The novel nondestructive testing (NDT) methodology described in this proposal will develop and demonstrate the application of guided ultrasonic waves for structural health monitoring (SHM) in complex, multi-layered structures, such as aircraft wings. The potential for the rapid and cost-efficient permanent monitoring of large surface areas of complex technical structures will be shown, allowing faster and more frequent inspections and monitoring at a lower cost, thus improving the reliability and safety of the inspected structures for a large number of industries. The proposed programme of work will tackle the real NDT problem in aerospace industry of the detection and monitoring of fatigue cracks in aircraft and demonstrate the proposed novel SHM technology for aerospace industry. In addition, fundamental research on guided ultrasonic waves propagation in complex, multi-layered structures will be conducted, laying the base for the application of the methodology for applications in a range of industries important to the UK economy, such as oil and gas exploration.Aircraft and other technical structures are subject to cyclic loading, e.g., during take-off, landing, manoeuvring, and adverse weather conditions. Such operating conditions can lead to the development of faults during the lifecycle of the structure. The skin of aircraft wings consists of multi-layered structures, connected using fasteners and rivets. Due to the stress concentration at the bolt holes of the fastener, fatigue cracks can start to develop from the edge of the hole during the service life of the aircraft. This damage can lead to the malfunction and ultimately failure of such structures, endangering lives. Therefore the integrity of such structures needs to be tested regularly or monitored using NDT methods. The novel methodology proposed here will result in the rapid and cost-efficient inspection and permanent monitoring of large surface areas of complex technical structures using guided ultrasonic waves. Guided waves can propagate over large distances in thin structures, allowing faster and more frequent inspections and monitoring of large surface areas at a lower cost, thus improving the reliability and safety of the inspected structures for a large number of industries. The significant, step change proposed here is to work on real, complex, multi-layered aircraft structures and to investigate and find ways to deal with the complexity of the guided wave propagation and scattering. Fatigue crack growth during cyclic loading will be monitored. Appropriate measurement equipment for guided waves in multi-layered structures will be developed and tested. The sensitivity and reliability of the proposed method for the detection of fatigue cracks at fastener holes in the different layers will be investigated and ascertained. Due to the propagation of the energy of the guided wave along the structure, the potential exists to improve the detection sensitivity in the middle (2nd) layer in such a multi-layered structure and to reduce inspection time and thus costs. The technology will be applied to real aircraft components and structures, and practical application experience gained, demonstrating the potential for the increase of inherent safety and cost reduction in further applications. In addition to the verification of the proposed SHM technology, fundamental research on guided ultrasonic waves propagation in complex, multi-layered structures will be conducted, laying the base for the application of the methodology in other important industries.

The potential for exploiting synthetic jet actuators to delay and control boundary-layer separation in conditions akin to those on aircraft components operating in high-load conditions has attracted much interest in recent years. However, the fundamental mechanism by which synthetic jets interact with incipiently separated turbulent boundary layers subjected to strongly adverse pressure gradient is yet to be fully understood before cost-effective operational flow-control solutions can be sought. This proposal seeks funding for a joint programme of work between groups at Manchester University and Imperial College London, which would exploit complementary strengths and facilities at the two universities. The programme aims to employ a combined experimental (Stereo PIV and other conventional measurement techniques) and computational approach (LES and LES/RANS hybrid modelling) to study the detailed interaction mechanisms, so as to derive generically valid guidelines on optimal separation control in a practical setting. The outcome of the research would be of value to both the academic community and aerospace industry, the latter striving to evolve engineering solutions to flow management with a minimum of moving parts and energy input.

