Acoustic Wind Tunnel Braunschweig Research Articles

Numerical studies on small rotor configurations with validation using acoustic wind tunnel data

This paper addresses the acoustic and aerodynamic characteristics of small rotor configurations, including the influence of the rotor–rotor interactions. For this purpose, a Rotor/Rotor/Pylon configuration is chosen for both the test and numerical simulations. The wind tunnel experiments on various rotor configuration were performed in DLR’s Acoustic Wind Tunnel Braunschweig (AWB). The experiments involve isolated rotors, and rotors in tandem and coaxial configuration in hover and forward flight. For numerical simulations, an unsteady free wake 3D panel method (UPM) is used to account for aerodynamic non-linear effects associated with the mutual interference among the Rotor/Rotor/Pylon configurations. The effect of the pylon is simulated using potential theory in form of a panelized body. Finally, the sound propagation into the far field is calculated with DLR’s FW–H code APSIM, using UPM blade surface pressure as input. The validation effort is supported by CFD TAU steady simulations on selected hover test cases. The experiments and numerical results indicate that the noise at the blade passing frequency (BPF) and its higher harmonics is the dominant source of the noise for the present rotor selection. The extra subharmonics between two BPFs appearing in the results are caused by the small geometric discrepancy between the blades as well as the motor noise. Broadband noise is also observed in the experiment, but its contribution to the overall sound pressure is very small and can be neglected. The simulation of the acoustic scattering from the rotor support system for the isolated rotor cases indicated an influence about 1–3 dB on the overall sound pressure of the polar microphones. In both the coaxial and the tandem configuration, the acoustic interferences are particularly well visible in the numerical simulations and cause a more complex noise directivity. There is almost no change in time-averaged inflow by applying phase angles. In the coaxial condition, in hover, the phase delay between rotors does not change the maximum noise level. In forward flight, the phase delay can influence the maximum level of the noise radiation. In both coaxial and tandem configuration, the position of the downstream rotor is key for the noise radiation, and therefore, avoiding the interaction with upstream wake can reduce the noise radiation.

Identifying the flap side‐edge noise contribution of a wind turbine blade section with an adaptive trailing edge

AbstractActive trailing‐edge technology is a promising application for localized load alleviation of large‐diameter wind turbine rotors, accomplished using one or more control surfaces in the rotor blade's outer region. This work focuses on identifying noise contributions from the flap side‐edge and the trailing edge in a laboratory condition. Measurements were conducted in the Acoustic Wind Tunnel Braunschweig (AWB) at the German Aerospace Center's (DLR) Braunschweig site. The small‐scale model has a span of 1,200 mm and a chord length of 300 mm. The control surface, a plain flap, has a span of 400 mm and a chord length of 90 mm. Far‐field noise was measured using a phased‐microphone array for various flow speeds, angles of attack, and flap deflection angles. Due to the size of the model and assumed closeness of the sound sources, two noise reduction addons were installed interchangeably: trailing‐edge brush and flap side‐edge porous foam for sound source identification. Analysis of the far‐field noise reveals that, while changes to the flap deflection angle alter the far‐field noise spectra, the trailing‐edge noise remains the predominant noise source at deflection angles and . No additional noise level was observed from the flap side edge within the measurable frequency range at these angles. The flap side‐edge noise has an increased role for frequency larger than 2 kHz for the larger flap deflection angles of and .

Noise simulations of flap devices for wind turbine rotors

AbstractThe rotor of a large diameter wind turbine experiences more substantial and more dynamic loads due to the fluctuating and heterogeneous wind field. The project SmartBlades 2.0 investigated rotor blade design concepts that alleviate aerodynamic loading using active and passive mechanisms. The present work evaluates the acoustics of the two load alleviating concepts separately, an inboard slat and an outboard flap, using the Fast Random Particle Mesh/Fast Multipole Code for Acoustic Shielding (FRPM/FMCAS) numerical prediction toolchain developed at DLR with input from the averaged flow field from RANS. The numerical tools produce a comparable flap side‐edge noise spectrum with that of the measurement conducted in the Acoustic Wind Tunnel Braunschweig (AWB). The validated FRPM/FMCAS was then used to analyze the self‐noise from a slat at the inboard section of a rotor blade with a 44.45 m radius and compared with that from the outboard trailing edge. Furthermore, the rotational effect of the rotor was included in the post‐processing to emulate the noise observed at ground level. The findings show an increase in the slat's overall sound pressure level and a maximum radiation upwind of the wind turbine for the case with the largest wind speed that represents the off‐design condition. In operational conditions, the slat adds at most 2 dB to the overall sound pressure level. The toolchain evaluates wind turbine noise with conventional or unconventional blade design, and the problem can be scaled up for a full‐scale analysis. As such, the tools presented can be used to design low‐noise wind turbines efficiently.

Experimental investigation of porous materials for trailing-edge noise reduction

The introduction of quiet short take-off and landing for civil aircraft operations in close proximity to the population poses important technological challenges. One critical aspect is the realization of extreme lift augmentation at low acoustic emissions. The aircraft concept selected to achieve this goal is a high-lift system equipped with an active flow-control non-slotted flap and a droop nose. For this specific configuration, trailing edge noise becomes a dominant noise source. Porous materials as a passive means for trailing-edge noise reduction are selected and characterized. Results of extensive experimental investigations in the acoustic wind tunnel Braunschweig are presented and discussed to point out the potential and limitations of the selected porous devices. Practical issues related to material manufacturing and integration into the wind tunnel model are addressed. The noise reduction potential of passive porous trailing-edge devices is found to strongly depend on both these aspects. Issues related to the characterization of the porous materials properties are described. Although porous materials are found to be successful at reducing trailing-edge noise emissions, the results indicate that there is still a need for more generic investigations to further clarify the parametric dependencies between noise reduction and material properties.