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The New Pendolino family: higher acoustic comfort and reduced environment impact

N. Manzi1– D. Miranda2

1ALSTOM Ferroviaria Acoustics & Aerodynamics Responsible, Savigliano, Italy, 2ALSTOM Ferroviaria Design Platform Not Articulated Train Responsible, Savigliano, Italy

Abstract

ALSTOM is developing a new generation of tilting trains. During the design stage of these trains, special attention is given to the acoustic comfort and the environmental impact. Thus, acoustic signature is a key parame ter in designing the new Pendolino. From the experience obtained from the previous version of tilting trains, it is known that the main component of noise, when the trains are operated at high speed, is generated by the aerodynamic effects. This component has an important contribution to the noise level inside the car and on the environmental impact. Up to now, the aero -acoustic noise has been controlled through the aerodynamic optimisation of the train shape. For the New Pendolino project, a new methodolo gy of analysis has been developed in order to be able to evaluate the impact of the retained train design not only from a qualitative point of view but also from quantitative point of view. Thus, this methodology does not only give the localization of sour ces but does also quantify the main aero -acoustic sources and also does give an evaluation of the contribution of aero -acoustic noise sources on the noise level inside the car as the environmental impact of the train. The aero-acoustic prediction of noise level is based on a combination of CFD (Computational Fluid Dynamic) numerical models and physical tests on scaled mock -ups in wind tunnel. The geometrical models at scale 1:5 and at full -scale for the CFD calculations is obtained from the 3D CATIA model of the train, used for structural design. The geometrical scale 1:5 model is used also to produce the mock -up at scale 1:5 for test in the wind tunnel. The comparison of the analytical results and the experimental measurements carried out with the mock -up at scale 1:5 demonstrates that reliable CFD calculations with the full -scale model can be obtained and thus the influence of the aero -acoustic sources on the inside and outside noise can be quantified.

1 Introduction

The problem of the noise generation duri ng the running of the train is a difficult design aspect to face during the development of a new project, especially when the train can run at 250km/h or more. The noise generated during the train movement is radiated outside, with an environment impact on persons and things near the line, and transmitted inside with an impact in terms of passenger comfort and crew health. For the driver point of view the noise impact has an importance in term of safety being important to limit the noise tiredness effects that can prejudice the necessary level of the attention during the work.

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2 Aerodynamic noise of rolling stocks

The noise problem for the railway transport is separated in two parts according to its impact on people. These two parts are defined in external an d internal noise. The noise produced by the railway vehicles is due to the numerous noise sources present inside and outside. These sources can be divided in three different categories:

- equipments (traction and auxiliary); - rolling (noise generated by the wheel/rail contact); - aerodynamic sources.

Such noise sources have a different contribution on the internal and the external noise and change also with the train speed. During the starting and at low train speeds the main source are the equipments, after t he rolling noise becomes the principal, at speed of around 200 km/h the aerodynamic noise becomes the dominant. In the Figure 1 there is a typical example of variation of the sound pressure level with train speed.

Figure 1: Typical variation of the sound pressure level with the train speed.

The noise control in the development of a new project due to the equipments and the rolling have an important role on the noise produced by the roll ing stock, but today such sources are more simple to manage than the aero -acoustic sources. For this reason Alstom is working to develop a new methodology to control these particularly sources. Based on previous experiences it was possible select and loca te the principal aerodynamic sources produced on the train and test their influence on the internal and external noise. Figure 2 shows the result of a noise simulation of external noise produced by train with distributed power like Pendolino. Such calculation was validated with experimental pass -by of the train on the Italian high speed line “Direttissima” that connect Florence Rome. The measurement and calculation was carried out at a distance of 7.5m from the centre line of the track. The simulation allows to separate the contribution of the different typical sources. The fine continuous curve represent the rolling noise contribution. The main peaks are present in correspondence with the bogies passage.

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The squares r epresent the equipment contribution. In this case the main equipments are located near the motor bogie (gearbox and traction motor). The circles represent the aero -acoustic sources contribution. In particularly such sources are located in:

o The train head and first bogie where there is the first noise peak o The intermediate bogies and gangway, sequence of peak after the passage

of the nose and before the passage of the last vehicle o The pantograph area (at the end of head vehicle). Its contribution is

present at the beginning of the passage of end vehicle. o The train tail where there is the last important peak.

The thick curve represents the sum of all the previous contributions.

3 4 5 6 7 8Time [s]

LpA

dB

(A) r

e 2*

10-5

Pa

Total Rolling Equipments Aero-acoustic

RollingTraction equipmentsAero-acoustic

RollingTraction equipments

RollingTraction equipmentsAero-acousticRolling

Figure 2: Pass-by at 7.5m from the centre line of the track – 250km/h of train speed.

