This work describes the development of a test bench that allows for a complete assessment of the aerodynamic characteristics and the acoustic emissions of a rotor in flight-like operating conditions. The rotor is named TUC-TUC, after the TU delft Characterization model for roTor aeroacoUstiCs. Its design is driven by the ability of precisely control its configuration and load distribution while facilitating a holistic set of aeroacoustic measurement techniques to take place. The design rationale, technical developments, experimental plans, estimated performance and noise emissions are shown in this study.

Acoustic annoyance is a keen factor in the social acceptance of novel urban air mobility concepts. Although regulations and certification requirements exist for such operations, they rely on measurements of the vehicle under controlled, and mostly steady, conditions. These conditions differ significantly from real envisioned operations, where turbulence from the urban environment, rapid maneuvers, system automatic control, and gusts can affect the vehicle’s noise emissions. To assess such differences, this work focuses on the study of rotors, commonly applied to urban air mobility and transport vehicles, under varying rotational speeds. An experimental campaign is carried out in the anechoic wind tunnel of the Delft University of Technology, where an unsteady rotational speed of the rotor is prescribed. Acoustic measurements are carried out along with the integral loads of the rotor. The work explores both the aerodynamic effects of such an operation and its impact on noise emissions. The final goal is to create a global picture of the relevance and physics of rotor noise under non-steady rotational speeds.

This work focuses on the assessment of the accuracy of numerical prediction and experimental campaigns on providing the noise emissions of an isolated benchmark propeller. An experimental campaign is carried out with a model low-Reynolds propeller of 0.3 m diameter operating at high RPM, equivalent of a tip-Mach number (M ₁) of 0.37 and an advance ratio (J) of 0.4.Measurements are conducted on an open-test section wind tunnel, surrounded by an anechoic chamber. Simulations are carried out with the commercial software PowerFLOW and aim at reproducing the propeller geometry and conditions. BEMT-based noise estimations are also used to demonstrate the expected results. The discussion is focused on the uncertainties of the experimental campaign, and the current accuracy of numerical and analytical predictions, creating a complete picture of the discrepancies expected when predicting propeller noise levels and potential sources of errors. Results point to an accurate ability of the three methodologies to assess the overall noise emissions. Nevertheless, precise description and measurements of the higher harmonics of the tonal emissions and of the broadband noise levels is still lacking and require improvements in experimental conditions and a detailed assessment of the flow over the propeller.

The present study focuses on the application of finlet rails as a passive technique of flow control to mitigate trailing-edge noise. Finlet rails are small cylinders whose axes are aligned along the streamwise direction, transversally positioned with respect to the trailing edge. In the first part of this study, the effects of finlet geometry on the aeroacoustic emission of a NACA 63₃−018 airfoil are investigated using an array of microphones. It is observed that reducing the transversal spacing of finlet rails leads to increasing the maximum noise reduction, found to be of 4 decibels at relatively low frequencies. An optimum for the height of the finlets was determined, equivalent to 1.6δ^∗, where δ^∗ is the displacement thickness of the boundary layer. With the aim of unveiling the underlying physical mechanism for finlet rails, PIV at high spatial resolution is applied around the surface treatment. It is found that the turbulence energy is lifted-up and moved away from the scattering edge, which attenuates the wall-pressure fluctuations. The observed attenuation of the wall-pressure fluctuations occurs at the energy-containing scales, which is an important difference with finlet fences. In the region underneath the finlet rails, the transversal size of the energetic structures diminishes when the surface treatment is applied. The combination of the lift-up of the turbulence structures, that reduces the wall-pressure fluctuations, with the smaller turbulence scales is responsible for the noise reduction observed for finlet rails.

This work addresses the importance of considering the coherent organization of turbulence structures when predicting the noise from serrated trailing edges. Turbulent flow is the source of broadband trailing-edge noise. The existence of coherence in the turbulent flow departs from the state-of-the-art assumption for noise prediction based on the sum of the different wavenumbers excitations in an incoherent manner. This study addresses whether the latter hypothesis is the underlying cause of the noise underestimation observed from theoretical models for serrated trailing edges at high frequencies. A simplified hairpin model in the form of a bounded vortex filament is used as input to the scattering solution. The vortex filament is used to compute the turbulent velocity and wall-pressure fluctuations induced by the modelled coherent structure. This coherent wall-pressure structure is given as input to a numerical acoustic solver of the diffraction problem, yielding the scattered acoustic field. Results focus on the differences between coherent and incoherent assumptions. It is demonstrated that the acoustic scattering of coherent structures differs from the incoherent sum of the wavenumber spectrum of such structures, showing more consistent results with experimental evidences. The results indicates that modelling of the noise from serrated trailing edges can be improved with a detailed description of the turbulent flow.

