Challenge B: An environmentely friendly railway

Assessment of micro-pressure wave emissions from high-speed railway tunnels

M. Hieke, C. Gerbig, T. Tielkes, K. G. Degen Deutsche Bahn AG DB Systemtechnik Munich, Germany

ABSTRACT When a high-speed train enters a tunnel a compression wave is generated which travels through the tunnel and results under certain conditions in the emission of a so called micro-pressure wave at the opposite portal. Those micro-pressure wave emissions can cause strong acoustic noise which may contradict to an environmentally friendly railway operation. With increasing train speeds, those emissions also play a significant role in the acoustic verification process. Appropriate counter-measures may be applied to the infrastructure or to the rolling stock to limit the emissions to an accepted value. In this paper we present the prediction methods and the acoustic assessment procedures currently in use at Deutsche Bahn AG. Compared to former prediction methods determining only the pressure amplitude, we now calculate full pressure-signals in the time domain and perform an acoustic assessment of the micro-pressure waves using distinct sound pressure levels. The relevant tools and procedures have been developed during the last 4 years, after the first acoustic perceivable micro-pressure wave was emitted at the new high-speed line Nuremberg-Ingolstadt. After that event, all German high-speed tunnels in planning or under construction needed to be evaluated again with the newly developed methods regarding the micro-pressure wave emissions. If necessary, appropriate counter-measures are applied to lower the emissions below the current threshold values. As present standards demand for twin tube, single track tunnels for high-speed lines, many new high-speed tunnels will be equipped with portal hoods. The established prediction and assessment procedures including the acoustic threshold values are currently being transferred into official regulations for the design of new German high-speed tunnels. 1. INTRODUCTION In December 2005, when first test runs were carried out on the new high speed line Nuremberg-Ingolstadt, significant sonic boom incidents occurred at the portals of the 7700 m long Euerwang tunnel and the 7260 m long Irlahll tunnel. The original tunnel planning contained ballasted track but finally a slab track was installed without further adjustments in the tunnel design. This modification in the track bed layout led to a decrease of wave attenuation in the tunnels and, as a consequence, to significant acoustical emissions at the tunnel portals. The micro-pressure wave (MPW) emissions could be reduced successfully by equipping both tunnels with acoustical track absorbers [1] to attenuate the wave steepening within the tunnels. The line went into operation in May 2006 without delay and without restrictions. For future German high speed tunnels the topic will be of increased interest because for safety reasons new railway tunnels shall be built for single track operation in two tubes with a cross section of around 60 m2 each and will be equipped with slab track. In order to assess the properties of these tunnels in the early planning stage and to predict potential micro-pressure wave emissions, accurate prediction models are needed. With these, appropriate countermeasures can be applied earlier in the planning process thereby decreasing construction costs. For the assessment of MPW-emissions in Germany, sound pressure levels are currently in discussion as limiting values instead of pressure magnitudes. To calculate the associated sound pressure levels, the pressure signals in the time domain need to be known. It will be shown that complete MPW signals can be predicted with existing numerical approaches. Empirical matching factors are presented which are derived from comparisons of simulations with full-scale measurements in German tunnels. 2. GENERATION OF THE COMPRESSION WAVE There are several methods to obtain the pressure signal of the compression wave when a train enters a tunnel. On one hand the pressure can be either measured in full-scale for existing tunnels or in model-scale experiments for new portal concepts. On the other hand the generation of the compression wave can be investigated by numerical simulations.

Challenge B: An environmentely friendly railway

There are one-dimensional approaches like the one presented by Howe [2] or more complex and detailed approaches like the three-dimensional flow analysis in [3]. In this paper some results are presented using the commercial CFD-software ANSYS-CFX 12 to simulate the compression wave generation. The computational domain used in the CFD-simulations consists of stationary as well as moving sub-domains connected via fixed domain interfaces (see Figure 1). Detailed configuration and simulation setup is described in [4].

Figure 1: left simulation domain with train, environment, a 25 m long portal hood and

tunnel; right view in direction of train travel with symmetry, interface between moving train mesh and stationary tunnel mesh

In these simulations the train is an ICE 3 which is currently the fastest German train travelling up to 300 km/h on the German rail network. The train model is simplified by closing the intercar gaps and by removing the bogies and the pantographs. A constant train cross section behind the train nose is generated by extrusion up to the end of the moving train sub-domain. The blockage ratio was set to R = Atrain / Atunnel = 0.161. The pressure signal is extracted from the simulations at a point 100 m inside the tunnel behind the tunnel-portal interface. Here, the pressure wave has turned into a one-dimensional wave front, and the interferences due to reflections at the opposing tunnel end do not yet interact with the pressure wave, see [4] for details. A comparison of the entry pressure waves for different portal modifications is shown in Figure 2.

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Figure 2: Compression waves of different portal types (ICE 3 with 300 km/h entry speed)

The vertical portal represents the worst case scenario causing the highest pressure gradients. The rise time of the compression wave is the shortest of all, because the train nose enters the full tunnel cross section abruptly. Every variation from this scenario lengthens the entry process and decreases the entry pressure gradients. A sloped portal increases the entering time before the train nose enters the full tunnel cross section. A hood also increases the entering time and in addition to this, the shape of the entry pressure wave can be adjusted by pressure relief openings in the hood. By varying size

Challenge B: An environmentely friendly railway

and position of these openings the shape of the entry pressure wave can be further adjusted in order to obtain a fairly uniform increase over the entire length of the hood. The full pressure signal in the time domain is transferred to the next stage tool. 3. STEEPENING OF THE COMPRESSION WAVE There are several methods known to calculate the steepening process of the compression wave. They commonly use one-dimensional wave propagation models but differ in the way how frictional losses are modeled. DB Systemtechnik has developed a simulation tool including steady and unsteady friction to predict the wave propagation in German tunnels [5]. The tool solves the one-dimensional Euler equations augmented with quasi-steady and unsteady friction terms as described in [6]:

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Here dh = 4Atunnel/Utunnel denotes the hydraulic diameter of the tunnel and W(t ) is a weighting function which depends on the non-dimensional time = 4/dh2 based on the reversed time = t . For the typical time scales encountered in tunnel aerodynamics ( is in the order of magnitude of 10-7), the weighting function does not depend on the Reynolds number. Roughness of tunnel walls and trackbed is considered by adjusting the parameter us. The friction parameters and us have to be adjusted to the specific tunnel interior and track layout. Full-scale measurements in the 8 km long tunnels Euerwang and Irlahll between Ingolstadt and Nuremberg were carried out to obtain these friction parameters. Both tunnels have an approximately semi-circular cross section of about 92 m2 and are equipped with the slab track system Rheda 2000. Additional acoustic absorbers consisting of porous material are mounted inside and outside the rails in order to attenuate wave steepening [1]. Figure 3 shows some details of the Euerwang tunnel interior.

Figure 3: Interior of the Euerwang tunnel

During the homologation tests in 2006 the pressure wave propagation was measured by installing pressure probes at intervals of 1460 m starting 200 m inside the tunnel. By using different train types (ICE 3 and ICE-S test train) and different entry speeds a wide range of propagating compression waves could be recorded. Measurements were done with and without acoustical absorbers. The measured pressure signals were low-pass filtered by a 4th order Butterworth LP-filter with a cutoff frequency fcut 80 p/tmax / p in order to allow for a reliable determination of the wave steepening. The validation focuses on gradients much smaller than 80 kPa/s which are relevant in practice. In

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