Djamel Rezgui

1. The prevailing aim in wind turbine design is to develop larger, more powerful systems that extend higher into the atmosphere to capture stronger and more consistent winds; however, increasing th...

While Distributed Electric Propulsion (DEP) architectures are critical for emerging eVTOL aircraft, the complex rotor-structure interactions they introduce remain difficult to predict and experimentally validate. However, standard aeroelastic models often oversimplify the propulsion unit as a rigid mass, neglecting the internal structural dynamics that govern rotor-structure coupling. Furthermore, limited experimental data exists to validate how these gyroscopic and inertial mechanisms manifest on a representative flexible wing. To address this gap, this study presents the development and validation of a dynamically scaled Multirotor Test Rig. Inspired by the Maxwell X-57, the rig’s structural design was sized using a nonlinear Sequential Quadratic Programming optimisation algorithm to replicate the aircraft’s specific modal frequency ratios and mode shape sequences. Although designed for multi-rotor applications, this specific investigation isolates a single tip-rotor configuration, referred to as the Flexible Rotor-Wing Demonstrator, to establish a fundamental baseline for gyroscopic and structural coupling effects. Experimental Modal Analysis was conducted in non-rotating and rotating states and correlated with two MSC NASTRAN models: a high-fidelity 3D solid model and a computationally efficient 1D beam element model. The results successfully captured critical dynamic phenomena, including resonance interactions, frequency splitting into forward and backward whirling modes, and modal veering. These findings validate the numerical framework, providing a verified baseline for future investigations into full multi-rotor dynamics.

In the present work we conducted a deep investigation on identifying geometry and orientation-dependent effects on three-dimensional wake dynamics and fluid–structure interaction. The study employs a coupled Lattice Boltzmann method and Immersed Boundary Method to investigate the flow past hexagonal cross-section cylinders under two different orientations with respect to the free stream: one with a flat face perpendicular to the freestream and one with a corner aligned in the direction of the flow. The analysis spans different flow regimes, with Reynolds numbers ranging from 20 to 103. The framework adopts a Large Eddy Simulation approach based on the Smagorinsky subgrid-scale model for the highest Reynolds numbers investigated (Re=500 and Re=1000). Validation of the numerical method involved comparing simulated drag and lift coefficients, as well as surface pressure distributions with benchmark data obtained from traditional Navier–Stokes solvers. The aerodynamic behavior of the hexagonal cross-section cylinder was then compared to the circular cylinder case to highlight the influence of cross-sectional geometry on wake dynamics, showing that the orientation of the hexagonal cylinder significantly alters the wake topology and promotes the early transition to a three-dimensional wake structure. The study further focuses on the Vortex Induced Vibration problem at Re=200. In this context, the solid-to-fluid density ratio is kept constant among the different test cases at 10, while the ratio among the pulse of the lift fluid force and the natural frequency of the system is set at two different values 0.5 and 0.9 to investigate how the wake dynamics is affected by the displacement of the obstacle.

Distributed Electric Propulsion (DEP) systems offer strong potential for sustainable aviation, yet unconventional rotor installations introduce new aerodynamic and acoustic challenges.This study experimentally examines how rotor–rotor separation influences the aerodynamic and acoustic performance of isolated and wing-mounted DEP configurations using 9′′, five-bladed P/D = 1 propellers. The presence of the wing provides a small but repeatable installation benefit, resulting in an approximately 4% improvement in propulsive efficiencyfor advance ratios J > 0.4. For the isolated configuration, separations up to 3% of propellerdiameter produce noise levels around 2 dB higher than those at larger spacings, while noisecharacteristics stabilise for 4% of propeller diameter. At the smallest spacing 1% of propellerdiameter, enhanced wake interaction leads to elevated velocity fluctuations and increasednoise. In contrast, the DEP configuration shows only minor sensitivity to separation, providing acoustic shielding that reduces noise above the propulsion system, while increasing side-field radiation. Under static thrust conditions (MT ≈ 0.21 to 0.315), the isolated andDEP configurations show nearly identical noise levels, indicating that tip speed dominatesthe acoustic response. Under inflow, the isolated configuration produces stronger broadbandand tonal noise, with increases of up to 4 dB and 3 to 5 dB, respectively, while the DEPconfiguration consistently remains quieter.

Advanced air mobility represents efficient and sustainable transportation through the use of electric vertical takeoff and landing (eVTOL) aircraft, which will see their propellers operating in edgewise flow for a significant portion of their mission profile, giving rise to challenging aerodynamic and aeroacoustic features, different from the conventional helicopter rotors. The present experimental study examines the effect of the blade tip Mach number on the acoustic signature emitted by a propeller operating in edgewise flight conditions. Both aeroacoustic and aerodynamic measurements were performed for a 12-in., two bladed propeller over a wide range of tip Mach numbers (MT) from 0.12 to 0.56, and inflow velocities from 0 to 24 m/s. The results confirm the highly sensitive nature of the propeller noise to the tip Mach number. The overall sound pressure level (OASPL) in the plane-parallel to the propeller's plane of rotation, scales with MT3 to MT4 at low tip Mach numbers (MT0.3, for the no inflow velocity case. Similar trends are observed for the plane perpendicular to the plane of rotation, but with noise scaling up to MT8. At the highest inflow velocity case, the OASPL scales with MT3 to MT7, depending on the observer location and MT, highlighting the increased complexity and sensitivity of noise directivity at higher inflow velocities. Tonal noise is the dominant source of noise at lower inflow velocities, with an increased contribution of broadband noise at higher inflow, possibly leading to the larger deviations of the tip Mach number scaling observed. These findings offer valuable insights into the acoustic behaviour of propellers during edgewise flight conditions, which can be leveraged to design more efficient, low-noise propellers in an eVTOL application.

The development of distributed electric propulsion systems aims to advance sustainable aviation; however, increased noise levels, particularly with unconventional rotor placements, pose new challenges. This study experimentally investigates the acoustic performance of overlapping propellers, focusing on the effects of lateral and axial separations under varying inflow conditions and rotational speeds. Experiments were conducted in an aeroacoustic wind tunnel using a twin side-by-side configuration with five-bladed propellers. Noise analysis reveals that tonal noise is the primary contributor to the increased overall sound pressure level, exhibiting significant sensitivity to the lateral distance across all advance ratios and directivity angles. At a lateral distance of approximately 40% of the propeller diameter, tonal noise levels are found to be significantly elevated—by approximately 14–18 dB—compared to the non-overlapping case. Results indicate that lateral separation distance is the dominant factor influencing acoustic behaviour due to blade-vortex and blade-wake interactions, while axial separation distance plays a secondary role.

Real-time hybrid testing is a method in which a substructure of the system is realized experimentally and the rest numerically. The two parts interact in real time to emulate the dynamics of the full system. Such experiments, however, are often difficult to realize as the actuators and sensors, needed to ensure compatibility and force–equilibrium conditions at the interface, can seriously affect the predicted dynamics of the system and result in stability and fidelity issues. The traditional approach of using feedback control to overcome the additional unwanted dynamics is challenging due to the presence of an outer feedback loop, passing interface displacements or forces to the numerical substructure. We, therefore, advocate for an alternative approach, removing the problematic interface dynamics with an iterative scheme to minimize interface errors, thus, capturing the response of the true assembly. The technique is examined by hybrid testing of a bench-top four-storey building with different interface configurations, where using conventional hybrid measurement techniques is very challenging. A case where the physical part exhibits nonlinear restoring force characteristics is also considered. These tests show that the iterative approach is capable of handling even scenarios which are theoretically infeasible with feedback control.