Andreas Lauterbach

Aeroacoustic Measurements of a Scaled Half Model at High Reynolds Numbers

The measurement of airframe noise on small-scale models is well known and common practice in conventional wind tunnels. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements during the development process of modern aircraft are often performed in cryogenic and/or pressurized wind tunnels which are capable of higher Reynolds number flows. Thus, the characteristics of the moving fluid are better adapted to the scale model. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. A microphone array consisting of 144 microphones was designed and constructed for this purpose. In this paper, acoustic array measurements performed in a cryogenic wind tunnel are described for various Reynolds numbers using a 9.24% Dornier-728 half model. Additionally, the background noise of the empty test section was measured within the range of the measurements performed on the Dornier-728 half model. Our results seems to indicate a Reynolds number dependency of the measured sound power for various sources.

Microphone array wind tunnel measurements of Reynolds number effects in high-speed train aeroacoustics

The present study focuses on the Reynolds number dependence of high-speed train aeroacoustic sound sources. To cover a wide range of Reynolds numbers the experimental investigations are carried out on a 1: 25 scale-model of the high-speed train Inter City Express 3 by conducting microphone array measurements in two wind tunnels. The latter are the Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick, providing nearly perfect acoustical conditions, and the Cryogenic wind tunnel (DNW-KKK) of the DNW (German -Dutch wind tunnels) in Cologne, allowing measurements at higher Reynolds numbers. Two types of sound sources with different characteristics at Reynolds numbers of up to Re = 0.46 × 106 have been identified by measurements in the AWB. It was found, that the aeroacoustic noise from the bogie section is dominant for frequencies f < 5 kHz and can be characterised by cavity mode excitation. Further, the pantograph is the dominant sound source above f = 5 kHz with an Aeolian tone characteristic. Additional aeroacoustic measurements have been conducted in the cryogenic wind tunnel DNW-KKK in order to analyse the Reynolds number dependence of the noise generated at the first bogie, for higher Reynolds numbers of up to Re = 3.70 × 106. The DNW-KKK admits varying the Mach and Reynolds numbers independently. These measurements reveal only a weak Reynolds number dependence of the noise source generated at the first bogie.

Simultaneous Multiplane PIV and Microphone Array Measurements on a Rod-Airfoil Configuration

The influence of the elastic rotor blade deformation and the aerodynamic interference from the fuselage on the rotor aerodynamics as well as rotor noise characteristics was studied. A BO105 Main Rotor /Tail Rotor/Fuselage (FUS) configuration is chosen for the numerical simulations. A coupling of unsteady free wake 3-D panel method (UPM) and Airbus Helicopters’ (formerly: Eurocopter) rotor code HOST was conducted to account for the effect of elastic blade deformation as well as compressibility correction. The effect of fuselage is simulated by using two fuselage models in UPM, (1) potential theory in form of a panelized fuselage and (2) an analytic fuselage influence formulation derived from isolated fuselage simulation based on (1).Measurement techniques based on microphone-arrays are well-known and common practice on scaled models in wind tunnels with closed test section. Usually, full-scale Reynolds numbers are not achieved. To increase the Reynolds number measurements are performed in cryogenic and/or pressurized wind tunnels. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. In order to use a microphone array in a cryogenic environment, coming to grips with cold hardiness and ensuring long term stability of the array fairing and the electronic devices, especially the microphones, are the primary challenge. In a first step measurements of the radiated noise from a single rod configuration have been conducted in the cryogenic wind tunnel DNW-KKK using a prototype microphone array designed for cryogenic environment. Measurements were carried out with a wide range of operational flow parameters. The measured sound radiation results showed a very good agreement with theory and a Reynolds number dependency of the measured and predicted sound power was shown.

