I introduction of fluids
Fluid is a general term for liquid and gas. It has fluidity, compressibility and viscosity. A fluid composed of a large number of molecules that are constantly moving thermally and have no fixed equilibrium position has certain compressibility. The compressibility of liquid is small, while the compressibility of gas is large. When the shape of the fluid changes, there is also a certain movement resistance (viscosity) between the layers of the fluid.
When the viscosity and compressibility of fluid are very small, it can be approximately considered as an ideal fluid, which is an ideal model for studying the movement and state of fluid.
Second, quality and density.
Like other substances, liquids have mass and weight. The mass per unit volume of fluid is called fluid density, which is expressed by ρ. If the density of any point in a fluid is the same, the fluid is a uniform fluid, and the density of a uniform fluid is expressed as ρ = m/v. For a non-uniform fluid, the density of each point is different.
Fluid characteristics and fluid classification:
1, fluid characteristics
At rest, the active surface of a solid can bear both shear stress and normal stress. However, fluid can have both normal stress and tangential stress only when it is in motion, and can only bear normal stress, that is, compressive stress or static pressure when it is at rest.
Solids will deform under the action of force, and the relationship between deformation and force obeys Hooke's law within the elastic limit, that is, the deformation of solids is proportional to the magnitude of force. The angular deformation velocity of fluid is related to shear stress, and the relationship between laminar flow and turbulent flow is different. In laminar flow, the relationship between them obeys Newton's law of internal friction.
2, fluid classification
According to the viscosity of fluids, fluids can be divided into two categories, namely ideal fluids and actual fluids. All fluids existing in nature are viscous, which are collectively called viscous fluids or actual fluids. A fluid without viscosity at all is called an ideal fluid. This fluid is just a hypothesis, but it doesn't actually exist. However, it is of practical significance to introduce the concept of ideal fluid.