Keywords: high-speed train, aerodynamics, aeroacoustics, cryogenic wind tunnel, high Reynolds number, acoustic camera, microphone array, laser-Doppler anemometry.

Modern high-speed trains must allow for operational speeds of up to 350 km/h, which also results in a sharp increase in the emitted sound. For this velocity range, aerodynamic noise exceeds all other sources of sound, such as engine-gearbox noise, noise from aggregates, general noise from the bogies and interaction between wheels and rails. For the development of modern high-speed trains the aeroacoustics becomes an increasingly important topic. The wind tunnel studies, presented here, focus on the Reynolds - and Mach number dependence of typical aeroacoustic sound sources of high-speed trains: the noise emitted by the first bogie. The knowledge of the aeroacoustic scaling is essential for a correct interpretation of the results obtained by measurements on down-scaled models. For the sound source localization and quantification a microphone array technique is applied.

To achieve comparable aerodynamic conditions, the Reynolds number realised with the scaled model must be similar to the one obtained for the full-scale train. Therefore, cryogenic wind tunnel measurements in the Cryogenic Wind Tunnel Cologne (DNW-KKK) were conducted for which the Reynolds number was increased by cooling the fluid. By cooling to 100 Kelvin, the Reynolds number can be increased by a factor of 5. In addition, this facility permits the variation of the Mach and Reynolds numbers independently, which is very valuable to gain insight into the aeroacoustic scaling, without changing the scale of the model. The investigations were carried out on a 1:25 scaled model of the Next Generation Train, the DLR's technology concept of a future train. The model can be equipped with a aerodynamic fairing, which covers the open bogie cavities.

In order to achieve a reasonable simulation of the flow underneath the train in a wind tunnel, a double model setup is used. The investigations of the Reynolds number influence reveal a significant influence on the tones in the spectra, whereas the influence on broadband noise is less intensive. The sound emitted by the bogie with mounted fairing contains more tonal components of lower amplitude compared with the configuration without fairing, and again these peaks are significantly decreased for the higher Reynolds number. The powerlaw exponent according of a aeroacoustic dipole of alpha equal to 6 cannot be found experimentally for all the investigated Mach numbers. The influence of the Mach numbers on the sound pressures show a significant inconsistency.

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