A model is a simplified description of a real world thing, phenomenon, process, or system, or the imitation of some of its properties. In the general sense, it refers to the prototype made by imitating the shape of the physical object or the structure in the design, and its size can be divided into reduced type, physical type and large. Some models are exactly like the real thing down to the details, while others just mimic the main features of the real thing. The significance of the model is that it can understand the image of the physical object through vision. In addition to the value of art appreciation, it also has great utility in education, scientific research, industrial construction, civil construction and military. With the progress of science and technology, people regard the object of research as a system and study it from the whole behavior. This kind of systematic research is not to enumerate all the facts and details, but to identify the factors and interrelationships that have a significant impact in order to grasp the essential laws. Various models can be established for the studied system by means of analogy and abstraction. This is called modeling. Models can take many different forms, and there is no uniform classification principle. According to the form of the model, it can be divided into physical model, mathematical model and structural model.

Physical model

Also known as solid model, it can be divided into physical model and analog model. ① Physical model: according to the similarity theory, the physical object made according to the scale of the original system (it can also be enlarged or the same size as the original system), such as the aircraft model in the wind tunnel experiment, the hydraulic system experiment model, the building model, the ship model, etc. (2) Analog model: in different fields of physics (mechanical, electrical, thermal, fluid mechanics, etc.) the respective variables in the system sometimes obey the same law, according to this common law can be produced physical meaning completely different analogy and analogy model. For example, under certain conditions, the pressure response of a pneumatic system composed of a throttle valve and a gas volume has similar rules to the output voltage characteristics of a circuit composed of resistors and capacitors, so the pneumatic system can be simulated by a circuit that is easier to experiment.

Mathematical model

A class of models described in mathematical language. A mathematical model may be one or a set of algebraic, differential, difference, integral, or statistical equations, or some appropriate combination of them, by which the interrelationships or causal relationships between the variables of a system are described quantitatively or qualitatively. In addition to mathematical models described by equations, there are also models described by other mathematical tools, such as algebra, geometry, topology, mathematical logic, etc. It should be noted that the mathematical model describes the behavior and characteristics of the system rather than the actual structure of the system.

Structural model

The model mainly reflects the structural characteristics of the system and the causal relationship. One important class of structural models is graph model. In addition, the atrioventricular models commonly used in biological system analysis also belong to structural models. Structural model is an effective means to study complex system.