15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
Authors
Abstract
In recent years, multiple studies have focused on an innovative aircraft design space defined by boundary layer ingestion (BLI) propulsive systems with hybrid-electric powertrain. A key knowledge gap in BLI aircraft performance prediction is modelling of interactions between the propulsive system and the airframe since the coupling between the two is highly non-linear. Previous works by the authors presented a framework for modelling of a small, low-loading propulsor on the wing suction side near the trailing edge of a small aircraft based on Daher TBM 900. The resulting performance is characterised in terms of system-level metrics such as lift, drag or power consumption. While the preceding developments were done for a single propulsor configuration, the objective of the current paper is to present the advances made in modelling of a pair of propulsors in order to study their interaction and any other emerging phenomena. To that end, the study employs Body Force Modelling (BFM), a medium-fidelity approach that represents a turbomachinery stage by the azimuthal mean effects it exerts on the fluid, transcribed as source terms in the mass, momentum and energy conservation equations. That way, computation time-saving benefits of avoiding blade geometry meshing and computation thereof are leveraged; this comes at the cost of losing visibility of the blade-scale phenomena which are averaged out. It nevertheless remains a perfect candidate for this study since the information on the propulsor effect on the flow, which depends on the local flow characteristics, is preserved throughout the propulsor assembly. Models of such assemblies are commonly controlled by a parameter representing the rotor rotational speed. In this work a different formulation was developed, which enables the user to control the power input of the propulsor under consideration instead. As a result, it is now also possible to study coupling of multiple wing propulsors, with each at a different power level. Thus, deeper insight can be gained into questions often asked in the context of distributed propulsion systems, notably the feasibility of loaded wind-milling for energy harvesting. The presented simulations were performed using the CFD software StarCCM+, selected for its powerful unstructured mesher that simplifies grid generation around complex geometries. For the investigated test cases the results align with intuitions developed so far, indicating a gain in lift, a loss in overall drag and an increased operability. A strong coupling between two adjacent propulsors is demonstrated. The developed power-based formulation is demonstrated to be more suitable than the rotational speed when investigating propulsor energy recovery. The simulated behaviour correlates well with theoretical predictions for free windmilling speed developed previously by other members of the research group. The presented results and overall modelling framework potentially allow to gain insight into synergies that emerge beyond the local subsystem-level (here aero-propulsive) performance gains, such as feedback on aeroplane lateral control and stability subsystems sizing.
ETC2023-264
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