A full-scale wind-tunnel experiment for studying the noise induced by the A-pillar vortex in a vehicle under different flow conditions

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A full-scale wind-tunnel experiment for studying the noise induced by the A-pillar vortex in a vehicle under

different flow conditions

Alexandre Da Silva, Vincent Valeau, François Vanherpe, Guillaume Bonnavion

To cite this version:

Alexandre Da Silva, Vincent Valeau, François Vanherpe, Guillaume Bonnavion. A full-scale wind- tunnel experiment for studying the noise induced by the A-pillar vortex in a vehicle under different flow conditions. e-Forum Acusticum, Dec 2020, Lyon, France. pp.1523-1526, �10.48465/fa.2020.0549�.

�hal-03215256�

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A FULL-SCALE WIND-TUNNEL EXPERIMENT FOR STUDYING THE NOISE INDUCED BY THE A-PILLAR VORTEX IN A VEHICLE UNDER

DIFFERENT FLOW CONDITIONS

A. Da Silva

¹

V. Valeau

¹

F. Van Herpe

²

G. Bonnavion

^1,2

1

Institut PPRIME UPR 3346, CNRS-Université de Poitiers-ENSMA, France

2

PSA groupe, Vélizy-Villacoublay Cedex, France

vincent.valeau@univ-poitiers.fr - francois.vanherpe1@mpsa.com

ABSTRACT

This study aims to experimentally investigate the aerodynamic and acoustic phenomena related to the A-pillar vortex, with a particular focus on the effect of the turbulence of the oncoming flow on the A-pillar vortex dynamics and on the noise in the cabin. In this purpose, a full-scale wind tunnel experiment with an 3/4 open jet test section type has been set up, using a production vehicle. The turbulence of the oncoming flow is varied by using a bluff body generating a wake upstream of the vehicle under study.

In a synchronized manner, aerodynamic and wall-pressure measurements on the left front glass of the vehicle were performed, as well as sound pressure measurements in the vehicle. The influence of the turbulence generator on the power spectral density of the data are analysed, showing that some turbulent flow conditions enhance the noise level inside the cabin at low and high frequencies, but are also responsible of significant noise fluctuations.

1. INTRODUCTION

The noise perceived inside motor vehicles has become an essential criterion of comfort and safety for the passengers, as much as a concern for the manufacturers. Among the sources of noise heard inside the vehicle cabin, aerodynamic noise gets dominant for speeds higher than 90 km/h.

In particular, George [1] has identified the A-pillar vortex as an important source of aerodynamic noise. This structure is particularly sensitive to the unsteadiness of the flow due to oncoming wind turbulence or to the wake of other vehicles.

The effect of the unsteadiness of the oncoming flow on vehicles has been studied in numerous aerodynamic studies (eg., [2, 3]). Studies addressing its effect on aerodynamic noise are more scarce. Helferet al[4] showed that wind unsteadiness is responsible for an amplitude modu- lation of cabin noise, this noise being broadband and particularly dominant in the mid and high frequency range.

Such phenomena has been studied by Lindener et al [5]

with both road and wind-tunnel tests. The purpose of the present paper is to report an experiment aiming at generating unsteady flow conditions in a full-scale wind-tunnel, in order to investigate its effects on the A-pillar vortical

structures and its consequence on the noise heared inside the cabin. In particular, the experiment has been designed in order to carefully assess the simultaneous effect of the unsteadiness of the oncoming flow on the vortex dynamics and on the inner noise. The experiment is presented in Section 2, followed by some results (Section 3) and the general conclusions (Section 4).

2. DESCRIPTION OF THE EXPERIMENT The car model used in this study is a Peugeot 308 phase II. This car car was installed in the full scale aeroacoustic wind-tunnel S2A (Souffleries Aéroacoustiques Automo- biles) located in Montigny-le-Bretonneux (France), fully described in [6]. The door gaps of the vehicle were carefully and fully taped in order to avoid spurious sources of noise. The front engine cooling air-intake was sealed as well and windshild wippers were removed. Full covers were placed on the wheels and cabin aerators were closed.

The inner acoustic field was measured by using two microphones embedded in the ears of an artificial binaural head measurement system (Head Acoustics). Some aerodynamic sensors were installed under the A-pillar vortex (Fig. 1). Four surface microphones (model GRAS 40PS-1 CCP) were used for measuring the wall-pressure fluctuations on the driver’s side window (#1 behing the side mirror and # 2-4 under the A-pillar vortex). One hot-wire (labelled A, model Dantec 55P11) was located at the basis of the driver’s side window in order to measure the velocity fluctuations of the oncoming turbulent structures.

