Transpiration Cooling for Sharp Leading Edges on Hypersonic Vehicles

Typically, high speed vehicles are designed with smooth bodies for aerodynamic efficiency and to avoid hot spots incurred from steps, cavities and gaps. However, there are many reasons why in a practical vehicle these are unavoidable including:
- Allowance for thermal expansion between different components - for example the tiles of the thermal protection system on the Space Shuttle
- Actuation of control surfaces
- Inclusion of instrumentation - for example for data transmission and guidance of the vehicle
- Tolerance in manufacture and joining of different components
- Attachment points during launch and separation of stages
- Interfaces that are required for separation of deployable components
Entry vehicle design is usually conservative with simple geometric aerodynamic shapes being preferred. However, gaps, interfaces and protuberances may be necessary due to the reasons outlined above. Features like this on the surface of the vehicle can cause various effects such as increased localised heating rates and premature boundary layer transition. Flows like this are difficult to analyse, meaning that any design process needs to carry high safety margins.
Currently, only basic rules of thumb exist for sizing gaps, interfaces and protuberances such that they will not be significant in terms of aerothermodynamic effects. Most commonly, these relate the size of the feature to expected boundary layer length scales and are not always applicable to true flight vehicles.
The aim of this project is to extend the current knowledge base to include more general laws for the effect that these surface features have on the flow over an aircraft body at the conditions expected during flight. Extending the library of available databases and engineering level correlations will enable consideration in these areas to be addressed more accurately in early design stages, ultimately leading to carrying less design margin in later phases. This will be achieved by combining experimental testing with numerical simulations.
The experimental testing will take place in the hypersonic test facilities at the Oxford Thermofluids Institute which have unique European capabilities to perform high Reynolds number and heat transfer experiments. Numerical simulations will primarily take place at Fluid Gravity Engineering (FGE) who will be working in collaboration with the University for this project. FGE is a European leader in the development and application of numerical simulation tools for high speed entry vehicles.
Experiments will consist of aerothermodynamic testing of simplified vehicle models (for example flat plates or cones) featuring steps, gaps and protuberances in different configurations. Models will be highly instrumented for the investigation of various flow effects such as heat transfer and changes in surface pressure in areas of interest. Results from these experiments will be used to produce open generic correlations which can then be included into software for carrying out numerical simulations where they can be used in the design of real vehicles.
This project falls within the EPSRC Fluid Dynamics and Aerodynamics research area.