Comparison and analysis of chemical models, methodologies and parameters in hypersonic rarefied flows

The aim of the thesis is the study of some problems of hypersonic rarefied Aerodynamics. More specifically the subjects considered are: a) Development of a new parameter to detect non-equilibrium region. As already pointed out, the identification of non-equilibrium regions is important for an accurate solution of a flow field. More specifically an hybrid code needs a parameter to determine what method (DSMC or CFD), has to be used in the solution of the flow field. In this thesis, a new parameter to detect the non equilibrium region is proposed. This parameter is based on the Crocco theorem. The assumption on which the new parameter relies is that a theoretical relation, based on the hypotheses of equilibrium, as the one of the Crocco theorem, is not verified in non-equilibrium. The new parameter has been computed as the difference of the terms forming the Crocco theorem equation. Thus, one can expect that the higher is non-equilibrium, the larger is the mismatch between the terms and therefore the higher is the parameter. b) Improvement of approximate methods (bridging formulae) for the evaluation of aerodynamic coefficients of a re-entry vehicle in high altitude flight. At the first stage of a design of a re-entry vehicle it could be important to determine in a fast way the aerodynamic forces coefficient. The achieve this goal the well known “bridging formulae” are used. In this thesis a “new” methodology (here called “new” bridging formula) has been developed. The “new” bridging formula, has been successfully tuned to sphere and it has been also tested on two current capsules: EXPERT and ORION. c) Analysis and comparison of several chemical models: 1) peculiar of a DSMC approach such as quantic models (classic and new), Fan-Shen and Bird, 2) “classic” models such as the Gupta and Park models with and without ionization reactions. It is well known that one of the most important problems in the design of a capsule is the evaluation of heat flux during the re-entry. This evaluation has to provide information about the design of the Thermal Protection System (TPS). To this regard it is important to underline that due to the endothermic characteristic of the reactions, the chemical model affects the computation of the heat flux. A very deep analysis about the difference in the computation of heat flux between a direct simulation Monte Carlo code (DS2V) and a computational fluid dynamics code (H3NS) has been carried out. To this purpose a method to implement the Park model in a DSMC code has been developed. d) Application of DSMC codes to evaluate the aerodynamic coefficients of a current capsule (EXPERT) and a future aerospace vehicle (FAST20XX). For the EXPERT capsule computer tests have been carried out in the altitude flight with particular attention to the longitudinal stability of the capsule. As for as FAST20XX, the demonstration and validation of the numerical tools able to predict aero-thermal loads on a space re-entry vehicle at high altitude conditions has been carried out. This goal is fulfilled by the characterization of the DLR low density wind tunnel V2G and by an aerodynamic analysis of the available model of a lifting body.