Hot-wires B and C were located 50 mm downstream of the surface microphones 2 and 4, to measure the velocity fluctuations under the A-pillar vortex (at a distance of approximately 5 mm from the side window). For the results shown in this study, the left side mirror was removed in order to eliminate the corresponding source of noise.

The operating flow speed of the wind-tunnel was varied from 110 to 160 km/h (ie., approximately, from 30 to 45 m/s); only results at 140 km/h are given in this paper. The yaw angle was varied in the experiments, but only results at a yaw angle of 0^◦are presented in this communication.

The effect of the turbulence of the oncoming flow was investigated in this study. It was generated by a so-called turbulence generator of width 1.5 m, simulating the wake of a

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Figure 1. Locations of the probes: A, upstream hot-wire, B, C donwstream hot-wires; 1 to 4: surface microphones.

vehicle located upstream, at 4 m from the car under study (Fig. 2). Two positions were considered for the turbulence generator, one aligned with the vehicle under study (position 1), while for position 2 the generator center is aligned with the left side of the car.

Figure 2. Definition of the positions (1 and 2) of the turbulence generator.

3. RESULTS BY SPECTRUM ANALYSIS This section shows some results based on spectrum analysis. The sampling frequency is 48 kHz, and all power spectral densities (PSD) are calculated by using the Welch’s method with 800 averages. For the microphone signals, the PSDs are normalized with the reference pressure, 2.10⁻⁵ Pa.

First, we investigated the probes’ spectra for a reference case corresponding to a flow speed of 140 km/h, a zero yaw angle and no turbulence generator (Fig. 3). Fig. 3.a exhibits the PSDs of the hot-wires. Signals are generally broadband, with the energy decreasing when the frequency increases. The velocity fluctuations have a weak energy for the upstream hot-wire (C); the estimated turbulence level is 0.4%. Conversely the PSDs are several orders of mag-

nitude above for the hot-wires located close to the side window (under the A-pillar vortex), with turbulence lev- els of 15 and 19 % respectively for hot-wires B and C, which shows that the turbulent velocity fluctuations measured close to the window and due to the A-pillar vortex are intense. The spectrum of the velocity fluctuations measured by hot-wire C (upper part of the window) also reveals a peak at frequency 160 Hz.

The spectra of Fig. 3.b concern the wall-pressure fluctuations measured by the surface microphones. The most intense wall-pressure fluctuations are found for sensors 3 and 4,ie., in the upper part of the window. The spectrum of microphone 2 is significantly lower, from 6 dB at low frequencies to about 3 dB at high frequencies. The most intense wall-pressure fluctuation are known to be located under the vortical structure generated by the A-pillar (pri- mary and secondary vortices) [7], so it is likely that microphones 3 and 4 are under the vortical structure while microphone 2 is located slightly outside. The spectra of microphones 2-4 also exhibit a peak at the same frequency as hot-wire C (160 Hz). This is most probably the frequency of the main instability convected by the A-pillar vortex.

Figure 3. a) PSDs of the hot-wire signals; (b) PSDs of the surface microphone signals; (c) PSDs pf the microphones inside the cabin.

The acoustic spectra as measured by the binaural head inside the car (Fig. 3.c) are broadband with local frequency

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peaks. The microphones located at the left and right ears have similar spectra, but a peak around 4 kHz is observed for the left ear, which corresponds to the coincidence frequency of the window and is due to the fact that the left ear is closer to this window. The frequency of the main instability of the A-pillar vortex does not appear in the acoustic spectra, so it can be concluded that it does not have a significant influence on the acoustic field in the cabin.

The influence of the turbulence generator is investigated in Fig. 4. In Fig. 4.a, the PSD of the upstream hot-wire is investigated. The results indicate that the presence of the upstream generator produces a great increase of the unsteady fluctuations of the oncoming flow. The turbulence intensity level, initially equal to 0.4 %(no generator), rises to 13%(position 1) and 18%(position 2). Po- sition 2, where the generator center is aligned with the left side of the car, produces the maximum turbulence intensity level. The influence of increasing the flow turbulence on the acoustic field measured inside the car is plotted in Fig. 4.b, which plots as a function of frequency the PSD level difference in dB, compared to a reference level corresponding to no turbulence generator ((Fig. 3.c)). It is found that the turbulence generator produces an increase of the sound pressure level at low frequencies (< 100 Hz, up to 4 dB) and high frequencies (> 1kHz, up to about 3 dB for position 2). In the mid -frequency range, the influence is weak for position 2, and even results in a drop of the acoustic level for position 1.

