When measuring the fluid flow in a tube, it is often necessary to understand its flow state and velocity distribution. **Reynolds number** is an important parameter to characterize fluid flow. The ratio of the inertial force Fg and the viscous force (internal friction force) Fm when the fluid flows is called the **Reynolds number** . Represented by the symbol Re. Re is a dimensionless quantity. Similar criteria for the effect of viscosity on fluid mechanics. Named in honor of O. Reynolds and recorded as Re. Re = ρvL / μ, ρ and μ are fluid density and dynamic viscosity, and v and L are characteristic velocity and characteristic length of the flow field. For outflow problems, v and L are generally taken from the forward flow velocity and the main size of the object (such as wing length or ball diameter); for inflow problems, the average flow velocity and channel diameter in the channel are taken. **Reynolds number** represents the ratio of inertial force to viscous force acting on a fluid micelle. If the Reynolds numbers of two geometrically similar flow fields are equal, then the ratio of inertial force to viscous force of the corresponding micelles is equal. The smaller the Reynolds number means the more significant the effect of the viscous force, the greater the greater the effect of the inertial force. Flows with a small Reynolds number (such as those in a lubricating film) have viscosity effects throughout the flow field. **For** the flow with a large **Reynolds number** (such as the general aircraft flow), its viscosity effect is only important in the boundary layer or the wake near the object surface. *Reynolds number* is the main similarity criterion in fluid mechanics experiments involving the influence of viscosity. However, the Reynolds number of many model experiments is much smaller than the Reynolds number of the real object. Therefore, it is an important subject of fluid mechanics experimental research to study the correction method and the development of high Reynolds number experimental equipment. **The** small **Reynolds number** means that the viscous force between the particles is dominant when the fluid is flowing. The particles of the fluid flow regularly parallel to the inner wall of the pipeline and are in a laminar flow state. A large Reynolds number means that the inertia force is dominant, and the fluid is in a turbulent flow state. Generally, the pipeline **Reynolds number** Re <2000 is a laminar state, Re> 4000 is a turbulent state, and Re = 2000 ～ 4000 is a transition state. Under different flow conditions, the fluid's movement law and velocity distribution are different, so the ratio of the average velocity υ to the maximum velocity υmax of the fluid in the pipeline is also different. Therefore, the size of the Reynolds number determines the flow characteristics of the viscous fluid. When the external conditions are geometrically similar (geometrically similar tubes, fluid flowing through geometrically similar objects, etc.), if their **Reynolds numbers** are equal, the fluid flow states are also geometrically similar (hydrodynamic similarity). This similarity is the basis for the standardization of flow **measurement throttling devices** . |