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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
20
50
100
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500
1k
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]:
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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“
4402 PROCEEDINGS OF ISMA2010 INCLUDING USD2010
“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
VEHICLE NOISE AND VIBRATION (NVH) 4403
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
4404 PROCEEDINGS OF ISMA2010 INCLUDING USD2010
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.
VEHICLE NOISE AND VIBRATION (NVH) 4405
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.
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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.
VEHICLE NOISE AND VIBRATION (NVH) 4407
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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-200
-100
0
100
200
300
400
500
600
700
800
900
0 0,5 1 1,5 2 2,5
Zeit [s]
Dru
ck
[P
a]
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.
4408 PROCEEDINGS OF ISMA2010 INCLUDING USD2010
-40
-30
-20
-10
0
10
20
30
40
50
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
-0,5
0
0,5
1
1,5
2
2,5
3
0 0,5 1 1,5 2 2,5
Zeit [s]
Bes
chle
un
igu
ng
[m
/s²]
-40
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-10
0
10
20
30
40
50
0 0,5 1 1,5 2 2,5
Zeit [s]
- [-
]
Surface Integral of Pressure [N]
37
42
47
52
57
62
67
0 0,5 1 1,5 2 2,5
Zeit [s]
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
VEHICLE NOISE AND VIBRATION (NVH) 4409
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
4410 PROCEEDINGS OF ISMA2010 INCLUDING USD2010
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
VEHICLE NOISE AND VIBRATION (NVH) 4411
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.
4412 PROCEEDINGS OF ISMA2010 INCLUDING USD2010
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
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