This work discusses the physics of noise reduction achieved from serrated trailing–edges and its impact by the serration design. An experimental campaign is carried out with a benchmark 2D model based on a NACA 63₃–018 airfoil. Different trailing-edge serrations are tested under several flow speeds and angles of attack conditions to build a complete dataset of acoustic measurements. Systematic modifications of a reference sawtooth serration design are made to its scale and geometry. Scale modifications are based on sawtooth serrations and comprehend carefully considered variations of the serration height (2h), wavenumber (λ), and aspect ratio (2h/λ). Geometric shape modifications are represented by concave–shaped and combed–sawtooth serrations. This study represents a unique sensitivity–based parametric analysis on the scaling and geometric properties of trailing–edge serrations where the impacts of each modification are studied separately. The results obtained are used to provide guidelines for serration design choices and their impact on broadband noise reduction.

Experimental results on trailing-edge (TE) noise from a NACA 63₃ –018 airfoil are presented for a chord-based Reynolds number Re_c range between 2 × 10⁵ and 3 × 10⁶. Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying Re_c, angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad Re_c range, two NACA 63₃ –018 airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers St at which the peak noise level is measured reduce as Re_c increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and Re_c. The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646.

The impact of aerodynamic loading on a serrated trailing edge is studied experimentally. Aerodynamic and acoustic measurements are conducted on a sawtooth-shaped trailing edge, retrofitted to a flat plate featuring a trailing-edge flap, and placed at incidence to the free-stream flow. The turbulent flow across the trailing edge is inspected by time-resolved three-dimensional velocity field measurements obtained from 4D-PIV, while the wall-pressure fluctuations are measured with surface-embedded microphones. Results discuss the relation between the velocity fluctuations over the serrations, the surface pressure fluctuations, and the far-field noise spectra. The aerodynamic analysis discusses the effect of counter-rotating vortex pairs, generated by the pressure imbalance across the edges of the serrations under loading. It is shown that the interaction of these vortices with the incoming turbulence affects the intensity of the wall-pressure spectrum at the outer rim of the serration surface. On the suction side, the intensity of the pressure fluctuations from the incoming boundary layer dominates over that induced by the vortex pairs. On the pressure side, instead, the velocity gradient prescribed by the vortex pairs produces a significant increase of the pressure fluctuations around the edges. The resulting spatial distribution of the wall-pressure fluctuations directly affects the far-field noise. Scattering predictions carried out with the wall-pressure fluctuations in the centre and root (on the suction side) exhibit good agreement with the measured noise in the low-frequency range, whereas using the surface pressure data at the tip of the serration (on the pressure side) yields a better prediction in the high-frequency range.

An experimental aero-acoustic characterisation of the NACA 63 ₃-018 airfoil is presented in this study, featuring trailing-edge noise emissions with and without serrations. Measurements have been carried out for a chord-based Reynolds number range between 0.18 × 10 ⁶ and 4.8 × 10 ⁶ . Two airfoil models with different chord lengths have been tested in five different wind tunnels. The goal is to compare the measurements in different facilities, quantify the uncertainties, and establish a validation database that can serve as a benchmark for computational studies. The tests have been performed with clean and forced-transition boundary layers for a variety of angles of attack. The effect on the spectral slope and peak levels is evaluated. Scaling laws have been applied to compare different test conditions. The quality and nature of the collapse, as well as the applicability limits of the scaling, are examined. Different serration geometries have been tested at different flap angles. The noise reduction dependence on the aerodynamic loading is discussed. This work is based on an initiative of Task 39 "Quiet Wind Turbine Technology" of the Technology Collaboration Programme (TCP) of the International Energy Agency (IEA).