Shock-induced separation of hypersonic transitional boundary layers

At high supersonic and hypersonic speeds, surface friction and surface heat transfer (kinetic heating) are dominant effects. These are consequences of the boundary layer, the thin layer of retarded flow near the body surface that arises through the action of viscosity. The limit states for a boundary layer are either the smooth-flowing laminar state or the random fluctuations of a turbulent boundary layer. Laminar boundary layers are characterised, amongst other properties, by low heat transfer and low skin friction. In a turbulent boundary layer typical values may be of order five times larger, so that it is critical to understand the likely boundary layer state. The phenomenon of transition is the process by which an initial laminar boundary layer changes to a turbulent one. Transition itself is a pacing item in aerodynamics and many aspects of transition are still not properly understood. A special feature of hypersonic flows is that the transition region may be very substantial in extent; it may cover a significant extent of the body surface so that critical flow interactions then occur in this transitional state. The effect of shock waves provides one such critical interaction. Shock waves, themselves, are an almost inevitable feature of supersonic flows. They arise when the flow is turned in such a manner as to raise its pressure significantly and are features of many practical configurations such as: intakes to air-breathing engines; deflection of control surfaces; the flow approaching leading edges of aerodynamic surfaces. If the pressure rise generated by a shock wave is sufficiently strong it can then generate a so-called separated flow. In this case the boundary layer no longer follows the surface; instead it rises above the surface with a 'trapped' recirculating flow beneath it. This is a complex flow; it may be unsteady and it is characterised, amongst other factors, by surface heat transfer rates that rise significantly above those of the attached boundary layer. Laminar boundary layers are much more readily separated than turbulent boundary layers and produce much larger separation zones. Transitional boundary layers therefore, potentially, occupy a midway state. Given that a transitional boundary layer may, in effect, be switching between laminar and turbulent states, the separated flow is likely to respond strongly to the instantaneous state of the approaching flow.This proposed study is therefore focussed on a benchmark experimental investigation of the separation of a transitional boundary layer by a shock wave. This is an interaction that can commonly occur but which, because of the challenging nature of transitional flows, has so far received only the most modest of attention, either experimentally or computationally. The objective here is to establish both the transitional boundary layer and the shock wave system with as high a precision as possible and in a manner that aids experimental repeatability; this requires the most careful design and management by the experimentalist. The ultimate objective is to generate high quality benchmark data that explains the basic flow physics and produces a well-defined test case for computational simulation and contributes to the engineering design database.

Many technical challenges must be addressed to sustain high-speed flight, ranging from design of air-breathing engines to developing the materials and cooling systems required to resist the intense heat transfer and high temperature environment. A fundamental understanding of the underlying physics is necessary to develop practical solutions for these problems and, in the current project, our goal is to explore a critical and (currently) unpredictable phenomenon that is responsible for the generation of severe aerothermal effects: Shock-induced separation in transitional hypersonic boundary layer. In addition to academia, other beneficiaries of this work include the Defence establishment and DSTL, the European Space Agency and European agencies that work closely with ESA, such as DLR in Germany. These are all involved in developing high flight-speed technologies and we have a long established record of collaboration with them. The data will be published in leading journals and will also be made available on a dedicated webpage to allow easy access to end-users . Apart from the above-mentioned targeted dissemination, we will also participate in the Aeronautics Department Research Colloquium which invites attendees from various UK Aerospace sectors including AIRBUS and QINETIQ. We are also actively pursuing new collaborations with other groups world-wide, including internationally renowned academics and other researchers from NASA, DLR, US AirForce and Japanese Space Exploration Agency. The long-term benefits of the current project to the general public should be recognised. In addition to addressing a critical phenomenon relevant to high-speed flight, thereby maintaining UK's presence as a leader in this area of research, this project also strives to prevent the erosion of hypersonic expertise in the UK. The training of a researcher and a student is crucial in this regard and will ensure a long-term availability of expert knowledge base within the UK in an area that will be critically important in the future. Moreover, high-speed aerodynamics is an attractive field of study and can serve as an inspirational tool in recruiting and retaining the best young minds in the country. Towards this end, we plan to utilise the department open day to showcase our capabilities through wind tunnel demonstrations. We also plan to upgrade our webpage to include general information on the status of hypersonic research and its importance.