approaches to analyse and predict sloshing noise of vehicle fuel tanks

Approaches to analyse and predict slosh noise of vehicle fuel tanks

C. Wachowski1, J.-W. Biermann

1, R. Schala

2

1 Institut fuer Kraftfahrzeuge – RWTH Aachen University, Acoustics Department,

Steinbachstraße 7, D - 52074, Aachen, Germany

email: [email protected]

2 Audi AG, Ingolstadt, Germany

Abstract Sloshing is generated by fuel motions in the tank. Depending on fuel type, filling level, tank geometry and

excitation slosh noises result. Passengers can perceive this phenomenon as airborne noise as well as

structure-borne noise.

Due to the increasing lightweight construction the vibro-acoustic properties of vehicle bodies change.

Thus, the significant noise contribution of sloshing might be transferred better into the passenger

compartment. Tank sloshing mainly occurs and is received due to manoeuvres like stop-and-go traffic or

parking. Arising hybrid vehicles turn off the combustion engine during these operating conditions, so that

no dominant combustion noise masks the slosh noises. Hence, fuel sloshing is investigated in a holistic

approach by the Institut fuer Kraftfahrzeuge of RWTH Aachen University (ika), the Forschungsgesell-

schaft Kraftfahrwesen Aachen mbH (fka) and the Audi AG.

The aim of this research is the development of methods for the acoustic fuel tank design process and

adapted vehicle integration. Hereby, a main topic is the evaluation of CAE-tools regarding their capability

in predicting valuable results. The particular challenges arise by reason of the multiplicity of sound

generating mechanisms and the complexity of the acoustic system.

The investigation starts with a detailed analysis of the sound source: A tank with sloshing fuel. The

measurements are performed on a special test rig and take place in a semi-anechoic acoustic chamber to

eliminate disturbing influences. By using an acoustic camera sound sources are detected on the tank

structure. CFD-simulations of the sloshing are performed in parallel, in order to gain a deeper under-

standing of the slosh motions.

Following structural and sound radiation analyses are performed and assessed aiming to clarify the

dependency on the tank structure properties of slosh noise. Again, the capability in predicting relevant

acoustic behaviour of attendant utilised simulation tools is proven. Main challenges here are the non-linear

material properties and the high material damping. A comparison of sloshing noise, eigenmodes and

transfer functions shows the correlation between them. The experimental structural and radiation analyses

are also simulated with CAE-methods in order to show the capability of those methods. Outgoing from

these results, approaches to optimise or rather decrease slosh noises can be developed.

1 Introduction

Because of fuel motion, sloshing can be generated in dependency on tank geometry, filling level, fuel type

and excitation. Parameters and physical relations contributing to this acoustic phenomenon are not totally

clarified by now [1]. Passengers can receive this as airborne and structure-borne noise and assess it as

comfort reducing. Figure 1 shows an exemplary measurement of slosh noise in the passenger

compartment.

4399

f/H

z

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2k

5k

t/s4 5 6 7 8 9 10 11L/dB(C)[SPL]35 40 55 60

L/d

B(C

)[S

PL

]

35

40

45

50

55

60

65

70

t/s4 5 6 7 8 9 10 11

Sloshing

Figure 1: Slosh noises in the passengers compartment

These slosh noises were measured in a vehicle by an artificial head placed on the co-driver’s seat. Because

this slosh noise has got deep frequency sound parts, the C-weighting is used instead of the common A-

weighting. The figure shows that the sloshing can remarkably be noticed in the passenger compartment. It

can be received comfort reducing and is sometimes interpreted as a vehicle or rather chassis damage.

Although diesel and gasoline engine driven vehicles mostly possess tanks, which are identical in

geometrical construction, tanks partly filled with diesel show a more critical sloshing behaviour. That

means, separate sloshing phenomena can remarkably be heard and felt in the passenger compartment.

Hereby saddle tanks possess a different sloshing behaviour in comparison to single chamber tanks, due to

their design (see Figure 2).

Figure 2: Single-chamber tank (left) and saddle tank (right)

In saddle tanks with two chambers of different length two separate slosh events can occur in short

temporal distance. This happens, because the sloshing eigenfrequency of a partly filled tank depends on

the filling level and the chamber length. It can be calculated by the equation (1), [2]:

4400 PROCEEDINGS OF ISMA2010 INCLUDING USD2010

2222

ijb

j

l

ihtanh

b

j

l

i

4

gf (1)

With is f – frequency

i, j – mode number in x and y

l – dimension in x-direction, length

b – dimension in y-direction, width

h – dimension in z-direction, height

Because of the complex noise generating mechanisms and the high dynamics, a deeper investigation is

necessary to understand the phenomenon and to develop measures to reduce slosh noises. Each acoustic

system is described by a source, a transfer system and a receiver. That is why, single components of this

acoustic system need to be analysed. Figure 3 shows the developed model of the acoustic system.