The results in Fig. 4.b have been calculated by using the Welch method, so that they indicate mean acoustic lev- els. We are now interested in the noise fluctuations heard inside the vehicle. On this purpose, we use the time- domain function (noted down Φ(t)) developed by Zumu Doli et al. [8], which is an indicator of the temporal vari- ations of the energy of an acoustic signal in a given third- octave frequency band. If the mean and standard deviation of theΦ(t)function are denoted byµΦandσΦ, one can show that the noise level fluctuations in dB are of the order of20log(1 +σΦ/µΦ). This is why in Fig. 5, we plot the quantityσ_Φ/µ_Φcalculated for a range of third-octave bands. The results show that the configuration generating the highest noise fluctuations is configuration 2, followed by configuration 1. The absence of the turbulence generator is favourable to the lowest noise fluctuations, while turbulent flow conditions produce noise fluctuations that increase at high frequency.

4. CONCLUSIONS

An experiment has been set up in the full scale aeroacoustic wind-tunnel S2A. A production car (model Peu- geot 308 phase II) has been installed in the wind-tunnel flow with different turbulent conditions. The turbulence of the oncoming flow was created by a turbulence generator located upstream the vehicle at two different positions.

The driver’s side window was instrumented with hot-wires (one at the basis of the A-pillar to measure the “input” conditions and two measuring the flow at a distance of 5 mm from the window) and surface microphones. The acoustic

Figure 4. a) PSDs of the upstream hot-wire signal (A), with no turbulence generator, and with the generator in positions 1 and 2; (b) inner acoutic field: difference in dB induced by the presence of the turbulence generator in positions 1 and 2.

field inside the cabin was measured by using an artificial binaural head.

The results show the broadband nature of the signals, with a dominant peak at 160 Hz for the sensors located within the A-pillar structure. Interestingly, this instability does not create a frequency peak in the spectrum of the acoustic field measured inside the cabin, which is of broadband nature with small local frequency peaks. The velocity fluctuations are very turbulent inside the A-pillar vortex, the turbulence intensity reaching 19%. The presence of the turbulence generator is shown to enhance the turbulence of the oncoming flow, the turbulence intensity rising from 0.4%(no generator) to 18%. Those turbulent flow conditions are shown to increase the noise level inside the cabin at low and high frequencies, but are also responsible of significant noise fluctuations.

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Figure 5. Ratio σ_Φ/µ_Φ for the inner acoustic field per 1/3 octave band, with no turbulence generator and with the generator in positions 1 and 2.

5. ACKNOWLEDGEMENTS

The authors warmly acknowledge Laurent Philippon for his invaluable technical support. This work is supported by OPENLAB FLUIDICS.

6. REFERENCES

[1] A. George, “Automobile aerodynamic noise,” SAE Technical Paper, 1990.

[2] J. Howell, J. Baden Fuller, and M. Passmore, “The effect of free stream turbulence on a-pillar airflow,”SAE Technical Paper, vol. SP-2226(2009-01-0003), 2009.

[3] N. Oettle, O. Mankowski, D. Sims-Williams, and R. Dominy, “Assessment of a vehicle’s transient aerodynamic response,”SAE Technical Paper, vol. 2012- 01-0449, 2012.

[4] M. Helfer, M. Riegel, and J. Wiedemann, “The effect of turbulence on in-cabin wind noise - a comparison of road and wind tunnel results,” inProceeding of the 6th MIRA International Conference on Vehicle Aerody- namics, Gaydon, UK, 2006.

[5] N. Lindener, H. Miehling, A. Cogotti, F. Cogotti, and M. Maffei, “Aeroacoustic measurements in turbulent flow on the road and in the wind tunnel,”SAE Technical Paper, vol. SP-2066 (2007-01-1551), 2007.

[6] P. Waudby-Smith, T. Bener, and R. Vigneron, “The GIES2A full-scale aero-acoustic wind tunnel,” inSAE Special Publication, no. 1874, pp. 283–295, 2004.

[7] J. Fischer, V. Valeau, L. Brizzi, and J. Laumonier,

“Joint acoustic and wall-pressure measurements on a

model a-pillar vortex„”Exp. in Fluids, vol. 61, Article 54, 2020.

[8] C. Zumu Doli, V. Valeau, L. Brizzi, J. Valire, H. Lazure, and F. Van Herpe, “Transmission des fluctuations de bruit dans un modle dhabitacle automobile gnres par un coulement instationnaire : tude en souf- flerie,”Acoustique & Techniques, vol. 88, pp. 26–33, 2018.