Investigating the Effects of Surface Topography and Roughness on Turbine Aerodynamic Performance

Reducing the SFC of aircraft engines has become an increasingly significant task in order to achieve Advisory Council for Aeronautics Research in Europe's (ACARE) 2050 flightpath plan to reduce aircraft emissions. A part of this challenge is the requirement to improve the overall aerodynamic efficiency of gas turbines by increasing machine stage efficiencies. Within aeroengines, entropy is a useful measure of inefficiency and approximately one third of the entropy generated within an aeroengine turbine is associated with the "aerodynamic friction" between the air flow and the blade surfaces. A major contributor to the rate of entropy production within a boundary layer is the "skin friction" between the air and blade which depends strongly on the roughness (topography) of the blade surface. When the roughness elements protrude through the laminar sublayer, the perturbations resulting from the roughness determines the rise in skin friction. In the rare instance that roughness is accounted for, the capabilities of determining the impact on the skin friction are constrained by insufficient correlations utilising coarse measurements of the centreline averaged roughness height, Ra, as the singular responsible geometric parameter. A surface with the same Ra can have various streamwise roughness distributions where the surface is akin to rolling hills or steep mountains. This determines whether the laminar sublayer perturbs over the topography or is protruded through. An added dimension to the PhD problem is the presence of a pressure gradient in turbine blades and particularly the region of diffusion on the suction surface.
The aim of this PhD is to develop understanding of the relationship between surface topography (roughness) and aerodynamic loss. One of the deliverable of this research will be the development of correlations for the effects of surface topography that can be applied to all blade surfaces (i.e engine-run, novel ceramic matrix composites and as-manufactured). An improved understanding of how surface roughness affects skin friction is not only important to current designs but essential for the successful implementation of new materials and manufacturing processes. The PhD will take an experimental and numerical approach. A flat-plate liner working section in the Rhoden wind tunnel (situated in the Low Speed Whittle Lab) will be used to simulate scaled-up engine representative boundary layer, allowing the impact of the surface topography on skin friction to be measured. A numerical tool will be used to determine which surfaces to be 3-D printed and experimentally tested. This will be the main method used to determine correlations which can be applicable to all real turbine blade surfaces. To broaden the scope of the research, high fidelity computational fluid dynamics (in particular, large eddy simulation) will be performed to see if the fluid-structures responsible for the roughness induced aerodynamic loss can be captured.

1. Impact on the UK Aero-Propulsion and Power Generation Industry
The UK Propulsion and Power sector is undergoing disruptive change. Electrification is allowing a new generation of Urban Air Vehicles to be developed, with over 70 active programmes planning a first flight by 2024. In the middle of the aircraft market, companies like Airbus and Rolls-Royce, are developing boundary layer ingestion propulsion systems. At high speed, Reaction Engines Ltd are developing complex new air breathing engines. In the aero gas turbine sector Rolls-Royce is developing UltraFan, its first new architecture since the 1970s. In the turbocharger markets UK companies such as Cummins and Napier are developing advanced turbochargers for use in compounded engines with electrical drive trains. In the power generation sector, Mitsubishi Heavy Industries and Siemens are developing new gas turbines which have the capability for rapid start up to enable increased supply from renewables. In the domestic turbomachinery market, Dyson are developing a whole new range of miniature high speed compressors. All of these challenges require a new generation of engineers to be trained. These engineers will need a combination of the traditional Aero-thermal skills, and new Data Science and Systems Integration skills. The Centre has been specifically designed to meet this challenge.

Over the next 20 years, Rolls-Royce estimates that the global market opportunities in the gas turbine-related aftercare services will be worth over US$700 billion. Gas turbines will have 'Digital Twins' which are continually updated using engine health data. To ensure that the UK leads this field it is important that a new generation of engineer is trained in both the underpinning Aero-thermal knowledge and in new Data Science techniques. The Centre will provide this training by linking the University and Industry Partners with the Alan Turing Institute, and with industrial data labs such as R2 Data Labs at Rolls-Royce and the 'MindSphere' centres at Siemens.

2. Impact on UK Propulsion and Power Research Landscape
The three partner institutions (Cambridge, Oxford and Loughborough) are closely linked to the broader UK Propulsion and Power community. This is through collaborations with universities such as Imperial, Cranfield, Southampton, Bath, Surrey and Sussex. This will allow the research knowledge developed in the Centre to benefit the whole of the UK Propulsion and Power research community.

The Centre will also have impact on the Data Science research community through links with the CDT in Data Centric Engineering (DCE) at Imperial College and with the Alan Turning Institute. This will allow cross-fertilization of ideas related to data science and the use of advanced data analytics in the Propulsion and Power sectors.

3. Impact of training a new generation of engineering students
The cohort-based training programme of the current CDT in Gas Turbine Aerodynamics has proved highly successful. The Centre's independent Advisory Group has noted that the multi-institution, multi-disciplinary nature of the Centre is unique within the global gas turbine training community, and the feedback from cohorts of current students has been extremely positive (92% satisfaction rating in the 2015 PRES survey). The new CDT in Future Propulsion and Power will combine the core underlying Aero-thermal knowledge of the previous CDT with the Data Science and Systems Integration skills required to meet the challenges of the next generation. This will provide the UK with a unique cohort of at least 90 students trained both to understand the real aero-thermal problems and to have the Data Science and Systems Integration skills necessary to solve the challenges of the future.