The example clearly demonstrated the importance of the aerodynamic sources for the exterior noise, especially when the maximum sound pressure level is an important design target (LpAFmax). As anticipated the noise of aero dynamic origin has an important contribution also inside the train. This is particularly evident on train with distributed power where the passenger compartment are present also in the leading vehicle and close to the driver’s cab. An example of aero -acoustic contribution inside the passenger compartment is presented in Figure 3 which shows the sound pressure level spectra measured in the same position of passenger compartment when the vehicle is the leading one an d when it is the last one. The noise measured when the vehicle is in the leading position is higher than the noise measured when it is at the end of the convoy. This is due to the different contribution of the aerodynamic sources, more important when the v ehicle is in the leading position.

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Frequency - 1/3 octave band [Hz]

LpA

dB

(A) r

e 2

10-5

Pa

Leading vehicleEnd vehicle

Figure 3: Sound pressure level spectra measured in the same position in the

passenger compartmet when the vehicle is the leading and when it is the end. These simple considerations confirm the importance of the aero -acoustic sources for a train with distributed power like the Pendolino. The origin of the aerodynamic noise is based mainly on the following phenomena:

- Flow detachment on the vehicle surfaces with the generation of macro vortex (narrow band component normally at low frequencies);

- Turbulent boundary layer (broad band in the range of the medium and high frequencies);

- Cavity effects. In principle a good streamline shape of the train is a condition necessary for a low aero -acoustic noise level. Normally to evaluate the aerodynamic behaviour of a train, wind tunnel tests with scale models were carried out. This method has some limitations in term of geometrical detail, mock-up scale factor, Reynolds number and extrapolation of the res ults to the real vehicle (scale 1:1). When the aim of the test is the measurement of aero -acoustic sources an anechoic or a quasi -anechoic wind tunnel is necessary. In the last 10 years numerical simulations CFD (Computational Fluid Dynamics) have gained consideration in the evaluation of the aerodynamic behaviour of the vehicles. Special tools have been developed to study the aero -acoustic phenomena. These tools are able to consider a very detailed geometry of the vehicle and thanks to the fast parallel CPUs they are able to carry out the aero -acoustic simulation in a reasonable time. For the development of the new Pendolino family, Alstom decided to use both methods, test in wind tunnel and CFD calculation, in order to define a methodology to evaluate and localize the main aero -acoustic source on the train.

3 Project procedure in ALSTOM Ferroviaria

In order to obtain reliable aero -acoustic calculations Alstom settled an experimental -numerical procedure that permits to obtain simulations of the aero -acoustic behaviour of a new conception train.

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Preparation of the 3D CATIA model of the vehicles

Validation of the numerical CFD model in scale 1:5

CFD calculation with the real geometry of the vehicles

Internal and external noise prediction

Phase (1)

Phase (2)

Phase (3)

Phase (4)

Guide line to the preparation of the geometry and calculation set-up for

the full scale model

The procedure consists in the following four phases:

- Phase 1 – Preparation of the vehicles geometry - Phase 2 – Numerical model validation in scale 1:5 - Phase 3 – CFD calculation with the real geometry of the vehicl e in scale 1:1 - Phase 4 – Noise predictions

3.1 Preparation of the vehicles geometry – Phase 1

This phase consists on the preparation of the geometry necessary to realize the physical model in scale 1:5 to be used in the wind tunnel and the mathematical models in scale 1:5 (Figure 4) and 1:1 (Figure 5) for the CFD calculations (phase 3). The 3D models are simplified in order to eliminate all the geometrical parts not useful for the executions of the calculation. The various parts of the train are transformed in surfaces that represent the external envelope of the vehicles. These are the surfaces directly in contact with the external airflow. During the modelling, particular attention was given to those zones that are potential sources of aerodynamic noise. Such zones are:

- Train nose, where the airflow have strong acceleration - The under coach cavity of the bogie - Discontinuity between the vehicles - Pantograph cavities on the roof - Other discontinuities

Figure 4: Geometry in scale 1:5 for the mock -up and the CFD model.

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Figure 5: Geometry in scale 1:1 for the CFD model.

3.2 Numerical model validation – Phase 2

The objective of this phase is to validate a CFD calculation model with experimental results obtained due to tests in wind tunnel with a mock -up in scale 1:5.

Test in wind tunnel with model in scale 1:5

Comparison between test and calculation results

CFD calculation with the same geometry adopted in

the wind tunnel test

Is the comparison OK?