Systematic Comparison of Microphone Array Measurements in Open and Closed Wind Tunnels

A systematic comparison between microphone array measurements in closed and open test sections of wind tunnels is presented. The study focuses on the beamforming accuracy in both wind tunnel types with respect to source position, relative and absolute levels of beamforming results and integrated spectra using two difierent sound sources. At flrst, an in-∞ow calibration sound source incorporated in an electromechanic driver and with known characteristics is used. This kind of source allows to investigate the in∞uence of the difierent ∞ow conditions on the sound propagation and their in∞uence on beamforming results. In the second part of the test a plate with various cavities serves as an aeroacoustic sound source. In both wind tunnels the same phased microphone array is used for the measurements, whereby the aim is to keep the experimental setup and conditions as identical as possible in both cases in order to be able to examine the test section-dependent efiects. The beamforming results of the two difierent sources, obtained from measurements in both test sections, show a high qualitative and quantitative agreement provided that the SNR is su‐ciently high enough. The wind tunnel dependent deviations of the integrated source spectra do not exceed 4.7 dB.

Investigations of Aeroacoustics of High Speed Trains in Wind Tunnels by Means of Phased Microphone Array Technique

The present study focuses on the analysis of the main aeroacoustic sound sources of a high speed train, measured in a wind tunnel. The experiments using a 1:25 Inter City Express 3 model were carried out in two different wind tunnels: The Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick and in the Cryogenic wind tunnel (DNW-KKK) of the DNW (German Dutch wind tunnels) in Cologne. The AWB is a Goettingen type wind tunnel with open test section which is surrounded by an anechoic chamber. The advantage of this facility is its low background noise level and its nearly anechoic test section. The maximum Reynolds number, based on the wind speed and the width of the train, achieved is 0.46 million. In order to obtain higher Reynolds numbers a second measurement campaign has been conducted in the cryogenic wind tunnel, using another array for cryogenic in-flow applications. The DNW-KKK enables higher Reynolds numbers up to 3.7 million by cooling down the fluid to 100 K. The DNW-KKK has a closed test section and the microphone array is mounted on a side wall inside the wind tunnel, and therefore the measurements are affected by the turbulent boundary layer. Drawback of this facility is that it is not optimized for aeroacoustic experiments and reflexions as well as the high background noise level can disturb the results. Differences of the two different experimental setups on the results and primarily, influence of the Reynolds number on the aeroacoustic of a high speed train will be discussed.

A New Type of Line-Array for Acoustic Source Localization at Drive-By Tests of Trains

The microphone array method together with the beamforming technique, required for the post-processing of the recorded acoustic data, has been optimized for measurements on moving rail vehicles. In order to increase the spatial resolution along the moving vehicle a new type of microphone array was developed. This comprises a line array in combination with a two-dimensional elliptical acoustic mirror. Acoustic source were separated successfully using the properties of the elliptic mirror along the trajectory of the train by applying the beamforming methods for the vertical axis. The combination of both techniques allows a three-dimensional reconstruction of the source distribution. The design was successfully tested on a high speed train track in Germany, which is used by different type of Inter City Express and other passenger trains. It was possible to improve the quality of the results significantly and individual wheel sets and aeroacoustic noise caused by roof structures can be identified in the source maps. The resulting acoustic source data obtained with the microphone array technique, provides input data for noise prediction methods, which can fill gaps in existing noise source databases thus assisting in the goal for a deeper understanding of rail vehicle noise.

Scaling of the Aeroacoustics of High-Speed Trains

The present study focuses on the scaling of aeroacoustic sound sources of a high-speed train. To cover a wide Reynolds number range the experimental investigations are carried out with a 1:25 Inter City Express 3 model by measuring in two wind tunnels by means of microphone array technique. The facilities are the Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick, which provides nearly perfect acoustical conditions, and in the Cryogenic wind tunnel (DNW-KKK) of the DNW (German - Dutch wind tunnels) in Cologne, which allows measurements at higher Reynolds numbers. Two sound sources with different characteristics are identified at Reynolds numbers of up to Re = 0.46 ×106. The aeroacoustic noise from the bogie section is dominant for frequencies f < 5 kHz and can be characterised by cavity mode excitation. The pantograph is the dominant source of sound above f = 5 kHz with a Aeolian tone characteristic. Additionally aeroacoustic measurements at higher Reynolds numbers of up to 3.70×106 have been conducted in the DNW-KKK. By cooling down one can increase the Reynolds number, and besides, this wind tunnel admits to vary the Mach and Reynolds numbers independently. Drawback of this facility is that it is not optimised for aeroacoustic experiments and reflexions as well as the high background noise level can disturb the results. These measurements revealed only a weak Reynolds number dependence of the noise source generated at the first bogie.