Analysis of weakly ionized hypersonic flow

Pişkin, Tuğba
The knowledge of physical phenomena and numerical computation skills are required to simulate flow field around the re-entry capsule due to extreme conditions resulted by high speed and high energy of capsule. The re-entries of spacecraft occur generally at high hypersonic speed where reactions take place actively, and where plasma field around geometry as well as radiation from the surface affect characteristics of the flow field. In these conditions, ideal gas assumption fails; thus, real gas assumption is used to define species state characteristics. The main consideration of this study is to compute the components of the flow field by considering thermal, chemical and vibrational nonequilibrium. There are various methods developed to model flow field with the examination of real gas effects and nonequilibrium in each modes. Mainly, these methods differ according to chosen temperature model; the simplest one is one temperature model, where all energy modes are defined with the same temperature. Because the assumptions for one temperature are not sufficient to characterize hypersonic flow field region, the most accepted and the most applied method is two temperature model by defining translational – rotational and vibrational- electronic temperature. In this model, dissociation reactions and vibrational energy levels of species are associated with Park’s approach. Different forward reaction rate constants and two different calculation of equilibrium constant are examined and compared. Moreover, the influences of number of species and reactions are also studied in the modelling chemical nonequilibrium. Three dimensional Apollo Command Module is chosen as geometry. Newton- GMRES method is used to solve flow field, whereas Newton method is used to compute vibrational and translational temperature. In the numerical modelling part, different CFD considerations are examined such as mesh refinement, order of spatial discretization and different splitting of flux vectors. For the mesh refinement study, flow field is simulated by using three different meshes; coarse, medium and fine. In addition to first order discretization, MUSCL schemes are used to achieve higher order spatial discretization; therefore, different flux limiters are examined to prevent oscillations. Moreover, Van Leer and Steger Warming flux splitting schemes are developed with the consideration of real and ideal gas assumptions. Briefly, the aim of this study is to enhance knowledge about high temperature gas flow and to apply various models with comparisons of each other and that of experimental data..