Sound generation in

the tank

Airborne noise in

passengers' compartment

Transfer via sound

propagation

Structure-borne noise in

passengers' compartment

Slosh motion

in the tank

Transfer via

suspension strapps

Transfer via body

contact points

Vibrations of the

tank wall

Structure-borne noise

Airborne noise

∑

Vehicle body

Figure 3: Model of the acoustic system

As the figure shows, noise is generated directly as airborne noise and as structure-borne noise. In any case

the vibrations are transferred through the tank structure. From here the noise propagates as secondary

airborne noise and as structure-borne noise into the passenger compartment.

This means for the acoustic investigation that the NVH behaviour of the tank, its suspension and the

vehicle body has to be investigated. Aim of the investigation steps is the development of methods and

approaches for acoustical optimisation of the tank and its suspension as well as the validation of

countermeasures.

VEHICLE NOISE AND VIBRATION (NVH) 4401

2 Sloshing and slosh noises

In [1] the two sloshing noise types “splash” and “hit” are distinguished. The performed investigation

shows, a third noise type called “clonk” should be introduced. Summing up, slosh motion can generate

sloshing noise. “Splash”, “hit” and “clonk” describe special sloshing noise types.

2.1 Types of slosh noise

All three types are presented in the following.

„Splash“

In [1] the sloshing of fluid waves into each other is considered as noise generating mechanism for “splash”

noise. Figure 4 shows a schematic diagram.

Figure 4: Schematic diagram „splash“

Two wave fronts sloshing into each other are presented. The red lines symbolise the occurring noise. In

general “splash” has got a lower sound intensity than “hit” under a comparable excitation. Further on,

higher frequencies are more pronounced and sparkling bubbles contribute to the overall noise.

„Hit“

Beyond that, [1] declares “hit” noise as second sloshing noise type. It is generated by wave fronts hitting

the tank wall (see Figure 5).

Figure 5: Schematic diagram „Hit“


“Hit” is received less “swooshy” and tends to a higher sound pressure level in comparison to “splash”. In

addition, lower frequencies are more pronounced. Due to the direct excitation of the wall, the sound

characteristics are supposed to depend on the acoustic properties of the wall such as damping and

eigenmodes.

„Clonk“

In order to classify a third type of sloshing noise, “clonk” is proposed. It describes a dark and clean

sounding impact. Among the presented phenomena “clonk” possesses the sound with the lowest

frequency. The name is derived from the term “gear clonk”, because of its similar sound. Performed

concept tests let arrive the conclusion, that “clonk” is generated, when sloshing liquid compresses air

volumes abruptly (see “impact bubble” [3] and Figure 6).

Figure 6: Schematic diagram „clonk“

In some degree the phenomenon can be provoked by a comparatively low excitation. A geometry, which

abets an inclusion of air at certain filling levels and an according wave form of the sloshing fuel are

determining. After presenting the noise generating mechanisms their acoustic differences are investigated

in the following.

The three phenomena are compared by a wavelet analysis. Here the wavelet analysis provides a higher

time resolution and frequency resolution relating to the capability of a Fast-Fourier-Transformation. That

is why, the wavelet analysis is suitable to visualize transient and abrupt noises (for example: door slam

noise). Figure 7 presents the analyses of the three phenomena and relevant criteria for their description.

Figure 7: Comparison of the three phenomena, wavelet analysis


Further on, a single phenomenon can be divided into different frequency ranges, to investigate parallel

occurring effects. In particular “splash” is suitable to separate single part noise phenomena, due to its

longer duration (see Figure 8).

Highpass: 7000Hz

Lowpass: 500Hz Bandpass: 750 - 2000Hz

No filter

Sparkling of little

air bubbles

Liquid – in – liquid

sloshingWall hits

Figure 8: Separation of “splash” into part noises

The analysis demonstrates that even the description of an apparently well-defined phenomenon is

complicate, due to overlapping of single noise generating effects. These relations are rather in a scientific-

academic focus, though. Thus, the terms “clonk”, “hit” and “splash” represent a sufficient categorisation

for the development process at the moment.

