction of movement. Kinematic viscosity refers to the dynamic viscosity at the same temperature. The ratio of the density of the fluid under pressure. Viscosity is usually expressed by kinematic viscosity, and the unit is mm2/s.
Conversion of dynamic viscosity unit:
1 poise (1P)=100 centipoise (100cP)
1 centipoise (1cP) = 1 mPa·s (1mPa·s)
1 mPa·s (1mPa·s)=1000 microPa·s (1000μPa·s)
Conversion of dynamic viscosity and kinematic viscosity
μ=ν·ρ
Where μ---dynamic viscosity of the sample (mPa·s)
ν－－- Kinematic viscosity of sample (mm2/s)
ρ－－- The density of the sample at the same temperature as the measured kinematic viscosity (g/cm3)
Viscosity has a great relationship with the temperature of the fluid, not much with the pressure.
What does the higher the kinematic viscosity indicate? The greater the kinematic viscosity, the greater the viscosity value, and the greater the friction between the internal molecules. When the fluid moves under the action of external force, the energy consumed will be greater. If the external force acts on a certain situation, the distance of fluid circulation will be smaller. Falling ball viscometers, viscosity cups, and Ubbelohde viscometers are based on the characteristics of fluid kinematic viscosity, and use gravity as an external force to promote relative movement between fluids, which can be converted into fluid viscosity values by measuring time. Rotary viscometers and vibrating viscometers both use electrical energy to convert the friction between fluids into resistance or damping, and then convert them into viscosity values and display them on the display of the instrument. The greater the kinematic viscosity, the greater the displayed value, which represents the greater the viscosity of the fluid.