High Enthalpy Non-Equilibrium Shock Layer Flows: Selected Practical Applications

Kurzfassung

A space vehicle (re)-entering the atmosphere of Earth or a different planet is subjected to flows that place the vehicle under extreme physical conditions. High temperature effects such as dissociation, vibrational excitation, electronic excitation or gas radiation within the shock layer in front of the vehicle will have to be correctly modelled by computational fluid dynamics (CFD) tools to be used in the framework of the design process of future entry or re-entry configurations. One important step during the development of a CFD code is the validation of the physico-chemical models used to describe the high temperature effects. The strategy generally pursued is to validate these models with data obtained in ground based testing facilities and / or flight tests. The validated CFD tool can subsequently be used for ground-to-flight extrapolation and for the computation of the flow field past (re)-entry vehicles at free flight conditions. Due to the complexity of the physical and chemical phenomena which are observed during (re)-entry, this validation can only be performed for a certain range of the flight trajectory or for a certain range of conditions in ground testing facilities. Even here, due to the large number of phenomena which need to be considered and the interactions between them, the validation of each aspect of the physico-chemical model is very difficult. Due to the fact that the development of in-flight measurement techniques as well as the measurement techniques applied in ground based testing to provide additional and more detailed information about the considered high enthalpy flows is a continuously progressing research field, code validation is not a one step process. It is rather an ongoing interaction between experiment and CFD. The aim of this combined effort is to improve both the knowledge of the flight environment and the facility performance on one hand and the performance of the physico-chemical modelling on the other hand, ultimately leading to a reduction in the uncertainty of predicted flow quantities. Related to this combined experimental / numerical effort, two selected examples are discussed here. The first describes the investigation of the chemical relaxation process in a cylinder shock layer flow in the High Enthalpy Shock Tunnel Göttingen (HEG), and the second is related to the numerical prediction of the radiative heat flux on the Huygens spacecraft during its entry into Titan’s atmosphere.