2.2 Bench tests

In a preceding step driving tests were performed to measure sloshing noises and the exciting manoeuvres.

An exemplary manoeuvre is presented in Figure 9. In this diagram the longitudinal acceleration of the

vehicle and its derivation is shown. A manoeuvre is described by the following parameters:

MAX,Ba - maximal acceleration

MAX,Va - maximal deceleration

MAX,Ba - maximal acceleration gradient

MAX,Va - maximal deceleration gradient


Figure 9: Exemplary driving manoeuvre

An analysis shows that manoeuvres with a wide range of parameters lead to slosh noises. Using these

results the specification of the test bench was stated:

Replication of driving maneuvers

One-dimensional excitation and motion of the sled

Copy of the vehicle body suspension point

Enhanced accessibility

Use of an acoustic camera for sound source localisation

Electro-mechanical drive of the sled

The defined reference manoeuvre is presented in Figure 10.

Figure 10: Reference manoeuvre

The diagram shows the longitudinal acceleration as well as the according velocity and distance of the sled.

A concept was designed and at last the test bench was built up. The measurements are performed in a

semi-anechoic chamber. Figure 11 shows the design draft, which demonstrates the functionality of the test

bench.


Microphone array

Sled

Tank

Absorbing wedges

Linear guiding elements Linear motion system

Figure 11: Design draft of test bench

It is noticeable, that space for acoustic wedges is provided below the tank. Those wedges prevent ground

reflection of sound above 125 Hz. Due to sloshing the tank structure is excited abruptly, by what it

radiates broad band noise. The acoustic camera is pictured free floating, in order to ensure a good clarity

of the figure. It is carried by a frame on the real sled (see Figure 12). So, its position and distance to the

tank are fixed, what provides a high quality of the recordings.

Figure 12: Test bench with measurement systems

To perform measurements in the presented amount, three PC’s (conventional acoustic measurement

system, acoustic camera, drive system) have to be handled parallel and the two measurement systems have

to be synchronised.

In order to compare the results with the test drives, the same measurement setups are used. Because of the

better accessibility, sensors can be placed on other or more positions to investigate further points.

Triaxiale accelerometers are placed as shown in Figure 13.


Skizze des Tanks Messpunkte-Gitter

Figure 13: Tank structure and measurement grid

The blue elements represent the tank and the red elements represent the suspension straps. The airborne

noise during sloshing is recorded by an artificial head. For a comparison of different tanks the artificial

head is always placed in the same position. Beside this “conventional” acoustic measurement system, an

acoustic camera is applied. Acoustic cameras work according to the beam forming method. At this, the

position and the sound pressure levels of sound sources in a reference plane with a defined distance are

determined using an array of n microphones. The applied system possesses 32 microphones.

The resulting “sound pressure level map” is overlaid with an optic picture. Thus, a sound source

localisation on the test object is capable. Figure 14 exemplifies an acoustic picture, such as the acoustic

camera and the associated software generate.

Driving direction

Localised

sound source

Secondary

chamber

Main

chamber

Figure 14: “Acoustic picture“ of a slosh event, top view

The figure shows the analysis of a sloshing event. The localised sound source can be detected clearly and

the result matches the subjective reception.


2.3 CFD-Simulation of sloshing

The first attempts in calculating sloshing were performed for designing tank ships. Those analytic models

were refined and later also used to solve stability problems of aircrafts and space vehicles with liquid fuels

[4]. In order to perform more detailed and efficient analyses of complex tank geometries, up-to-date CFD-

methods are necessary. By placing virtual sensors in the tank model, physical values can be recorded, such

as pressure or velocity. For the analysis of the simulation results different visualisations are selectable.

Pressure and the pressure distribution of the liquid surface (ISO-surface) are considered as most important

values in the present context. Figure 15 shows a tank model with applied virtual sensors and the according

pressure-time diagram.

Pressure

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0

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0 0,5 1 1,5 2 2,5

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HK Front

HK PDome 1

HK PDome 2

HK Side

HK Top

NK Crossbaffle

NK Front

NK PDome 1

NK PDome 2

NK Top L

NK Top R

Pre

ssure

[Pa]

Time [s]

Figure 15: Virtual sensors and pressure-time diagram

The diagram shows, that in each tank chamber a slosh event appears, which is indicated by a pressure

peak. Furthermore, the pressure contribution on the ISO-surface can be visualised, in order to create a

quick and clear overview of the slosh events. Figure 16 shows an exemplary analysis of two slosh events

at different time steps.