Calculation model validated

YES

NOTModification of the geometric detail and/or the

fluid mesh

PHASE 1

PHASE 3

Phase (2a)

Phase (2b)

PHASE 2"Validation of the numerical CFD

model in scale 1:5"

Figure 6: Validation process. The FIAT Research Centre has performed a series of experimen tal measurements in the FIAT wind tunnel. Such measurements have been carried out in order to obtain useful information for the preparation and the successive validation of the CFD calculation model. The FIAT wind tunnel used to carry out the tests is a c lassic Göttingen tunnel with close circuit. The main characteristics are:

- Göttingen closed circuit ¾ open jet test section - Nozzle Exit Area of 31 m 2 - Nozzle Width x Height = 7 m x 4.6 m - Test Section Width x Height x Length = 12.2 m x 10.8 m x 10.5 m - Turntable Diameter = 5 m - Maximum Flow Speed = 195 km/h - Fan with 10 variable-pitch blades and 2 motors 75 & 150 rpm - 7-Components Balance Max Weight = 2000 N each component

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- Boundary Layer Control (BLC): Upstream suction - Boundary Layer Thickness at section centre w /o BLC δ95=35 mm - Turbulence Intensity at section centre < 0.1% - Acoustic treatment: Test section walls, Corner vanes - Background Noise out -of-flow = 75 dB(A) at 140 km/h - Test Section Reverberation Time (estimated) 1.6 s

The mock-up used during the wind tunnel test represents the head vehicle of New Pendolino in scale 1:5. In order to avoid influence on the mock -up of the wake generated by the tail and to guarantee the correct flow field on the leading coach a portion of tail vehicle of 2 m was added. The total length of the mock-up was 7.6m (see Figure 7).

5.6 m 2 m

0.859 m

Figure 7: Mock-up main dimensions.

The mock-up was mounted on two supports with an aerodynamic profile connected with a six components balance.

Figure 8: Mock-up tested

The measurements have been carried out with a wind speed of 160km/h and without upstream suction (boundary layer control) to avoid to influence the pressure fluctuation measurements with the noise gener ated by the suction device. During the test the following activities were carried out:

- Visualization of the air flow with paint; - Static pressure measurements; - Wall pressure fluctuation measurement.

In order to define the test conditions the following inf ormation have been recorded:

- Ambient pressure: 99600 Pa - Temperature: 20.6 °C - Relative Humidity: 13.4%

At the end of the tests the boundary layer total pressure profile on the floor upstream of the nose was measured (see Figure 9).

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Figure 9

The flow visualizations permitted to understand the qualitative behaviour of the airflow on the surface of the mock-up (see Figure 10).

Figure 10: Streamlines on the surface of the train.

Figure 11: Pressure tape positions.

In order to measure the static pressure on the surface of the mock -up 150 pressure probes were installed (see Figure 11) The pressure fluctuations have been measured on 15 points with flush mounted high frequency sensors.

Cab 1

Cab 2

Cab 3 Cab 4

Cab 5 Cab 6

Comp. 1 Comp. 2 Comp. 3

Comp. 7 Comp. 8 Comp. 9

Comp. 4Comp. 5

Comp. 6

Figure 12: Measurement positions of pressure fluctuations

The CFD calculations were carried out using the same geometry tested in wind tunnel and reproducing the same flow and ambient conditions (same Reynolds number). The aero-acoustic calculations have been done using the commercial code named Power FlowTM. The validation process adopted (see Figure 6) is iterative. At the end of each calculation a comparison with the wind tunnel test results was done. The various iterations have permitted:

- to establish the importance some structural train elements - to understand the level of detail of the geometry necessary - to localize the necessary fluid mesh refinement

550 mm

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The iterative process continued until a good aerodynamic simulation of the flow around the vehicle and a sufficient representation of the wall pressure fl uctuation have been achieved. In the following table are listed the calculation conditions used. At the end of the process 22’613’184 voxels (volume fluid cells) are used and the minimum size of the voxel was 0.5 mm.

Free stream condition Speed 160 km/h Density 1.204 kg/m3 Temperature 20 °C Viscosity 1.49 10-5 m2/s Reference dimension (height of the train)

0.859 m

Reynolds No. 2.56 106 Resolution Time step 1.463 10 -5 s Voxels min. dim. 0.5 mm

n. Voxels 22'613’184 The fluid modelling was don e considering different refinement areas. Figure 13 gives a view of the different areas of refinement. The volume mesh named VR0 up to VR7 are created by simple boxes. VR8, where cell size is 2mm, covers all the first carriage and the bogie cavity. Additional VR were put during the study to better resolve the flow field and aeroacoustic loads. They are presented on Figure 13 below: Cell size is 1mm in VR9, and 0.5mm in VR10.

