Determining the Effects of Competing Instabilities in Complex Rotating Boundary Layers

Fluid mechanics is of fundamental importance and underpins key developments in a range of disciplines, including aerospace, defence, energy and environmental research. For example, the efficient use of fuel has become an increasingly important factor in civil aviation, with the International Air Transport Association (IATA) committed to "reducing fuel consumption and CO2 emissions by at least 25% by 2020, compared with 2005 levels". Concurrently, aircraft engine noise has become a real and growing environmental issue, especially in the vicinity of airports around the world.

Given the global demand for increased air travel, energy efficient and quieter aeroengines are important targets for the aviation and aerospace industries. Aerodynamic improvements have the potential to contribute to the design of the next generation of energy-efficient aeroengines and tubomachinery. Specifically, an increased understanding of the underlying physics through active flow control can reduce aerodynamic drag and delay the transition to unstructured, turbulent flow, both of which are known to have negative implications for fuel consumption and noise emissions. The goal of delaying turbulent-transition can be achieved in a cost-effective way through detailed numerical simulations, as opposed to comparatively expensive experimental investigations. This project will help to achieve that goal by applying a novel computational approach that can model flow within the boundary layer over complex rotating geometries.

The boundary layer, a thin layer of fluid confining its viscosity close to a bounding surface, can influence the aerodynamics and drag characteristics of a fluid flow in a profound and significant way. Historically, the boundary layer flow over a rotating disk was used to model air flow over a swept-wing due to the similarity between their velocity profiles. Today, continuing developments in aeroengines, turbomachinery, spinning projectiles and, more recently, electrochemical applications, has created the need to understand boundary-layer flows over rotating bodies, such as disks, spheres and cones. Indeed, rotating 3D boundary-layer flows are now known to exhibit numerous flow characteristics, governed by highly complex and often competing mechanisms that cause flow instability and eventual breakdown to turbulent flow. For a family of rotating cones, experiments have observed a continuous change of flow characteristics as the governing flow parameters are altered. The applicant has shown that this change arises from an interaction between various forces governing flow within the boundary layer. However, the nature of this competing interaction remains largely unknown. With this in mind, this research project will develop a complex computational modelling code capable of providing robust and accurate quantitative predictions of the interaction between competing flow instability mechanisms in the turbulent-transition process. Such predictions can help to accelerate aerodynamics research, and inform or form part of innovative flow control strategies to reduce drag, thereby improving fuel consumption, as well as decreasing harmful noise and CO2 emissions.

The long-term objective is to inform the advancement of innovative flow control strategies to reduce drag in a range of aerodynamic, industrial and environmental applications, through a complex modelling capability that provides accurate and robust numerical simulations of realistic models. Given the proposed benefits of hydrodynamic stability theory in generating precise, cost-effective predictions compared with experiments, along with revealing flow control and drag reduction strategies, the potential environmental and economic benefits (resulting from reduced noise, CO2 emissions and fuel consumption) are significant. The beneficiaries and impact pathways directly arising from this project, which are crucial to the long-term objective, are outlined below.

Academic beneficiaries: Impact activities are vital in the short term to achieve longer term economic and societal impact. Aerodynamics research scientists/engineers will benefit directly from the findings of the proposed research, which will inform the development of experimental methodologies and active flow control techniques and thereby accelerate the development of the next generation of noise- and fuel-efficient aeroengines. In addition to dissemination through aerospace journals/conferences, the impact of this research will be promoted through forging new collaborative links with experimental scientists/aerodynamics researchers/engineers in the UK, via the PI's links with the UKFN, and internationally. This will likely result in cross-disciplinary research partnerships that will serve to strengthen the UK's competitiveness and reputation as a world-leader in aerodynamics research. It is also possible that the spectral stability code output from this project will advance the state-of-the-art in computational tools and form part of active flow control strategies on rotating-body flow applications, for use by researchers and engineers.

Commercial beneficiaries: The findings of this project will be of interest to the commercial developers of aeroengine and turbomachinery components in aerospace, aviation and industrial applications. Companies manufacturing aeroengines and turbofan components (such as Airbus, Rolls-Royce and Dyson UK) will be interested in the outputs of this project (including the developed stability codes), which can offer insights into their research and development of improved airflow characteristics for rotating aerodynamics applications. The PI will work closely with the Research and Knowledge Exchange (RKE) department (an MMU organisation dedicated to supporting the transition of academic enterprises into the commercial and public sectors), as well as with the UKFN Special Interest Groups, to explore and initiate potential research commercialisation opportunities. A one-day workshop will also be organised towards the end of this project to which the relevant commercial and academic parties will be invited. Along with fostering external collaboration, this event will explore the potential commercial exploitation and industrial benefits of the project outputs.

Young researchers: Through his departmental role in the delivery of the MMath programme, the PI undertakes significant research-led teaching at Master's level, which is targeted at developing and training successful PhD students in fluid mechanics. The economic and societal impact of inspiring young people to become scientists/applied mathematicians is clearly a critical impact pathway. In this role, impact activities will include a PhD project on the PI's research into boundary-layer stability theory employing asymptotic methods, which is not only directly related to this project but also funded by his School. Such activities will be complemented by the ongoing online dissemination and feedback of the project's findings via the PI's research group's social networking sites, Twitter, Facebook, and YouTube, as well as the research group's homepage.