Extending the buffet envelope: step change in data quantity and quality of analysis

Next-generation aircraft are likely to require significant changes in technology to meet ambitious targets on fuel burn, CO2, NOX and noise emissions. Integrated computer-aided engineering is a key enabler to mitigate the risk coming with disruptive change and new design concepts. Moreover, the long-term vision of digital aircraft design and certification, to reduce reliance on wind tunnel and in-flight testing, requires leaps in highest-fidelity flow simulation. We revisit a grand challenge of aircraft aerodynamics using both state-of-the-art industrial and next-generation simulation tools enabled for the identification of coherent flow structures targeting the mechanisms leading to transonic wing shock buffet and constituting the instability, which despite intensive research efforts remains controversial. Shock buffet manifests itself as a flow instability in high-speed flight with detrimental effects on the aircraft performance, economic efficiency, and ultimately passenger safety. A vast amount of literature on flow instability exists, yet analysis of practical flows relevant to the aerospace industry is limited and often confined to simplified cases.
Two key technology demonstrations provide the background to the work. The first is a recent global stability analysis of transonic shock buffet flow with three inhomogeneous spatial directions on an industry-relevant test case using an industry-grade computational fluid dynamics (CFD) solver suite and a Reynolds-averaged Navier-Stokes (RANS) aerodynamic model. However, high confidence in industry-standard CFD solutions is given only in a small region in the operating flight envelope near the cruise point due to the unavailability of general models to predict turbulent separated flow. Hence, the second recent key achievement is high Reynolds number direct numerical simulation (DNS) of supercritical transonic aerofoil flow, which also provides access to global modes.
The premise of the work programme is that significant new elements, relying on high-performance computing and advanced numerical flow analysis, are in place to develop next-generation buffet prediction schemes suitable for next-generation transonic wings. We investigate global and resolvent mode analysis across the range of aerodynamic models (from RANS to DNS) applied to low-drag configuration (swept, laminar flow, supercritical) aerofoils and wings, culminating in a modern long-range, wide-body aircraft wing geometry. The practical aim is to develop robust, cost-effective methods to determine the buffet boundary of the wings of the future. Along the way, we will learn more about the physics of shock-induced unsteadiness and the mechanisms leading to shock buffet in the flow around transonic wings.

Fluid dynamics and aerodynamics is a key area to many UK-based industries. For instance, the UK aerospace sector, the world's second-largest, directly employs a highly skilled workforce of more than 120,000 people across the country in high value jobs. Airbus in the UK has been responsible for the wing design for the last 40 years. It is critical, particularly now facing post-Brexit uncertainty, to maintain this strong position in the potential aftermath of such a European company's trans-national reorganisation. The research programme underlines how UK academics are instrumental in Airbus' strong research position. Along with physics-focussed leadership, we demonstrate the key role of high-performance numerical analysis in science and engineering, and the critical access to the number one national supercomputing facility can signify the importance of such strategic research-infrastructure investment to the decision makers.
The research unequivocally contributes to the UK's prosperity with direct outcomes in a more productive, more resilient and more connected nation, and indirectly, in the long-term, also to a healthier nation. The scientific insight gained from the research programme can define a step change in technology for aircraft manufacturers, but not only, to enable the virtual design and certification of the wings of the future with superior aerodynamic characteristics. Such bring along more lightweight structures, enhanced flight performance, more economic aircraft operation with reduced fuel burn, and lower CO2, NOX and noise emissions to benefit a sustained future of air transport. Besides the aerospace sector, the envisaged data quantity and quality of analysis of turbulent flow will benefit, amongst many, automotive, energy and consumer-goods industries just as well.
On a scientific level, the outcomes of the research programme will benefit academics working on questions of stability of complex physical systems. Despite vigorous work spanning several decades of effort and a vast amount of literature on fluid stability theory, analysis of practical flows relevant to industry is still limited and often confined to low Reynolds number flows with canonical geometrical complexity. Our approach will shed light on the urgent questions surrounding the topic in dealing with scale-resolved high Reynolds number turbulent flow and in pushing the capability through to practical engineering problems. This will allow the engineering and scientific community to step beyond the mere prediction and understanding of complex system behaviour and towards enabling the design and control of desirable engineered solutions.