E​RCOFTAC PC Netherlands and PC Belgium

LBM/vLES Numerical Simulation of the Flow Interactions and the Acoustic Propagation Occurring
to a Multi-Rotor eVTOL in Transition Maneuver

A​uthors: Federico Di Verniere, Emanuele Sticchi, Daniele Ragni, Damiano Casalino
(Flow Physics and Technology Department, Delft University of Technology, Delft, 2629HS, the Netherlands)

Riccardo Zamponi
(Flow Physics and Technology Department, Delft University of Technology, the Netherlands)
(Department of Environmental and Applied Fluid Dynamics, von Karman Institute for Fluid Dynamics, Belgium)

A 1.8-billion-element LBM–VLES CFD simulation of a full-scale multirotor eVTOL in transition flight conditions. Coherent vortical structures are identified using the λ₂ criterion, highlighting strong aerodynamic interactions. The dilatation field is then visualized on the three main aircraft planes, illustrating the propagation of acoustic waves. This visualization provides valuable insight into the key mechanisms governing noise generation and propagation which are emerging with the new rotorcraft designs for urban air mobility.

Scientific Abstract:

Community acceptance is widely recognized as one of the key enablers for the successful integration of UAM operations into urban environments. Among the various public concerns, noise remains the most critical factor. Continuous exposure to high or fluctuating noise levels has well-documented adverse e􀆯ects on human health and well-being, including sleep disturbance, stress, and anxiety. Therefore, minimizing and accurately predicting UAM noise emissions is essential to ensure both regulatory compliance and societal acceptance.

Since UAM vehicles are electrically powered, their overall acoustic footprint is dominated by aeroacoustic sources, i.e., noise generated directly by the aerodynamic interaction of rotors, blades, and airframe components. The magnitude and characteristics of this aeroacoustic noise strongly depend on the specific vehicle configuration, rotor arrangement, and operating conditions. The absence of a standardized UAM design results in high variability in acoustic behavior, which, in turn, limits the generalizability of existing studies and the availability of comprehensive noise databases.

Owing to the complexity of the flow interactions present in a full-scale vehicle, the vLES/LBM CFD simulation performed with the software PowerFLOW o􀆯ers a suitable trade-o􀆯 between computational cost and resolution accuracy. The simulation in this video is characterized by 1.8 Billion mesh elements, which represents a practical upper limit at the moment.

The analyzed case considers an aircraft flying at a freestream velocity of 40 knots in a transition attitude. In this condition, the rotor axes are tilted with respect to the cruise configuration, in which they are aligned with the freestream. Visualization of the vortex structures through λ₂-criterion isosurfaces clearly and immediately reveals the complexity of the aerodynamic interactions present in this scenario: from the impingement of the rotor wakes on the main wing to the distortion of the inflow experienced by the tip-mounted rotors. These phenomena are of fundamental importance both for aerodynamic performance and for noise generation.

Since noise is the primary focus of this study, the dilatation fields on the three main aircraft planes are presented. These fields provide crucial insight into the purely acoustic phenomena governing noise propagation. In particular, the visualizations show how the fuselage shielding e􀆯ect prevents part of the noise from propagating toward the ground, while also illustrating how di􀆯erent noise sources emit acoustic waves that interact (constructively or destructively) before propagating away from the aircraft.

These videos are exceptionally rich in information and are still under detailed analysis. When combined with results from the far-field propagation using solid and permeable FWH formulations, they are being used to identify and rank, by relative importance, all aeroacoustic noise sources generated by the aircraft in this flight condition.

Related Publications:

Di Verniere, F., Sticchi, E., Ragni, D., Zamponi, R., and Casalino, D., “Aeroacoustic Analysis of a Transitioning Vectored-Thrust/Tilt-Rotor UAM Vehicle Using the LB/VLES Method,” 51th European Rotorcraft Forum, Venice, Italy, 2025.

Acknowledgments:

This project has received funding from the European Research Council (ERC) under the European Union's Horizon research and innovation programme (MORASINA, grant agreement No. 101124038). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. This work used the Dutch national einfrastructure with the support of the SURF Cooperative using grant no. EINF-14048.

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