CAR MIRROR INDUCED AERODYNAMIC NOISE
Wagner Duarte Machado
FIAT - Chrysler - LATAM, Rodovia BR381, KM429, 32501-970, Betim. MG, Brazil
Eduardo Bauzer Medeiros
Dept. Engenharia Mecânica, Universidade Federal de Minas Gerais-UFMG, Av. Antonio Carlos
6627, 31270-901 Belo Horizonte, MG, Brazil email: [email protected]
The development of road test procedures to determine the wind induced noise produced by
motor car side mirrors is considered. The sound pressure levels were initially obtained in a
motorway varying the vehicle speed from 60 km/h to 110 km/h, using a set of surface pressure microphones attached at the outside surface of the car window, close to the mirror.
These results were also compared with measurements taken inside the cockpit. Another set
of results was obtained without the effects produced by air flow around the mirror by means
of a set of simulated track
1.
Introduction
Road vehicles have become considerably quieter in recent years as a result of research[1], and
the traditional noise and vibration sources in a vehicle such as the engine have become less important. At the same time other effects have gained in importance as a major nuisance consideration
such as electric pumps and the air conditioning fan. Structural coupling, together with its side effects has also become a matter of concern, and now the noise path which has to be taken into account, (structural or airborne) should be carefully examined [1].
A source of noise which has gained in importance in the last few years, particularly at higher
speeds, is the aerodynamic noise generated by a variety of car components such as side mirrors, the
object of the present study.
2.
Aerodynamic noise overview
2.1 General Description
Aerodynamic automotive noise is the result of pressure fluctuations in the air flow field around
the vehicle as it moves along a road. These modifications in the flow field become apparent as perceivable noise inside the vehicle as speed increases. The origin of these flow disturbances are either
the result of geometric discontinuities, such as wipers, mirrors and others, or air leakages which
modify the flow field, the latter introduced by door or window gaps, modifying flow transitions and
boundary layer detachment and generating oscillating flows.
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Considerable amount of research has been dedicated to this area of Aeroacoustics, where intensive wind tunnel testing and computational simulation, mainly CFD methods, have been used to
identify noise sources and improve aerodynamic streamlining. It should be mentioned incidentally
that aerodynamic noise figures also provide an insight into fuel consumption and efficiency, an additional and most important consideration for this development.
Aerodynamic noise becomes more apparent at higher speeds [4], as the pressure gradients increase, introducing wide band noise. Now mirror noise is mainly composed by lower frequency
component associated with alternate vortex formation downstream of the mirror.[6].
Other authors such as Chen et. al. have carried out wind tunnel and numerical studies in order to
describe the flow field around car side mirrors and the associated noise. The general idea of this
experiment can be observed by observing Fig. 1, where the vortex formation is associated with the
relative local flow field downstream of the mirror, and how it can be associated with microphone
positioning.
Figure 1. Flow Field around a mirror and microphone position.
3.
Experimental setup and procedure
The experimental procedure consisted of two independent parts, one comprising the measurements carried out in a highway in a compact car, the other measurements carried out inside an insulated chamber with a roll dynamometer with the same vehicle.
3.1 Experimental Setup
Four G.R.A.S. 40PS microphones were the chosen sensors for the sound measurements. These
microphones have been designed to enable sound measurements inside air flow, taking into account
that sound pressure levels are the desired results instead of a combination of turbulence pressure
fluctuations and sound pressure levels. The geometry of this microphone, which is shown in Fig 2,
also enables this sensor to be attached over a flat surface by means of an adhesive collar with the
sensing element levelled with the reference surface of the measurement field, therefore minimizing
flow interference.
Figure2. Surface Microphone 40PS.
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A set of four surface microphones was attached to the front window in a 2x2 matrix arrangement, close to and downstream of the mirror as shown in Fig. 3.
Figure 3. Surface Microphones Arrangement (outside the window).
Microphone positioning with respect to the flow wake is also very important to reduce the possibility of pure fluid flow parameters introducing false microphone readings. The chosen position for
the microphones were however inside the vortex flow generated by the mirror, according to what
has been previously mentioned in this text.
Figure 4. Microphone positioning inside the cockpit.
As an additional measure against the possibility of false (that is turbulence pressure fluctuations
instead of sound pressure levels) an additional set of microphones was positioned inside the cockpit
where normally the ears of the driver and the passenger would be positioned. This set up, which
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was used afterwards for a different experiment, is shown in Fig. 4. The readings taken inside the
cockpit were compared to the readings taken outside for all cases.
Microphones 1 and 2 were positioned 150 mm behind the mirror flat surface, and microphones 3
and 4 also 150 mm from the other microphones. The vertical distance between microphones 2 and 4
and the lowest part of the mirror was set to 50mm. Microphones 1 and 3 were positioned 100 mm
higher than the lowest part of the mirror. The gaps between the window glass and the car body were
sealed with adhesive tape, to reduce flow interference effects.
The microphones were connected a LMS Scadas Mobile digital analyser.
