Fluid is defined as the physical body,able to change its shape with an arbitrarily small impact on it. Usually there are two main types of liquids: droplet and gaseous. Drip liquids are liquids in the usual sense: water, kerosene, oil, oil, and so on. Gaseous liquids are gases that, under normal conditions, are, for example, gaseous substances such as air, nitrogen, propane, oxygen.
These substances differ in molecularstructure and mind of the interaction of molecules with each other. However, from the point of view of mechanics, they are continuous media. Because of this, some general mechanical characteristics are determined for them: density and specific gravity; as well as basic physical properties: compressibility, thermal expansion, tensile strength, surface tension forces and viscosity.
Under the viscosity understand the property of a liquid substance.to resist sliding or shifting its layers relative to each other. The essence of this concept consists in the appearance of a friction force between different layers inside a liquid during their relative motion. There are concepts of "dynamic viscosity of a liquid" and its "kinetic viscosity". Next, we consider in more detail what is the difference between these concepts.
Basic concepts and dimension
Internal friction force F arising betweenmoving relative to each other adjacent layers of the generalized fluid, is directly proportional to the speed of movement of the layers and the area of their contact S. This force acts in a direction perpendicular to the movement, and is analytically expressed by the Newton equation
F = μS (∆V) / (∆n),
where (∆V) / (∆n) = GV is the velocity gradient in the direction normal to moving layers.
The coefficient of proportionality μ is the dynamic viscosity or simply the viscosity of the generalized fluid. From Newton's equation, it is
μ = F / (S ∙ GV).
In the physical system of measurement unit viscosityis defined as the viscosity of the medium in which, at a unit velocity gradient of GV = 1 cm / sec, a friction force of 1 dyn acts on every square centimeter of the layer. Accordingly, the unit dimension in this system is expressed in din д s ∙ cm ^ (- 2) = g ∙ cm ^ (- 1) ∙ s ^ (- 1).
This unit of measurement of the dynamic viscosity is called the poise (P).
1 P = 0.1 Pa ∙ s = 0.0102 kgf ∙ s ∙ m ^ (- 2).
Smaller units are also used, namely: 1 P = 100 cP (centipoise) = 1000 mP (millipoise) = 1000000 μP (micropoise). In the technical system, the unit of viscosity is the value of kgf ∙ s ∙ m ^ (- 2).
In the international system, the unit viscosityis defined as the viscosity of the medium in which, with a unit velocity gradient of GV = 1 m / s per 1 m, a friction force of 1 N (newton) acts on every square meter of the fluid layer. The dimension of the value of μ in the SI system is expressed in kg ∙ m ^ (- 1) ∙ s ^ (- 1).
In addition to such characteristics as dynamicviscosity, for liquids, the concept of kinematic viscosity is introduced as the ratio of the coefficient μ to the density of the liquid. The coefficient of kinematic viscosity is measured in stocks (1st = 1 cm ^ (2) / s).
Viscosity coefficient is numerically equal to quantity.motion carried in a moving gas per unit of time in a direction perpendicular to the movement through a unit of area, when the speed of movement differs per unit speed in gas layers spaced apart by unit length. The viscosity coefficient depends on the type and state of the substance (temperature and pressure).
Dynamic viscosity and kinematic viscosityliquids and gases are heavily dependent on temperature. It was noted that both of these coefficients decrease with increasing temperature for droplet liquids and, conversely, increase with increasing temperature for gases. The difference in this dependence can be explained by the physical nature of the interaction of molecules in droplets and gases.
Physical sense
From the point of view of molecular kinetic theory,the phenomenon of viscosity for gases is that in a moving medium, due to the random motion of molecules, the velocities of the various layers are equalized. So, if the first layer moves in a certain direction faster than the second layer adjacent to it, then faster molecules move from the first layer to the second, and vice versa.
Therefore, the first layer tends to accelerate the movementthe second layer, and the second - to slow down the movement of the first. Thus, the total amount of movement of the first layer will decrease, and the second one will increase. The resulting change in the amount of movement is characterized by a viscosity coefficient for gases.
В капельных жидкостях, в отличие от газов, internal friction is largely determined by the action of intermolecular forces. And, since the distances between the droplet liquid molecules are small compared to gaseous media, the interaction forces of the molecules are significant. Molecules of a liquid, as well as molecules of solids, oscillate near equilibrium positions. However, in liquids these positions are not stationary. After a period of time, the liquid molecule drastically moves to a new position. In this case, the time during which the position of the molecule in the liquid does not change is called the time of its "sedentary life."
The forces of intermolecular interaction significantlydepend on the type of fluid. If the viscosity of a substance is small, then it is called "fluid", since the flow coefficient and dynamic viscosity of a liquid are inversely proportional to magnitude. Conversely, substances with a high viscosity coefficient may have mechanical hardness, such as resin. The viscosity of a substance in this case significantly depends on the composition of impurities and their quantity, and on temperature. With an increase in temperature, the amount of time "sedentary life" decreases, as a result of which the mobility of the liquid increases and the viscosity of the substance decreases.
The phenomenon of viscosity, like other phenomenamolecular transfer (diffusion and thermal conductivity), is an irreversible process, leading to the achievement of an equilibrium state corresponding to the maximum entropy and minimum free energy.
