Distributed Electric Propulsion systems are an emerging technology. Aerodynamic interactions between propellers in close proximity can, however, cause periodic variations in the blade loading. Together with acoustic interference, these installation effects can form a dominant noise source in such systems. In this contribution, we investigate a low-cost computational modeling approach to predict the unsteady loading of the propeller blades, and thereby the interaction noise of an array of side-by-side propellers. To inform this low-cost model, a numerical campaign of scale-resolving Lattice Boltzmann/Very Large Eddy Simulations (LBM/VLES) has been performed on the Dutch National Supercomputer Snellius. The goal of this model development is to gain a better understanding of the blade-to-blade interaction mechanisms and to determine to which extent the model can be applied for purposes like preliminary design, uncertainty quantification, or control, for which the computational cost of high-fidelity simulations is prohibitive. As a practical example, the optimal relative phase angle in an array of propellers is determined and validated.@en

Distributed electric propulsion systems are an emerging technology with the potential of revolutionizing the design and performance of aircraft. When propellers are located in close proximity, they can be subjected to aerodynamic interactions, which affect the far-field noise. In this paper, we study an array of three co-rotating and adjacent propellers to describe both the aerodynamic and acoustic installation effects. A scale-resolving CFD simulation based on the Lattice-Boltzmann/Very-Large-Eddy-Simulation method is used to solve the flow field around the propellers. An acoustic analogy integral approach calculates the far-field noise. Findings show that the helical vortical structures, generated at the tip of each blade undergo a flow deformation at the location of interaction. This causes the loading of each blade to vary during the rotation. Consequently, the unsteady loading noise becomes a dominant noise generation mechanism, driving the noise levels and directivity. It is also shown that introducing a non-zero relative phase angle between the propellers results in a reduction of the unsteady thrust, leading to a mitigation of the unsteady-loading tonal components along the rotation axis. Additionally, the relative phase angle causes constructive/destructive acoustic interference, as demonstrated by analyzing the noise emitted simultaneously by the three propellers.@en

Sound emissions of an automotive engine cooling system are studied using both single-microphone directivity measurements and a rotating beamforming technique. These measurements provide reference acoustic data on such a system and some new understanding of the effect that the radiator induces on the distribution of sound sources. Indeed, the beamforming results indicate that, above the frequency limit allowed by the Rayleigh criterion, it is possible to localize and quantify the noise sources even through the heat-exchanger core. Moreover, for the investigated operating points along the fan performance curve, the sources are always distributed at the tip of the blades and, in particular, at the leading edge. The present evidence, confirmed by the similar trends of the frequency spectra with and without the heat exchanger, leads to the conclusion that the dominant sound mechanism is the turbulence-interaction noise. Nevertheless, this turbulence is produced within the gap between the fan ring and its casing rather than generated by the radiator core. The latter appears to induce negligible acoustic transmission losses but, more significantly, is found to have a minimal influence on the aerodynamic modification of sound sources for all the analyzed operating conditions.

@en

Aerodynamic noise emitted by low-speed axial fans has been receiving increasing attention in various sectors of high societal impact, such as automotive and HVAC systems. In this framework, turbulence interaction, flow non-uniformities, trailing-edge boundary layer fluctuations and blade-tip leakages are different mechanisms generating aeroacoustic sources on the rotating blades and contributing to the overall emitted sound. An accurate localization of the sound sources on the surface of the blade is instrumental in separating and isolating these contributions and, therefore, in designing novel sound mitigation concepts. The main objective of this paper is to present an inexpensive and efficient way to isolate and quantify the noise generating mechanisms on rotating blades by means of a irregularly shaped microphone array. The technique is based on ROtating Source Identifier (ROSI) and has been implemented and validated at the von Karman Institute for Fluid Dynamics (VKI). Simulated benchmark datasets that refer to rotating point sources emitting white noise have been considered for the validation of the method. The accuracy in the source localization and in the source strength reconstruction has been evaluated for a fixed and a variable angular rate. Moreover, the algorithm implementation has been parallelized with the purpose of reducing its computational time, which represents the main drawback of ROSI. Finally, the developed technique has been applied to measure the noise sources generated by a forward-skewed subsonic axial fan operated at maximum efficiency. In this case, it has been possible to successfully localize and characterize the major noise sources on the blades. Although further investigation will be necessary to gain better insight into the topic, the present work constitutes an important step for a better understanding of the physical phenomena occurring in the noise generation of an axial fan.

@en

An automotive cooling module working at three different operating conditions is investigated using a microphone array method. Experiments on an open rotor and on a full module configurations were conducted in the ALCOVES anechoic chamber of the von Karman Institute for Fluid Dynamics, Belgium. The acoustic data are post-processed using the ROtating Source Identifier algorithm. As first step, the method was applied for the fan-alone configuration at the nominal operating point. The sound maps of one-third octave band are discussed with respect to the minimum resolving frequency emerging by the Rayleigh criterion. Integrated spectra of the whole beamforming maps are used to validate the method whereas acoustic characteristics of the fan are identified by evaluating integrated spectra of each blade separately. Subsequently, the ROSI method was applied to both configurations, demonstrating the acoustic transparency of the automotive heat exchanger. Depending on the adjusted mass flow rate, sound sources are located on the leading or trailing edges of the fan blades. To complement the analysis, sound directivity measurements have been carried out in the anechoic wind tunnel at the University of Sherbrooke, Canada, on an another sample of the same engine cooling module.

@en