A detailed experimental study on the aerodynamic performance and noise emission of airfoils and fan blades with permeable leading edges under disturbed inflow conditions was performed. The airfoils...

Additive manufacturing processes, such as coreless filament winding with fiber composites or laser powder bed fusion with metals, can produce lightweight structures while exhibiting process-specific characteristics. Those features must be accounted for to successfully combine multiple processes and materials. This hybrid approach can merge the different benefits to realize mass savings in load-bearing structures with high mass-specific stiffnesses, strict geometrical tolerances, and machinability. In this study, a digital tool for coreless filament winding was developed to support all project phases by natively capturing the process-specific characteristics. As a demonstration, an aluminum base plate was stiffened by a coreless wound fiber-composite structure, which was attached by additively manufactured metallic winding pins. The geometrical deviations and surface roughness of the pins were investigated to describe the interface. The concept of multi-stage winding was introduced to reduce fiber–fiber interaction. The demonstration example exhibited an increase in mass-specific component stiffness by a factor of 2.5 with only 1/5 of the mass of a state-of-the-art reference. The hybrid design approach holds great potential to increase performance if process-specific features, interfaces, material interaction, and processes interdependencies are aligned during the digitized design phase.

An experimental investigation on scaled wind turbine models in a wind tunnel with a microphone array is presented. Our study focuses on the localization and quantification of aerodynamic noise sources on rotating wind turbine blades with the aim of identifying the contributing factors that have an impact on the source spectra. Therefore, wind tunnel measurements were conducted for three different blade geometries (NACA 4412 shape, Clark‐Y shape, and sickle shape), five pitch angles between −2° and +8° and five wind velocities between 5 and 13 ms^̂−1. For the localization of rotating sound sources with a microphone array, a rotating beamforming method based on the acoustic ray method is used. The Clean‐SC deconvolution method was used to improve the resolution of the acoustic sources, and integrated spectra were calculated for the individual blades. The sound sources were localized at the wind turbine blades and assigned to the leading edge and trailing edge subregions. The results show a high dependency on the sound source distribution and the source strength with regard to the observed one‐third octave bands, wind velocity, and blade geometry. Hence, the localization of rotating sound sources with a microphone array is a suitable method for the development of wind turbine blades that emit less noise.

Acoustics in a moving fluid is a simple model of an analogue space-time. The analysis shows that the null geodesics of the metric g(mu nu) in that space-time correspond to the rays in the eikonal approximation of geometrical acoustics. The physical connection to general relativity is the propagation of sound along the null geodesics that corresponds to the propagation of a scalar field in a background gravitational field. In this framework, we derive a three-dimensional acoustic ray method in a rotating frame of reference and calculate an acoustic metric, acoustic rays, travel times, and amplitudes for the fluctuating part of the pressure. Further the application to rotating beamforming, i.e., localization and estimation of the strength of rotating sound sources with a microphone array, is presented. Analytical calculations are compared with the spherical harmonic series expansion method for the sound propagation in the rotating frame of reference. Two benchmark cases on rotating sound sources with generic time-data and measured time-data of an axial fan demonstrate its applicability to rotating beamforming. (C) 2020 The Authors. Published by Elsevier Ltd.

Invar alloys are used for high‐precision applications due to their invariance of the coefficient of thermal expansion over a temperature range up to about 250 °C. The present study determines the mechanical and corrosive properties of Invar 36 (64 % Iron – 36 % Nickel) specimens produced by laser powder bed fusion. Tensile specimens were fabricated to investigate the mechanical properties. The influence of a horizontal and vertical building orientation is considered. An apparent mechanical anisotropy depending on the respective building is observed. Based on this, further tensile specimens were stored in a hydrochloric acid pickling solution, a sulphate buffer solution and a sodium chloride solution to analyse the corrosion behaviour. The findings after eight weeks in the individual media show that outsourcing the Invar 36 specimens in a sulphate buffer solution results in significant changes in the mechanical properties, especially fracture elongation.

A detailed experimental analysis of the aerodynamic and aeroacoustic properties of flat-plate axial fans with slitted leading edges is performed. The sound emissions of five slitted leading edge designs are measured at a constant rotational speed and at a constant total-to-static pressure rise of the fans. For both cases, the fan blades with slitted leading edges reduce the turbulence interaction noise and lead to a reduction of the overall sound pressure level for volume flow rates above 0.6 m³ s⁻¹ compared to an axial fan with solid leading edges. The far-field noise analysis shows that the slits result in a noise reduction for frequencies below 2 kHz and a noise increase above 2 kHz. In addition, sound source localization is conducted with a microphone array and rotating beamforming methods are applied. The identified sound source distributions prove that slitted leading edges reduce turbulence interaction noise, but generate broadband noise in the fan blades’ trailing edge regions. The maximum sound reduction due to the slits could be detected at a dimensionless frequency of $ fh/\bar{w}\approx 0.5$, where f is the frequency, h is the height of the slit and $ \bar{w}$ is the mean relative inflow velocity. The noise reduction mechanism on axial fan blades corresponds well to previous investigations on flat-plate airfoils with slits.