Sigfried Loose

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.

Particle image velocimetry of the underfloor flow for generic high-speed train models in a water towing tank:

The underfloor flow field of three different 1:50 generic high-speed train models hauled through a water towing tank at a speed of 4 m/s (Reynolds number = 0.24 × 106) over smooth ground, rough ground, and ground with sleepers is measured by means of two-component particle image velocimetry. The reference train model, consisting of four cars, features inter-car gaps and two bogies per car. These features are removed and all gaps are closed to create the smooth train configuration whereas the rough train configuration has all the gaps but no bogies. The lowest underfloor flow velocities and velocity gradients at the ground are observed for the smooth train model on the ground with sleepers. Furthermore, the measurements reveal that any additional irregularity of the underbody leads to regions characterised by flow acceleration in the otherwise Couette-like flow and this increases the possibility of ballast flight. Therefore, it is concluded that a smooth underbody can lower the risk of ballast flight. The results obtained for the reference train model are compared with those of a full-scale measurement. It is shown that the results of the water towing tank experiments with the scaled train model reflect the main characteristics of the underfloor flow of the full-scale measurements.

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.

Considerations on active control of crosswind stability of railway vehicles

The DLR research project Next Generation Train deals with concepts, methods and technologies for a very high-speed train in double-deck configuration and light-weight design. Due to these three key features, crosswind stability is a particular subject of study. It is shown that conventional approaches here fall short of guaranteeing safety in high-wind occurrences according to the given homologation standards. Therefore, this paper discusses the feasibility of different approaches to ensure crosswind stability by means of active control. Four different concepts are overviewed, the most promising one is then chosen und examined in detailed multibody simulations that are based on data from wind tunnel measurements of the Next Generation Train.

An Experimental and Numerical Investigation of the Near Wake Field of a Tractor-Trailer Configuration

An experimental and numerical study was performed with a 1/15th scaled model of a typical European tractor-trailer configuration. The objective was to develop a new and practicable passive method which reduces the aerodynamic pressure drag by controlling the unsteady velocity field behind the trailer. The experiments included the measurement of the pressure distribution on the trailer base, the velocity wake based on particle image velocimetry and the resulting aerodynamic loads. A trailer underbody equipped with a full fairing and a rear diffuser with side plates is shown to reduce the aerodynamic drag coefficient by 2.4 %. The direct influence on the trailer wake and the resultant base pressure of the trailer is presented. The results of the also conducted Computational Fluid Dynamics (CFD) predictions confirm the effect of the drag reducing add-on devices on the velocity and pressure field in accordance to the experimental data.

Experimental Investigation of the Flow Field Underneath a Generic High-Speed Train and the Effects of Ground and Train Roughness

Results obtained from two component Particle Image Velocimetry (PIV) measurements on three different 1:50 generic high-speed train configurations hauled through a water towing tank over a smooth (Plexiglas) and a rough (grinding belt) ground at a speed of 4 m/s are presented. Principally, the three different generic high-speed train configurations are based on the same model (front car, two cars and tail car). The smooth generic high-speed train configuration (smooth GHSTC) reflects no bogies and covers the bogie cut outs and inter car gaps, the rough generic high-speed train configuration (rough GHSTC) is obtained by removing the bogies and leave the bogie cut outs and inter car gaps open and for the generic high-speed train configuration (GHSTC) the bogies are not removed but the inter car gaps are left open. A PIV set-up was chosen that the light sheet defines a vertical plane (XZ) between the ground and the train in the symmetry line of the train. Comparing the PIV results obtained for the GHSTC with full scale measurements, it was found that the same flow structures develop in the vicinity of the head and the tail of the train. But, the measured underfloor U-velocity of the downscaled model measurements did not reach the same value as those measured for the full scale high-speed train. The reason which is ascribed to the better aerodynamic underframe of the train model in the downscaled model measurements. The flow field underneath the GHSTC was fully developed at the beginning of the second car, in agreement to the full scale measurements. For the three train configurations three different flow fields underneath the train were obtained. The lowest velocities were found for the smooth GHSTC and the highest for the rough GHSTC. Further, the ground roughness changed the flow fields underneath the different train configurations.

Wall Shear Stress Measurements on a Double-Decker Train

The aerodynamic drag is the principal factor of the driving resistance of modern railway vehicles. To accurately predict the energy consumption, it is essential to estimate the drag that is fundamental for the development of new vehicles. One way to quantify the aerodynamic drag of a full-scale railway vehicle is to measure the wall shear stress since friction drag dominates. The goal of this study was to improve the prediction of the aerodynamic drag of railway vehicles by measuring the wall shear stress. Experiments were performed on a double-decker multiple-unit train KISS from Stadler to measure the wall shear stress under real operating conditions. Oil-film interferometry was used as a non-intrusive measurement technique with the setup installed inside the train to avoid flow disturbances induced by measuring probes. The obtained results are in good agreement with experimental full-scale data from the literature. The comparison to theoretical predictions based on pipe and flat plate experiments reveal clear deviations.

Particle Image Velocimetry of the underbody flow of generic high-speed trains

Two component Particle Image Velocimetry (PIV) of the underbody flow of three different 1:50 generic high-speed train configurations hauled through a water towing tank over a smooth and a rough ground at a speed of 4 m/s (Re = 0.24 Mio) have been performed. The reference train model (RTM) consisting of 4 cars, features bogies and inter car gaps. All this features are removed for the clean train configuration (CTM), while the open train configuration (OTM) reflects all gaps but no bogies.

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.

High-speed PIV of the Underfloor Flow of a Generic High-speed Train Model

Two component high-speed particle image velocimetry of the underfloor flow of three different 1:50 generic high-speed train models hauled through a water towing tank over a smooth, rough and a ground with sleepers at a speed of 4 m/s (Re = 0.25 Mio) were conducted. It was found that any additional irregularities at the underbody accelerate the flow which increases the chance of ballast flight. Therefore it is concluded that a smooth underbody can lower the risk of ballast flight. The results were compared to full scale measurements. The main characteristics of the underfloor flow are captured in the water towing tank for downscaled train models.