Figure 16: Visualisation of pressure distribution on ISO-surface

The red areas are remarkable. They represent a high pressure appearance. Besides these visualisations, the

surface integral of the pressure on ISO-surface can be calculated. The resulting force-time diagram allows

a simple identification of slosh events.

In the following a comparison between the results of simulation and bench test is performed. That is why a

tank model is investigated having the same filling level and being excited by the same manoeuvre (see

Figure 10) as in the relating bench test. Outgoing from the surface integral of pressure on the ISO-surface

the slosh process is divided into five relevant periods. Figure 17 shows the division of periods.


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0 0,5 1 1,5 2 2,5

Zeit [s]-

[-]

Surface Integral of Pressure [N]

Halt

Mounting upof liquid

Maximum displacement

Slosh main chamber

Slosh secondary chamber

Time [s]F

orc

e [N

]

• Halt

• Mounting up of liquid

• Maximum displacement

• Slosh in main chamber

• Slosh in secondary

chamber

Figure 17: Surface integral of pressure, division in five periods

In a concluding step the results of driving tests, bench tests and simulations can be merged. The next

picture (Figure 18) contains three diagrams:

1. Longitudinal acceleration of the tank

With this value the simulation and the bench test results can be synchronised.

2. Surface integral of pressure on the ISO-surface

This calculated value is used to identify slosh events.

3. A-weighted sound pressure level

The measured sound pressure level is recorded during the bench tests.

In addition to the diagrams, two “acoustic pictures” recorded by the acoustic camera are presented. Here,

only the two main slosh events are shown. At last, a side view of the simulated slosh process is displayed.

-1

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0

0,5

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1,5

2

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Bes

chle

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igu

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/s²]

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37

42

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67

0 0,5 1 1,5 2 2,5

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SP

L [

dB

(A)]

Working noises

Critical points

identified in

simulation

Simulation – Integrated pressure

Acceleration Profile – Identical for

simulation and test

Test bench – Noise

Figure 18: Comparison of simulation and testing


Besides the slosh noises, other noises can be noticed in the sound pressure level diagram (right, top). At

0.2 s a peak occurs in the run, which results from a trigger signal for synchronisation of the two

measurement systems. Between 0.4 s and 1 s the working noise of the test bench is dominant. The actual

slosh noise arises at 1.2 s (main chamber – red) and 1.4 s (secondary chamber – blue).

In the run of the surface integral of pressure on the ISO-surface are two peaks at 1.2 s and 1.4 s noticeable

(diagram bottom left). Relating to this, level two peaks arise in the run of the measured sound pressure,

caused by sloshing noises. Utilising the acoustic camera, according sound source on the tank can be

localised. The resulting locations match with the points considered critical points of the simulation. In

summary the comparison of test bench and simulation results shows a good match in time and location of

the slosh events. Thus, the simulation can provide a suitable forecast of test bench results and remarkably

complements them.

As the reliability of the simulation results has been shown, the proceeding can be applied to develop

optimised geometry versions. Figure 19 compares two different geometries.

Version A

Version B

Figure 19: Comparison of two tank geometries

As one can see, version B possesses horizontal and vertical baffles. The baffles have got holes, to allow a

reflux of the liquid in its position of rest. The following diagram (see Figure 20) shows the according

simulated results.

Figure 20: Comparison of the surface integral


The diagram shows that the remarkable peaks in the run of version A at 1.3 and 1.5 s are lacking in the run

of version B. Outgoing from the previous validation of the simulation it can be stated that a real tank with

the properties of version B will show a lower slosh noise level.

3 Structural analysis of a polymer fuel tank

An experimental modal analysis is performed in order to demonstrate the contribution of the structural

properties of the tank to the slosh noise. In this part of the present investigation the non linear material

characteristics and the high damping rate of PET represent a challenge.

3.1 Definition of the test setup

For this investigation simplifications are necessary:

The influence of liquid is not considered, for it is not possible to rebuild the liquid distribution in

the tank at the moment of sloshing.

The fuel pump and all pipes are removed to avoid noises like rattle.

The tank is mounted by rubber ropes.

Figure 21 shows the resulting measurement grid for the modal testing, which is transferred to the real tank.

x

y

z

x

y

z

vehicle coordinate system

vehicle coordinate system

Figure 21: FE-model of the investigated tank and measurement grid

The figure demonstrates two perspectives of the investigated tank and the 34 sensor positions. Point 1 is

meant for excitation. For this an electro-dynamic shakers is used. The excitation signal is white noise with

a frequency range from 15 to 1800 Hz. Its amplitude is adapted in order to gain a good correlation

between excitation and response.

