Hypersonic modelling and testing of space debris during planetary entry
Keywords: heat transfer experiments
space debris
atmospheric re-entry
hypersonic
machine learning
wind tunnel
direct simulation Monte Carlo

In the study of atmospheric entry, some of the most challenging conditions to model numerically manifest in the slip and transition regimes. Denser than free molecular gases and sparser than continuum flows, these regimes are typically characterised by Knudsen numbers in the range 0.001 <= Kn <= 10. The complex interaction between gas particles in these sparse flows make the determination of aerothermal phenomena especially difficult, as traditional Navier-Stokes based CFD solutions cease to be accurate, and the Newtonian-style flow which characterises the free molecular regime does not manifest until Kn ~ 10.
This thesis investigates this complex regime in the context of destructive satellite re-entry. Modern space debris demise codes often utilise an "object-oriented" approach to modelling intricate spacecraft during atmospheric entry, whereby simple geometric shapes such as cuboids, spheres, and cylinders are used to represent electronics, tanks, reaction wheels, etc. This approach is widespread in the debris demise community, allowing many current codes to utilise tumble-averaged aerodynamic and blunt-body heating correlations to assess the aerothermal loads acting upon the aforementioned debris analogues. However, while these methods are extremely computationally efficient, they tend to suffer from inaccuracies inherent in simplifying such complex phenomena into compact expressions. This in turn increases the likelihood of errors in estimations of the survivability of debris during atmospheric entry.
There are several alternate approaches which may be used to calculate the aerothermal loads in debris re-entry calculations, many of which are capable of providing greater insight into these loads than the aforementioned tumble-averaged and blunt body correlations. These methods differ in both complexity and accuracy, ranging from panel methods (which are simple enough to be evaluated using hand calculations) all the way to large scale CFD and DSMC simulations (which typically require computation on powerful workstations or large clusters). Additionally, it must also be noted that there is currently very little experimental data regarding the aerothermal behaviour of sharp-edged geometries in these regimes. This prevents experimental validation of numerical methods specific to debris re-entry problems, hence further increasing uncertainty.
The present work addresses this uncertainty in multiple ways. Firstly, panel methods are used to generate high resolution aerodynamic profiles for common debris shapes (cylinders and cuboids) with varying freestream Mach and Knudsen numbers, aspect ratios, and angles of incidence relative to the flow. These are then correlated using various techniques in order to create highly detailed aerodynamic models which may be queried in an extremely computationally efficient manner. Hence, the first source of inaccuracy mentioned – specifically, the use of tumble-averaged aerodynamic correlations – is addressed and mitigated.
Secondly, high fidelity simulations of the flow around cylindrical debris analogues are performed using DSMC codes, allowing the thermal distributions on these shapes to be studied. These data are then compared with experiments performed at the same conditions in Oxford's Low Density Wind Tunnel (LDWT), whereby heat transfer coefficients are compared between simulation and experiment. Various correlation techniques are also utilised here in order to normalise and reproduce surface distributions of heat transfer coefficient. As such, the second and third sources of uncertainty, i.e. the used of blunt body correlations and a lack of experimental validation, are similarly mitigated.