Simulation Volume VR0

VR1

VR2 VR3

VR4

VR5 (16 mm)

Floor

Train

VR5 (16mm)

VR6 (8mm)

VR7 (4mm)

VR8 (2mm)

Figure 13: Fluid mesh description of model in scale 1:5.

A first comparison with experimental data is obtained by the flow visualization. During the test, the mock -up was covered with special red painting and after blowin g on it the painting keeps the flow streamlines that can be used to compare with the simulation results. Overall comparison shows excellent agreement between simulation and experiment.

VR9 (1mm) VR10 (0.5mm)

NUMERICAL WIND TUNNEL

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Figure 14: Streamlines comparison on the surface of the train.

A second comparison with experimental data can be done with the pressure probes on the surface. Results on two lines (y=600mm and z=1300mm, scale 1:1) are plotted on Figure 15 and Figure 16. They show a very good agreement between computation and measurement. This assures that this set -up is good enough to get the right flow patterns around the geometry from the external aerodynam ic point of view.

0

1

2

3

4

5

6

7

8

9

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12

x [m]

z [m

]

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cp

Vehicle profile at y = 600mm Cp calculated V=160 km/h Cp calculated V=160 km/h Cp measured V=160 km/h

0.2

Figure 15: Pressure tape comparison on line y=600mm.

0

1

2

3

4

5

6

7

8

9

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12

x [m]

z [m

]

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cp

Contour line z = 1300mm Cp calculated V=160 km/h Cp calculated V=160 km/h Cp measured V=160 km/h

0.2

Figure 16: Pressure tape comparison on line z=1300mm.

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The acoustic field is correlated via microphones put on the surface. Compa rison is made in terms of the resulting Sound Pressure Levels. Care was taken to be sure that post -processing of both numeric and experimental signals was made utilizing exactly the same parameters of the FFT, to ensure the validity of the comparison. Results are separated in areas to help in the analysis. Some examples of the comparison are show in the following figures (see Figure 17 and Figure 18).

100 1000 10000

Frequency [Hz - Res. 2Hz]

Soun

d Pr

essu

re L

evel

[dB

re 2

10-5

Pa]

Cab6 measured Cab6 calculated

cab 1

cab 2

cab 3cab 4

cab 6

cab 5

Figure 17: Comparison of pressure fluctuation on cab 6.

100 1000 10000

Frequency [Hz - Res. 2Hz]

Soun

d Pr

essu

re L

evel

[dB

re 2

10

-5 P

a]

Comp5 measured Comp5 calculated

comp 1

comp 3

comp 2

comp 5

comp 6

comp 4

comp 7

comp 8

comp 9

Figure 18: Comparison of pressure fluctuation on comp5.

The SPL are correlated well on most probes. The quality of results obtained was good enough to allow to go for the scale 1:1 simulation.

3.3 CFD calculation with the real geometry of the vehicle – Phase 3

At the end of the validation phase the guidelines for a robust modelling were defined. Based on these information the model in scale 1:1 of the train was prepared. Figure 19 gives the main dimensions of the model. 3 vehicles, two head and one intermediate vehicle composed this model. The train model was directly mounted on a model of the rails.

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28.2m82.6m

3.43

mRails

Figure 19: Main dimensions.

In the following table the calculation conditions used are listed.

Free stream condition Speed 250 Km/h Density 1.204 Kg/m3 Temperature 20 °C Viscosity 1.49 10-5 m2/s Reference dimension (height of the train)

3.43 m

Reynolds No. 15.99 106 Resolution Time step 7.2 10-6 s Voxels min. dim. 2 mm

Voxels 94'144’004 Also in this case the fluid modelling was done with different area of refinement. The Figure 20 gives a view of the different ar eas of refinement.

SimulationVolume VR0

VR1

VR2VR3

VR4

VR5 (64 mm)

Floor

Train

Figure 20: Fluid mesh description of model in scale 1:1.

From this calculation the following results are obtained:

- Velocity and pressure field - TBL data along train

• Friction velocity • Boundary layer displacement thickness • Boundary layer convection velocity • Power spectral density (PSD) on the train surface

- Sound power level and directivity of the main aero -acoustic sources These information have been used for the acoustic calculations (phase 4).

VR5 (64mm)

VR6 (32mm)

VR7 (16mm)

VR8 (8mm)

VR9 (4mm)

VR10 (2mm)

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3.4 Noise predictions – Phase 4

The acoustic calculations have been carried out in cooperation with the group Ødegaard & Danneskiold-Samsøe using themselves tools.

