State-to-State Transport in Hypersonic Entry Flows

Ana Catarina Garbacz Gomes

Key information

Authors:

Ana Catarina Garbacz Gomes (Ana Catarina Garbacz Gomes)

Supervisors:

Maria Luís Grácio Bilro Castela (Maria Luís Grácio Bilro Castela); Mário António Prazeres Lino da Silva (Mário António Prazeres Lino da Silva)

Published in

September 13, 2018

Abstract

This work studies nonequilibrium hypersonic flows surrouding space reentry vehicles. The design of such vehicles and its associated thermal protection systems relies on the accurate modelling of transport phenomena. In this thesis, dissociation and vibrational energy transfer relaxation processes are treated using a state-to-state kinetics approach. A numerical study is carried out to assess the impact of using state-specific transport coefficients in CFD simulations of hypersonic external flowfields. The Gupta-Yos/CCS macroscopic transport model, available in the SPARK code, has been transposed to state-to-state species, considering in a first approximation that state-specific transport properties are equivalent to the macroscopic ones. Additionally, a state-dependent collisional cross-section model has been implemented, taking into account the increase of collision cross-sections for vibrationally excited molecules. The code was successfully applied to the simulation of a 7 km/s nitrogen flow past a sphere. Different physical models were considered: Euler and Navier-Stokes state-to-state, Navier-Stokes one and two-temperature. Considering transport phenomena in a state-to-state approach resulted in smoother variations of flow properties, with a 15% lower peak temperature and similar shock position. Comparing the viscous state-to-state simulations with the two-temperature ones, the state-to-state approach yielded respectively a 10% and 5% larger shock standoff distance and peak temperature. A comparison of the standard state-to-state transport model with the one considering enchanced collision cross-sections yielded minimal differences in the obtained results, in agreement with the assumptions from the litterature. The effects of dissociation and vibrational excitation processes were confirmed to be emphasised for higher freestream temperatures.