The accurate and efficient estimation of aircraft loads is an important problem in the aerospace industry. The extreme values of these loads are typically the key parameters used for sizing the aircraft's structure and, therefore, have a significant influence on the performance of an aircraft. Inaccurate estimations of these loads will lead to poor design choices, most typically a conservative structure design that is overweight, increasing energy consumption, toxic emissions, and noise pollution. This research will develop tools for the efficient identification of uncertainties and nonlinearities in aircraft structural components and investigate how they affect aircraft loads. Only nonlinearities and uncertainties in the joints will be considered in this project, but the developed framework can be used for other sources of nonlinearities and uncertainties, such as aerodynamics and flight control systems. Developing such a framework is crucial in order to overcome the predominant practice whereby aircraft designs are restricted to a deterministic and linear regime, despite rapidly increasing demands to consider unavoidable uncertainties and nonlinearities from technological, economic and safety considerations. In this project, a detailed and sophisticated numerical model of an aircraft structure will be used to explicitly identify the possible range of variations in the dynamic response due to geometric and material uncertainties and nonlinearities of a wing-store joint. These data will then be used to identify the parameters of reduced-order models (ROMs). ROMs are extremely useful for computational efficiency, and are used to efficiently predict extreme unsteady loads. The validated reduced-order structural model will be incorporated into the aeroelastic model and will allow the designer to include the effects of nonlinearities and uncertainties in the process of load calculations. The goal is to move away from the conventional safety-factor-based approach and explore the wide horizon of stochastic nonlinear aeroelastic analysis. This project will be carried out in partnership with Airbus, which will provide the data necessary to undertake the research. Since it has been observed that multiple sources of nonlinearities and uncertainties affect aircraft performance, the study aims to investigate the following questions: 1- How can detailed numerical models be used in the identification of sources of nonlinearities and uncertainties? 2- How can the structural model updating tools (the tools that will be used to validate the numerical model based on experimental data or detailed finite element predictions) be used to obtain ROMs that are capable of including the nonlinear and uncertain effects estimated by detailed numerical models? 3- How can these validated ROMs be used to efficiently investigate the effects of uncertainties and nonlinearities in aircraft load predictions (important parameters that have significant impact on aircraft performance)? The aforementioned questions will be answered by considering an important source of nonlinearities and uncertainties in the aircraft structure, i.e. structural joints.

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.

Air transportation is becoming more accessible to a greater number of people who can afford to travel by air. The air transportation sector forecasts that passenger and freight traffic will increase at an average rate of 4-5% per annum over the next two decades, leading to a doubling of the aircraft fleet by 2034 with respect to 2015. Formidable progress has been made since the introduction of jet-propelled aircraft about 65 years ago, but much of this improvement is offset by the huge increase in air traffic today. The Advisory Council for Aviation Research and Innovation in Europe (ACARE) "FlightPath 2050" calls for reductions of 75% in fuel burn, 65% in perceived noise, and 90% in oxides of nitrogen emissions by 2050 compared to the year 2000. NASA has proposed similar goals in the US for the "N+2" (service-entry 2025) and "N+3" (service-entry 2030-2035) generations of aircraft. These environmental constraints have generated a large effort to reduce the aerodynamic drag and to generate more efficient engines. A small reduction in fuel burn multiplied by the total number of transport aircraft leads to a significant reduction of emissions into the atmosphere. The FALCon project will develop new methods and tools to design a new generation of transport aircraft that take advantage of wing flexibility to improve the global aircraft efficiency. The project is specifically developed around a long-standing industrial challenge: assessing the impact of aerodynamic changes on the structural loads used for structural sizing of aircraft components is an expensive process (from several weeks when low-order methods are employed, to several months when employing higher-fidelity methods). Generally, the computing cost of an aeroelastic analysis is largely dominated by the aerodynamic analysis. The novelty of the FALCon project lies in the development of a computationally-efficient aerodynamic solver that reduces the computational time by, at least, 80% compared to current state-of-the-art methods. The FALCon project will contribute to: - Improved realism of predictions earlier in the aircraft design process, reducing the risk of not meeting customers' expectations and shortening the time to bring new aircraft on the market. - Reduce current conservatism in aircraft design by performing dynamic (time-domain) aeroelastic analysis, which are today neglected because of the computational costs. - Implement a paradigm change in aircraft design through the development of enhanced design tools for conceptual and preliminary phases, including aero-servo-elastic design constraints from the start of the design process. A new design paradigm is proposed, shifting the structural deformability from a performance limitation to a design opportunity. - A close collaboration with industry guarantees that the outcomes of the project will feed back to the UK and European aviation sector, strengthening its competitiveness. A seamless integration within the existing industrial design process is expected, adding no complications to current procedures.