3.4.1 Internal noise prediction

The internal noise of the train is calculated using a numerical pred iction program that works by dividing the car into a large number of elements such as floor elements, sidewall elements, window elements etc. Each element is defined separately by its geometry, position and acoustic data. Acoustic sources are taken into ac count by specifying the position and source strength of all relevant sources. Both external airborne sources (e.g. the noise from the wheel/rail contact), external structure -borne sources (e.g. the vibrations transferred from the bogie to the carbody) as well as internal airborne sources (e.g. an air conditioning fan) can be taken into account. For the airborne noise, the sound pressure level on the outer side of each element is calculated based on the geometry of the carbody and the position of the individ ual noise sources. From the sound pressure level outside each element and the airborne sound transmission loss, the radiated sound power from each element into the compartment is determined. Likewise, the structure-borne noise contribution of each element is calculated on the basis of the transmission loss of the carbody structure and the coupling loss from e.g. the carbody sidewall to the interior trim. For the structure -borne noise, the radiated sound power from each element is calculated on the basis of the structure-borne vibration level of the element and the radiation index of the element. The total sound pressure level inside the car is calculated by a summation of the airborne and structure -borne sound power from each element using a combination of a ray tracing method and mirror source techniques. The aero-acoustic noise is separated in two main source types:

- Noise due to the turbulent boundary layer (TBL) - Noise due to “discrete sources” as the bogie cavity, the bogie, the inter -car

cavity, etc.

Noise due to the TBL

The prediction of the noise due to the TBL is carried out in three steps as outlined below:

1. Calculation of TBL data along train using CFD § Friction velocity § Boundary layer displacement thickness § Boundary layer convection velocity

2. Calculation of resulting sound power into train

§ Carried out by acoustical finite element code MSC Actran TM § Results in sound power from each area

3. Calculation of resulting sound pressure level inside train

§ Carried out by ray-tracing method The calculation of the TBL characteristics along the train has been carried out with the CFD calculation. The calculation of the resulting sound power into the train due to the TBL is done by use of the acoustical finite element code Actran TM, which has the capability utilizin g the characteristics of TBL excitation as input and by modelling the structures exposed to the

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TBL, coupling them to the surrounding air, it is possible to obtain the relation between outside pressure and the radiated power inside the train. The transfer function (pressure outside to radiated power into car) is determined for several frequencies to find the average radiated power for each third -octave band. Step 3 (calculation of resulting sound pressure level inside car) is done in the same manner as for all other sources, i.e. the sound pressure level distribution is found by use of a ray tracing method. Acoustical rays are sent out from each source, i.e. each element in this case, into the compartment from all sources.

Noise from discrete sources

The aero-acoustic noise from discrete sources, e.g. the first bogie, is relevant for external noise as well as for internal noise since the noise is also radiated into the car as airborne noise. The source strength of each source is determined carried out as out lined below:

1. Calculation of total sound power of each source § Sound pressure level at reference surface around each source is

determined by CFD-calculation § Integration of sound pressure levels gives total sound power of

source § Same values as used for exter nal noise calculation

2. Estimation of resulting sound power into train

§ Incoming sound intensity on carbody elements close to the source estimated using combination of analytical and measured data for sound propagation around train

§ Results in sound power int o each element § Sound reduction index of element used to determine sound power

into car

3. Calculation of resulting sound pressure level inside train § Carried out by ray-tracing method

At the end the noise contributes of the TBL noise and of the discrete aero dynamic sources are combined with other internal noise contribution (equipments and rolling noise) and the values obtained are compared with the project noise targets.

3.4.2 External noise prediction

The external noise from a train is calculated by specifying al l relevant sources, their position and source strength as well as the noise propagation characteristics. A dedicated programme calculates the total sound pressure level at any given point in the terrain by using classical acoustical methods. The source strength of the aero -acoustic contribution from discrete sources has been updated according to the values obtained with CFD calculation. At the end the noise contribution of the discrete aerodynamic sources is added to the other external noise contributions ( equipments and rolling noise) and the obtained values are compared with the noise targets of the New Pendolino project.

4 Conclusions and future developments

This methodology based on CFD calculation validated by comparing measurements and simulations gave A lstom important information about the internal and external noise of the train before any test on line. But also permit us to identify potential problems and to define the necessary actions to solve them in order to design the best train in terms of noise level. The final step of the validation of the methodology will be done by comparing with measurements on the train obtained during the foreseen tests on line.