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The measurement of the noise sources is performed in the wind tunnel using a directional microphone arrangement and two microphone areas on both sides. Measurement object is an airfoil with brush extension.
© DLR, mit Genehmigung DNW-NWB
Wind energy - noise reduction
Projektinfo 08/2018

The measurement of the noise sources is performed in the wind tunnel (DNW-NWB) using a directional microphone arrangement (elliptic mirror) and 2 measurement arrays: The left field has a diameter of 3 m, 140 microphones and is facing the suction side. The right field has a diameter of 1 m, 96 microphones and is located next to the mirror on the pressure side. Measurement object is an airfoil with brush extension.
© DLR, mit Genehmigung DNW-NWB

Overview of the tested model variants without trailing edge modifications. The two left blade tips on the wind tunnel models show winglets bent up and down.
© DLR
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Testing low-noise rotor blades in the wind tunnel

The expected noise emissions and the resulting necessary distance to residential buildings are of great importance in the approval of wind farms. In order to reduce these emissions, scientists have designed a powerful rotor blade profile that is as low-noise as it is aerodynamically efficient and have tested the corresponding specimens in wind tunnels. These specimens were combined with passive reduction technologies on the trailing edge and modified blade tips. During the experiment, subject to the operating conditions, noise reductions of up to eight decibels were achieved.

Noise is a matter that affects new wind farms as well as the repowering of systems and has a huge impact upon acceptance among the population. To date, noise thresholds have often been the reason why wind farms have to maintain a considerable distance to settlements or have to restrict operations during the night. A reduction of the sound emissions by 1, 2 or 3 decibels (dB) numerically increases the possible number of wind turbines in a farm by 25%, 58% or 100%. This opens up freedom of design in the case of repowering. Scientists from the German Aerospace Center (DLR) in Braunschweig have jointly investigated the options for aeroacoustic improvement with partners in the research project, “BELARWEA – Blade tips for efficient and low-noise wind turbine rotors”. The aim is to reduce noise without the output suffering. The results are used for increasing aeroacoustic methods, in particular the development of simulation tools for the design-supporting evaluation of complex geometries (e.g. winglets) and for the demonstration of existing capabilities in the aeroacoustic profile design.

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Get simulation results on the test field

In modern wind turbines, the nacelles are so well encapsulated that the sound of the blades, particularly the trailing edges, are the dominant sources of noise. At the same time, the outer 20 – 25% of the rotor blade is important because it is there that the flow velocities are the highest.

For years, increasingly longer rotor blades have been used in wind turbines. It is expensive to test these blades on test rigs during the development phase. Therefore, an increasing large part of the development and testing work is performed with the aid of simulation software. This saves time and money. Validated software tools optimally show the results of previous tests so that – ideally – there is a possibility of comparing variants on the computer and making a pre-selection.

Aeroacoustic optimisation is a central development objective for a new blade design. Conventional industrial methods to predict a turbine’s total noise emissions are mainly empirical. They are based on measurement data with limited scope and primarily include noise estimations for the profiles present in the outer blade area. In the case of noise sources with line source characteristics, e.g. trailing edge noise, modelling of this kind based on 2D sections is possible. However, the objective is to increase the room for manoeuvre for design beyond the conventional degree and physicsbased, largely non-empirical 3D simulation methods must be applied in order to also assess e.g. innovative blade tip forms or noise reduction measures with complex flow already during the preliminary design stage.

This provides the starting point for the BELARWEA project: Phase 1 starts with the design of a new profile geometry, whereby non-empirical computational aeroacoustic methods (CCA) from aviation applications are already used to support the design process. The variant that came out best in the simulation was selected for a comprehensive test programme in various wind tunnel environments and combined with passive noise reduction technology. The primary objective of  the investigations was to get precise validation data for existing 2D and newly developed 3D simulation approaches. Furthermore, passive noise reduction measures were pre-selected for their later deployment in wind turbine rotors.

The new profile

The profile geometry is crucial for sound generation because it determines up to what point the air current has a laminar flow, from what point it becomes turbulent and where it separates from the profile. Also the flow angle on the leading edge, the flow velocity, potential small manufacturing defects and dirt on the blade have an impact.

The researchers in Braunschweig designed an aeroacousticly favourable profile named RoH-W-18%c37 (hereinafter: RoH). They compared it to the NACA 64-618 standard profile. For scientific comparisons, the socalled NREL 5MW reference wind turbine is often used;  ts geometry and performance data are accessible to the public. The aforementioned NACA standard profile is situated in the outer 20 – 25% of this wind turbine. Comparisons are first simulated on the computer and then verified by way of measurements. In the larger wind tunnel (DNW NWB), the outer 20% of the NREL 5MW rotor blade at a 1:6 scale was used as comparison geometry. Due to the lack of rotation in the wind tunnel test, the geometry was adjusted; in particular the blade twist was removed.

The blade tips of today’s modern wind turbines have little impact on the generation of noise on the rotors because in this area rotor output is often forgone for acoustic and structural reasons. Winglets or spiroids could increase the rotor output by up to 4%. Therefore, two winglet designs were also included in the tests and integrated in the wind tunnel model with RoH profiling.

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Addresses

Project management BELARWEA
DLR, Standort Braunschweig

Industrial advisory board BELARWEA
ENERCON GmbH

Industrial advisory board BELARWEA
GE Renewable Energy

Industrial advisory board BELARWEA
Nordex SE

Industrial advisory board BELARWEA
Senvion Deutschland GmbH

Industrial advisory board BELARWEA
Siemens Gamesa Renewable Energy

Project management ActiQuiet and ActiQuieter
Universität Stuttgart, IAG

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