**2. The description of multi-physics coupling problem**

As shown in **Figure 1**, the multi-physics coupling problem mainly involves fluid and solid in which a complex physical process takes place between aerothermodynamics within the fluid and thermo-structural dynamics within the solid through a fluid-solid coupling interface. From the view of systematic engineering, the multi-physics coupling problem constitutes a multi-physics, multi-size, and multi-variable coupling system with high coupling complexity. In the coupling system, aerothermodynamic is the active motivation including the coupling of aerodynamic force and aerodynamic heat, while the thermo-structural dynamics is the passive response including the coupling of heat transfer, thermal stress and deformation.

The high complexity of multi-physics coupling problems makes it very difficult to establish a complete coupling model simultaneously considering all the coupling relations and factors. It is necessary to split the problem into different coupling levels according to physical environment characteristics and the engineering application background. For the blunt leading edge and the fuselage with large heat insulation areas with large rigidity, the structural deformation is relatively weak and can be ignored so that the multiphysics coupling problem can reduce to fluid-thermal coupling problem or fluid-solid conjugate heat transfer problem. However, the structural deformation of some structures Modeling and Analysis of Fluid-Thermal-Structure Coupling Problems for Hypersonic Vehicles http://dx.doi.org/10.5772/intechopen.70658 113

**Figure 1.** Coupling mechanism of multi-physics coupling problems for hypersonic vehicle.

vehicles extremely complicated, which is characterized as complex flow fields, high enthalpy and long duration aeroheating with medium/low heat flux. Also, the interaction between the aerodynamic force/aeroheating flux of the external flow field and the heat transfer/thermal stress/deformation and other physical field of the internal physical field in the thermal protection system (TPS) will become extremely strong. Furthermore, the massive application of lightweight flexible materials and large thin-walled structure, especially the flight control rudder and other components will lead to another problem of aerothermoelasticity [3], which should consider the influence of sustained aeroheating. Therefore, the coupling between multi-physics such as flow field, heat and structure should be taken into account for a new-generation air-breathing hypersonic vehicle with the ability of hypersonic long-range

This chapter is to systematically study and analyze the coupling characteristics and mechanism of multi-physics coupling problems such as fluid-thermal-structural coupling of hypersonic vehicle, to construct a reasonable multi-physics coupling model, and to propose effective coupling analysis strategy based on computational fluid dynamics (CFD), computational thermodynamics (CTD) and computational structural dynamics (CSD), so as to provide theoretical support and analysis tools for further study of non-ablative thermal protection, aerothermoelasticity and other key issues. The following sections will focus on the modeling and analysis of several representative multi-physics coupling problems encountered on the

As shown in **Figure 1**, the multi-physics coupling problem mainly involves fluid and solid in which a complex physical process takes place between aerothermodynamics within the fluid and thermo-structural dynamics within the solid through a fluid-solid coupling interface. From the view of systematic engineering, the multi-physics coupling problem constitutes a multi-physics, multi-size, and multi-variable coupling system with high coupling complexity. In the coupling system, aerothermodynamic is the active motivation including the coupling of aerodynamic force and aerodynamic heat, while the thermo-structural dynamics is the passive response including the coupling of heat transfer, thermal stress

The high complexity of multi-physics coupling problems makes it very difficult to establish a complete coupling model simultaneously considering all the coupling relations and factors. It is necessary to split the problem into different coupling levels according to physical environment characteristics and the engineering application background. For the blunt leading edge and the fuselage with large heat insulation areas with large rigidity, the structural deformation is relatively weak and can be ignored so that the multiphysics coupling problem can reduce to fluid-thermal coupling problem or fluid-solid conjugate heat transfer problem. However, the structural deformation of some structures

maneuverable flight in the near-space atmosphere.

112 Advances in Some Hypersonic Vehicles Technologies

fuselage, inlet, and wing of hypersonic vehicles.

and deformation.

**2. The description of multi-physics coupling problem**

with low rigidity is no longer negligible and induces thermal stress and thermal deformation while the fluid-thermal-structural coupling problem mainly characterized as the coupling between aerodynamic force/heat and heat transfer/thermal stress/deformation. In particular, the aerothermoelastic problem behaves more prominently for the large thin-walled flexible structures such as the wings and flight control rudders in which the fluid-thermal-structural coupling should consider the inertial effect and vibration of the structures.

The modeling of so-called multi-physics coupling problem mainly refers to constructing the mathematical-physical model to describe the coupling behavior of the multi-physical fields, namely, the partial differential equation systems (PDEs) to describe the multi-physics coupling problem and the corresponding initial/boundary conditions. And then, the analysis is to solve the partial differential equations by numerical simulation method to obtain the physical properties and behaviors. This modeling and analysis can generally be divided into two different types [4, 5], that is, the monolithic coupling approach and the partitioned coupling approach. According to the characteristics of multi-physics coupling problems, the global strategy for modeling and analysis is shown in **Figure 2**. The monolithic coupling approach is used respectively for the aerodynamic force/heat coupling within the fluid and the thermostructural dynamic problems within the solid. In contrast, the partitioned coupling approach is applied for the fluid-thermal coupling and fluid-thermal-structural coupling problem through the fluid-solid coupling interface.

**Figure 2.** Global strategy for modeling and analysis approaches.
