1, which means that when fluid, gas and liquid flow, if the physical quantities such as pressure, velocity and density at any point in the fluid do not change with time, this flow is called steady flow, which can also be called "steady flow" or "constant flow". On the contrary, as long as any physical quantity such as pressure, velocity and density changes with time, the fluid is in unstable flow or time-varying flow.

2. Resistance: When water particles flow through a cylinder, the speed of flowing through the cylinder will produce friction resistance and pressure resistance, which are collectively called resistance, also called velocity force, and the proportion of friction resistance to the total resistance is often less than 5%. The resistance direction is the same as the flow rate. When the fixed cylinder is perpendicular to the flow velocity, it is generally believed that the fluid resistance is proportional to the square of the flow velocity.

3. Inertia force: There is only one expression of inertia force, but in fact it can be considered that inertia force is composed of two parts. Part of it is purely caused by the pressure gradient of the flow field. The pressure gradient is caused by the accelerated flow of fluid in the field. Because the characteristic scale of cylinder is small, it is considered that the flow of fluid at this time is not affected by the existence of cylinder, that is, when considering the movement of fluid, it is considered that cylinder does not exist. In the scale range of cylinder diameter, the pressure gradient can be reasonably regarded as a constant.