Adapting to non-carbon energies requires major changes in the architecture of transportation systems. In the case of an aircraft, this applies to numerous subsystems such as the propulsion or air conditioning unit. These developments are reflected in the components themselves and push the limits of some components, such as turbomachinery. The electrification process requires high-efficiency operating ranges for turbines and compressors that go beyond the current state of the art. The same difficulties are encountered in supplying air to fuel cells, which is holding up development.

The ANR CASTOR industrial chair is a research programme shared between Liebherr Aerospace, a recognised supplier of air systems to the aeronautics industry, and the DAEP at ISAE-SUPAERO. The aim is to increase the operating flexibility and performance of radial turbomachines, which are widely used in several industrial sectors because they are compact and robust, but whose geometric configuration leads to highly complex flows. In particular, we need to work on the operating limits of compressors, whose operability is limited by physical constraints, such as the compressibility of the gas, and above all by flow instabilities. This is a long-term issue in the specialist scientific literature, on which we need to make progress to better characterise and then push these  limits.

For decades, turbines have suffered from a strong bias towards the classical sizing approach, which is very effective for a single specification, but loses its validity when the specification becomes multiple, as required by more electric architectures. The detailed analysis of rotors dimensioned in an unconventional way is a good starting point for making this diagnosis, and adjusting the methods, correlations and models, while taking into account variable geometry devices, which ensure operating flexibility.

The scientific approach is based on the experience of the department's TMP (Turbomachines and Propulsion) team, which actively contributes to research work on turbomachinery flows and the associated methodologies, whether numerical or experimental. But the department's other skills are also put to good use, such as its expertise in numerical simulation methods, including multiphysics (aero mechanical and thermal), and flow stability analysis.

Two test benches enable advanced metrology to be deployed, giving access to the local structure of the flow. They are also used to work on metrological issues, which, in such a constrained environment, is a research subject in its own right. The same methodological work is being carried out on dimensioning tools and numerical simulations. The Chair's location and organisation means that the DAEP and Liebherr teams work closely together on these different subjects.

As a result, a team of around twenty people at the two sites and pooled testing resources contribute to this 4-year project. Direct costs of €1.25m are being split by the ANR and Liebherr. The increased flexibility expected from radial turbomachinery will help to meet aircraft electrification requirements. It will also contribute to the development of fuel cells and heat recovery cycles. It will encourage the emergence of new system architectures, which could have an impact on other industrial sectors.

Context and Research Objectives

ISAE-SUPAERO conducts advanced research in aeronautical propulsion and energy to support the sector's transition. Since 2016, its Aerodynamics, Energetics, and Propulsion department has been developing innovative architectures to improve the energy performance of aircraft. Thirteen patents have been filed in collaboration with Safran, reflecting a strong industrial base. At the same time, an exergy approach has been developed to quantify the recoverable energy in the interactions between the propulsion system and the airframe. This method, integrated into the Epsilon open-source software, is now used by several players in the aeronautical sector.

Technical Developments: the Multifan Concept

Since 2017, ISAE-SUPAERO has been working on a multifan system, a propulsion architecture that distributes thrust over several independent fans. This approach makes it possible to increase the engine bypass ratio while respecting ground clearance constraints. The main advantages of the multifan are:

• Improved propulsive efficiency and reduced drag
• Optimised interaction with the wing, improving stability and energy performance
• The possibility of thrust vectoring, reducing the need for a tail fin

2D and 3D CFD simulations have demonstrated aerodynamic benefits, notably via shock ingestion and vortex interaction, leading to several patents.

Exergy Applied to Aerodynamics

The exergy approach aims to quantify the useful energy recoverable from engine-wing interactions. Unlike conventional architectures, where the propulsion and airframe are separate, the multifan allows for greater synergy, in particular by exploiting the wake and boundary layers. Exergy distinguishes:

• Recoverable energy (exergy), which can be used to improve efficiency
• Lost energy (anergy), due to thermodynamic irreversibilities

These analyses have been validated experimentally in wind tunnels, demonstrating their relevance for optimising aeronautical configurations.

Numerical Tool: Epsilon Software

The exergy concepts have been integrated into the free Epsilon software, a plugin for Paraview. This tool enables data from numerical simulations and experimental tests to be analysed using an exergy approach. Since it was made freely available in 2022, Epsilon has been used by manufacturers such as Airbus and Safran, as well as by several international universities, reinforcing its impact on aeronautical research.

Student Training and Prospects

ISAE-SUPAERO is incorporating these advances into its training courses, particularly through a new teaching module on exergy in aerodynamics launched in 2023. This 20-hour course combines theory and practical applications. At the same time, a flying prototype project, supported by Airbus, was launched in 2022 with around ten students. Its aim is to design an A350 model equipped with the multifan system, offering a unique opportunity for applied learning.

Conclusion

The multifan and exergy approach developed by ISAE-SUPAERO provide innovative solutions for the future of aeronautical propulsion. Thanks to technological and numerical advances, this work is making an active contribution to energy optimisation in the sector.

The Institute is also pursuing its mission of excellence in education, integrating these innovations to prepare future engineers for the challenges of sustainable aeronautics.

The Genesis

When the SUPAERO and ENSICA research teams merged, the researchers working on the simulation of compressible flows chose to pool their activities in numerical methods and high-fidelity modelling of turbulent flows in the same calculation tool, on unstructured meshes, with the aim of retaining significant autonomy in the development of methods and models, but also maintaining the capacity to simulate more applied configurations in a massively parallel context.

The following paragraphs detail the methodological developments, as well as the projects aimed at producing results and studying the physics.

Today, this code is widely used in the department, by all three groups, although the D2F group remains the main developer and user.

In the department, 10 researchers are at least users, six of whom are developers or supervise developments.

Since 2023, an engineer has provided specific development and productivity support for research activities, in particular portability to external computers.

Methodological Developments

Given the main objectives (LES or DNS type simulation of compressible flows), the basic structure of the code is an unstructured mesh partitioning architecture with explicit Runge-Kutta type integration and non-blocking communications. The initial hybrid centred/upwind scheme with various sensors (shock sensors in particular) was completed by a complete restructuring compatible with SD and FR spectral schemes, by Lamouroux (2016) and then Saez-Mischlich (2021). At the same time, Saez-Mischlich (2021) has developed both Finite Volume (FV) and spectral (SD) ALE functionalities for moving meshes (here undeformed) and sliding meshes, which open up applications to rotors and turbomachinery. A specific development (in VF) by Rolandi (2021) makes it possible to force a stationary flow (Selective Frequency Damping method) and carry out a stability analysis using a Krylov/Finite Difference method, as well as carrying out a Floquet analysis for the secondary stability of a periodic flow.

Simulation at the Service of Physics

These developments have resulted in a reliable, high-performance tool for producing results for the physical analysis of flows. These include

• Supersonic flows: Shock wave/turbulent boundary layer interaction on academic configurations, with forcing (vortex generator) or on more applied configurations (supersonic air inlet)
• Turbulent flows on airfoils and rotors, for performance prediction, acoustic sources (e.g. effect of cavities, perforated walls) and identification of aeroacoustic coupling
• The stability of lift-off or wake flows at low Reynolds numbers
• Pulsed flows in turbines

Projects

From a methodological point of view, although spectral methods have been used in supersonic configurations, this is a particularly sensitive and severe situation for these high-order methods. A thesis is in progress to optimise the robustness/accuracy trade-off in shock capture situations in turbulent flows. The stability module is also being developed to include the adjoint operator and provide access to optimisation and control methodologies.

The main medium-term applications are supersonic flows with shock, in air inlets and, in the future, in nozzles and jets.