The tank is suspended in rubber ropes to simulate a mounting as free as possible. The vibrations are

measured with a triaxiale accelerometer.

3.2 Comparison of operational vibrations and modal behaviour

As one result of the experimental modal analysis the transfer functions of all measurement points are

calculated and summed up to one total transfer function. A modal model can be generated and the mode


shapes and modal damping values can be determined. In order to compare the modal structural behaviour

with the slosh vibration of the tank, FFTs of the slosh vibrations measurement are determined and

summed up in the same way as for the modal analysis. Figure 22 shows the correlation between the

summed up operational spectrum of a “clonk” sloshing and the summed up transfer function of the modal

analysis.

Figure 22: Comparison of summed spectrum (top) and transfer function (bottom)

Outgoing from these relations, the correlation between sum up frequency responses of dynamic bench test

and stationary modal analysis is shown. When the noise generating mechanisms were explained, it was

stated that “Clonk” noise is supposed to have a higher influence on the structural properties because of

missing splashes and the relatively direct excitation of the tank wall.

In this case the spectrum of a typical “Clonk”-measurement is taken into account. The comparison

demonstrates that there is some accordance between both curves. The difference results from different

effects:

In the dynamic bench test a tank with all of its parts, such as fuel pump or pipes, is investigated.

Those parts are missing in the modal analysis. They are removed in order to reduce rattling and

because only the structure is meant for investigation.

The influences of the liquid are missing. The liquid distribution in the tank cannot be modelled in

a static test. That is why, it is totally left out.

According to these limitations the results are in the scope of expectations. It can be stated that the slosh

noise type “clonk” depends to the structural-modal properties of the tank.


Following steps intend to validate an according FE-model and to employ tools for topography/topology

optimisation in order to lower the NVH-emissions.

4 Conclusion

In dependency on filling level, tank geometry and excitation fuel sloshing appears. Passengers receive this

phenomenon as airborne and structure-borne noise and may assess it as comfort reducing.

Thus, an investigation on slosh noise was performed. Outgoing from a detailed ascertainment of the

vehicle’s actual situation, a test bench was designed and constructed. Already during the design phase, it

was considered to perform measurements with an acoustic camera. Hence, a high quality of the

measurements could be provided. Furthermore, the test bench was equipped with an electromechanical

drive in order to obtain a high reproducibility of the excitation manoeuvre. A comparison of the bench and

test drive results demonstrates that the sloshing noise can be reproduced on the bench in a sufficient

degree. The analysis with the acoustic camera helps to understand and visualise the complex acoustic

phenomenon.

Another major part of this investigation was the simulation of the sloshing fluid by CFD-methods. A

model was set up and relevant parameters to assess the fluid motion regarding slosh noise were compared

with measured values. Altogether, the simulation can help to predict the sloshing behaviour of tank

geometry prototypes. In a following investigation the tank structure contribution to the slosh noise type

“clonk” was proven. In accordance to the previous assumptions there is some kind of dependency between

the “clonk”-characteristics. This leads to the conclusion that an optimisation of the tank structure can

achieve lower noise emissions and vibrations. In order to develop optimisations and countermeasures

CAE-tools can be utilised as presented.

Concluding it can be stated that the performance of up-to-date measurement and simulation tools were

presented. It was shown, in which way the developed approach leads to an efficient and productive

analysing and predicting of slosh noise.

References

[1] S. aus der Wiesche, Noise due to sloshing within automotive fuel tanks, Springer-Verlag,

Heidelberg (2005)

[2] H. Lamb, Hydrodynamics, Dover Publications, Sixth Edition, Dover (1932)

[3] B. Godderidge, M. Tan, C. Earl, S. Turnock, Grid Resolution for the Simulation of Sloshing using

CFD, 10th Numerical Towing Tank Symposium (NuTTS'07), Hamburg (2007),

http://eprints.soton.ac.uk/48789/

[4] X. Gou, T. Li, X. Ma, B. Wang, Forces and moments of the liquid finite amplitude sloshing in a

liquid-solid coupled system, Applied Mathematics and Mechanics, English Edition, Vol. 22, No. 5,

Shanghai University, Shanghai, 2001


approaches to analyse and predict sloshing noise of vehicle fuel tanks

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