3.2 Experimental procedure
The first part of the test was carried out in high speed highway having a smooth pavement, and
without any curves or hills in the part chosen for the test. The quality of the road surface is an important consideration because of the second part of the procedure. The measurements were carried
out at the following speeds: 60 km/h, 70 km/h, 80 km/h, 90km/h, 100 km/h, 110 km/h and 120
km/h. The car was driven in 5th gear and at a constant speed during the measurements. The sequence of measurements was repeated a couple of times at exactly the same road location to verify
dispersion and repeatability in the complete set of measurements.
The second part of the test was carried out with a roll dynamometer where all the conditions of
the first part of the test were simulated in an acoustically insulated chamber, without the effects of
aerodynamic noise.
During the two parts of the experiments the digital analyser was set up to store each set of data
for 15 seconds, and it used a linear averaging procedure with a Hanning window. The measurement
frequency bandwidth was set between 2,0 Hz and 10 kHz. The dedicated LMS software was used
for post processing of the results.
Finally the speed of the vehicle was monitored with and encoder mounted at one of the car
wheels.
4.
Results.
The results obtained for the tests are shown in Fig 5. They show the Power Spectral Density
(PSD) associated with the values measured by the microphone in Position 1. However it is important to mention that fairly similar tendency between the two sets of tests has been obtained with
the other microphones. The values obtained from the road measurement are shown in red and blue,
each representing one separate measurement, and the small difference between each set of results is
typical of all the set of measurements which have been obtained. Similarly, the lower lines in green
and pink represent two set of results for measurements carried out with the simulated road condition
using the roll dynamometer.
The yellow line represents a visual reference of a line drawn 10 dB bellow the road test values
obtained for the entire spectrum range, helping in the comparison of the obtained values between
the road test and the simulated road without aerodynamic noise. The difference of 10 dB has been
chosen in a somewhat arbitrary fashion, for easier comparison with some of the normative procedures in use by the Brazilian automotive industry. Even though there is still the need for some sort
of normalization, this dividing line suggests that it is possible to identify the contribution of aerodynamic noise introduced by the car mirrors, with the exception of the higher frequency components
measured at 60 km/h, indicated by a circle drawn over the corresponding Figure, where in any case
at this speed the aerodynamic contribution is not important.
The combined set of results of aerodynamic noise obtained for each microphone is shown in
Fig. 6, where it is possible to confirm unsurprisingly that the importance of aerodynamic noise increases with speed.
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(a)
(b)
(c)
(d)
(e)
(f)
Figure 5. Power Spectral Density Values obtained for the two testing conditions and for (a) 60 km/,
(b) 70 km/h, (c)80 km/h, (d) 90 km/h, (e) 100 km/h, (f) 110 km/h
Additional information is also to be obtained from the spectrogram shown in Fig. 7.These results
are being presently analysed, taking into account fluid flow structure, flow detachment and vortex
formation.
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(a)
(b)
(c)
(d)
Figure 6: Total aerodynamic noise for the mirror measured at position (a) P1. (b) P2, (c) P3, (d) P4
Figure 7: Sound pressure levels and the associated spectra
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These results have also been compared with the data produced by other authors, such as, and
they seem to provide a fair agreement between them.
5.
Main conclusions
The determination mirror induced aerodynamic noise has been considered. The proposed method
still under current development seems to have already produced consistent results with acceptable
dispersion and a desirable measurement repeatability, that is, the developed road test procedures did
provide useful information at a fraction of the time and cost of the more expensive wind tunnel testing.
It should be mentioned however that even though the dynamometer procedure to evaluate the
non-mirror contribution to the overall noise figure appears to confirm the feasibility of the proposed
method, additional testing is still required in the dynamometer chamber to provide more accurate
results, which is also currently being corroborated by computer simulation, presently under development by the authors. Also this procedure needs to be verified with other testing and modeling
information, introducing as a reference items such as tire noise developments.
Additional wind tunnel testing is currently being carried out by the authors and co-workers to
improve the presented set of results.
REFERENCES
1 Guimarães, G. P., and Bauzer Medeiros, E. The Use of Experimental Transfer Path Analysis
in a Road Vehicle Prototype Having Independent Sources, Proceedings of SAE Brazil Noise
and Vibration Conference, Florianópolis , Brazil (2008) – SAE 2008-36-0055.
2 Giorjão, T., Albuquerque, E., Cherman, A. Noise sources balancing on a vehicle. Development to improve customer satisfaction, Proceedings of the Brazilian Symposium of Vehicular
Acoustics, IX SIBRAV, São Paulo, Brazil (2007).
3 Cogotti, A. Evolution of performance of an automotive wind tunnel, Journal of Wind
Engineering and Industrial Aerodynamics, 96(1), 667-700, (2008).
4 Senthoran, S., Lepley, D., Hendriana, D., and Prazer T. Numerical Simulations and Measurements of
Mirror Induced Wind Noise, Proceediongs of SAE International Congress, Detroit, USA (2009).
5 Chen, K.H.. Johnson, J., Dietschi, U., and Khaligi B. Wind Noise Measurements for Automotive Mirrors, Proceediongs of SAE International Congress, Detroit, USA (2009).
6 Grahs, T., and Othmer, C. Evaluation of aerodynamic noise generation: parameter study of a generic
mirror. Evaluation of the aeroacoustic source strength, Proceedings of the European Conference on
Computational Fluid Dynamics, ECCOMAS-CFD, The Netherlands (2006).
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