Hypersonic aerothermodynamics of satellite demise
Creators: name: Rees, Thomas W
Publisher: name: Imperial College London
Year: 2020

description: The goal of the work presented in this thesis is to improve the aerothermodynamic heating models used in modern satellite re-entry analysis tools. The current generation of heating models are usually based on correlations developed for shapes such as flat plates and hemispheres. These overly simplified models can lead to inaccurate predictions of the ground casualty risk associated with a re-entry event. In order to derive new heating models for shapes more representative of satellite geometries, this work uses a combination of CFD and wind tunnel measurements to study the heat fluxes experienced by a cuboid at two different orientations in a Mach 5 flow and at a range of Reynolds number conditions. 2D and 3D CFD simulations showed that the hypersonic flow around a cuboid geometry has a strong dependence on Reynolds number, with a breakaway separation bubble forming from the windward expansion edge at very high Reynolds numbers. This separation bubble can significantly lower the heat fluxes underneath it. In addition to this separation bubble, the 3D CFD simulations showed that the sharp corners on a 3D geometry can generate streamwise vortical structures along the streamwise edges of a cuboid. The surface heat fluxes induced by these structures can be as high as the stagnation point heating value and could therefore play a significant role in satellite fragmentation during re-entry. Finally, the CFD simulations suggested that the highest heat flux values experienced by the cuboid occur at the sharp edges and corners of the geometry. However, the heat flux predicted by CFD in these regions is seemingly non-physically large. The CFD simulations were experimentally validated with wind tunnel measurements of the Stanton number distribution over a cube. The Stanton number measurements were obtained by recording the temperature history of the wind tunnel model using infrared thermography and then calculating the convective heat flux using a 3D inverse heat conduction solver. The experimental Stanton number values generally showed very good agreement with the CFD results, and comparisons of both the stagnation point Stanton number and the average heating experienced by the cuboid were favourable. However, in contrast to the CFD results, the experimental measurements did not show any region of significantly increased heat flux near the sharp edges of the cuboid. We propose that this is likely due to a breakdown of the continuum assumption in these regions, which cannot be captured with conventional CFD. The combined CFD and experimental results were then compared to two currentgeneration satellite re-entry prediction tools, DRAMA and SAM. These comparisons showed that DRAMA overpredicts the average heat flux to a cuboid by 65-75% depending the flow conditions and cube orientation. On the other hand, the average heat fluxes predicted by SAM agreed well with the experimental and CFD values. Despite SAM’s success at predicting average heat flux values, comparisons of Stanton number distributions over the surface of the geometry showed that the tool did not predict the regions of increased heat flux associated with the streamwise vortex structures generated by the 3D expansions around the cube corner, while at the same time over-predicting the heat flux to other regions of the cube geometry. Future generations of re-entry prediction tools will need to be able accurately predict the Stanton number distributions across entire satellite geometries. This is important because different fragmentation phenomena may occur depending on which satellite components fail first.