# Dynamic Modeling and Full-Process Simulation of the Core Engine Test Rig Main Test System
## 1 Introduction  
With the advent of the industry 4.0 era, the concept of a digital twin is gradually being applied to aerospace, automotive, military, energy, and other industries [1]. It is consensual that a digital twin is a virtual representation of a specific physical asset. A virtual representation means a machine learning model and/or a physics-based computational model [2]. Modeling methods based on machine learning do not need to consider the physical laws specially and can be used to solve complex engineering problems when the data is sufficient. [PERSON] et al. [3] used machine learning-based LightGBM algorithms to build a digital twin model of a petrochemical factory, which provides a solution to control problems in petrochemical production. However, machine learning models are often limited to the training domain and cannot be used to design a new system. In addition, they usually need more data for the training (calibration) to compensate the absence of an initial mathematical structure (materialized as a physical law). Therefore, it is more appropriate to establish a digital twin model using a modeling method based on physical laws for engineering problems with limited amount of experimental data and focusing on physical changes during the entire work process of physical entities. [PERSON] et al. [4]used COMSOL to establish a multiphysical simulation model of mangoes, as a digital twin of cold chain transportation of fruits, and studied the thermal behavior of mangoes in the entire cold chain. [PERSON] et al. [5] imported the passive biped robot structure drawn by CAD into the multibody software (MSC Adams) and established the multibody dynamics simulation model as the digital twin of the passive biped robot. Combined with the actual model of the passive biped robot, the design of the passive biped robot was completed. In the aerospace field, more attention is paid to the parameters and physical changes in the real-time working process of complex systems such as satellites and engine pipeline systems, and the establishment of complex simulation systems based on the physical laws is the key technology of digital twins. [PERSON] et al. [6] used Modelica based on the object-oriented declarative modeling language to establish a multi-domain integrated satellite system as a digital twin. Combined with the actual satellite system, fault diagnosis and health monitoring were realized. The modeling methods based on the laws of physics can be roughly divided into two types by application object: one is the high-dimensional modeling and simulation of a single component, such as the nozzle [7] in a liquid rocket engine system and the compressor [8] in an aircraft engine system, and this type of research generally uses computational fluid dynamics (CFD) software; the other is the simulation of the whole or partial system, and this type of research usually uses a one-dimensional (1D) or even zero-dimensional (0D) simplified model. It focuses on the overall characteristic of the system and the interrelationships among various components rather than the physical process within any single component. Common special or commercial software tools include GFSSP [9; 10], Amesim [11; 12], EcosimPro [13], Flowmaster [14] and so on. To meet the requirements of multipurpose general modeling, the system simulation usually follows the ideology of modularization, that is, to segment the system into several independent functional modules, and then combine them into a numerical model of the whole system according to certain rules. All the above-mentioned software for system simulation follow such modeling idea [15].