Elsevier

Applied Acoustics

Volume 197, August 2022, 108893
Applied Acoustics

Numerical investigation of whistling sound in narrow-gap flow of automobile side mirror

https://doi.org/10.1016/j.apacoust.2022.108893Get rights and content

Highlights

  • The generation mechanism of the narrow gap whistling of a side mirror was investigated.

  • A wind tunnel test was performed to find the flow conditions for the onset of the whistling sound.

  • The generation mechanism of the whistling sound was captured using compressible LES techniques with high-resolution grids.

  • The main aerodynamic sources were identified using the vortex sound sources.

  • The generation mechanism of the whistling sound based on the feedback mechanism was revealed.

  • The standing wave pattern in the narrow gap was shown to be coupled with the identified vortex sound sources.

Abstract

It is frequently reported that a high-frequency whistling noise is generated by airflow through the narrow gap of a vehicle’s side mirror under certain driving conditions. The aim of the present study is to identify the generation mechanism of the narrow gap whistling of a side mirror, which is considered one of the most challenging problems due to too the small dimensions and complex geometry of the gap. A wind tunnel test was performed to find the flow conditions where the whistling sound is generated. The relative directions and speeds of the airflow were identified for the onset of the whistling sound and its frequency. A numerical analysis of the external and internal flows of the side mirror was then conducted using compressible Large Eddy Simulation (LES) techniques with high-resolution grids to capture the generation mechanism of the whistling sound, which is generally based on a feedback mechanism between the vortex and sound waves. The numerical result was validated through a comparison of the predicted sound pressure spectrum with the measured ones. The main aerodynamic sources were identified using the vortex sound sources, and a generation mechanism of the whistling sound based on the feedback mechanism was revealed. This was done by visualizing the standing wave pattern in the narrow gap, which was coupled with the identified vortex sound sources.

Introduction

The contribution of aerodynamic noise increases in comparison to structure-borne noise as a vehicle’s running speed increases, especially for electric vehicles, of which production and sales are on the rise. Due to geometric discontinuity, the rear-view side mirror of an automobile is one of the biggest contributors to aerodynamic noise. Lounsberry et al. [1] experimentally investigated laminar-flow whistling over a vehicle’s side mirror housing. The boundary layer starts as laminar and then attempts to switch to turbulent but fluctuates at a high frequency. Flow visualization results identified the laminar separation region on the mirror housing, and boundary layer trips were shown to eliminate the whistling by introducing turbulence into the boundary layer.

Lee et al. [2] experimentally investigated whistling noise using a simplified side mirror for various angles of the geometry at a selected flow speed. A high-frequency tonal noise and its harmonics were clearly observed. They also found another narrow-band frequency and its harmonics. Werner et al. [3] extracted the length scales of a vertical structure where a shear layer develops near the trailing edge of a side mirror and its upper surface. They related the scales to the acoustic radiation. They also experimentally investigated the tonal self-noise emission of a vehicle’s side mirror and the associated flow field and showed that a distinctive pattern of tonal noise emission was associated with the shear layer instability modes [4].

Frank et al. [5] reproduced these experiments and observed an acoustic feedback loop using large eddy simulation and direct acoustic simulation. The simulation was based on the high-order discontinuous Galerkin spectral element method for non-conformal curved elements. These studies investigated aerodynamic noise due to external flow over side-mirror housings.

Recently, it was frequently reported that high-frequency tonal noise called whistling is generated by airflow through the narrow gap of a vehicle’s side mirror under certain driving conditions. The aim of the present study is to identify the generation mechanism of the narrow-gap whistling of a side mirror. This problem has been considered one of the most challenging aeroacoustic problems for vehicles due to the small dimensions and complex geometry of the gap. Whistling noise is generally induced in a specific flow structure that repeats periodically at a specific frequency. This phenomenon is caused by a feedback mechanism, which is one of the fundamental generation mechanisms of tonal noise.

Chanaud et al. [6] categorized the feedback mechanisms of whistling sounds into three classes, as shown in Fig. 1. The common excitation mechanism that triggers the feedback mechanism for all three classes is the flow instability originating from shear flow, which occurs when the flow speed reaches a certain critical speed. Class I refers to flows including vortex generation that affects the next vortex generation, which results in periodic vortex generation and causes tonal noise at the vortex shedding frequency. This feedback is completely hydrodynamic: one vortex induces instabilities of another vortex without any contribution from sound waves. A typical example of Class I is vortex shedding noise.

Classes II and III refer to flow structures involving sound waves causing the feedback phenomena. The difference between Classes II and III is the feedback distance of a sound wave. Class II refers to a flow structure where the feedback distance of a sound wave is very small compared to the wavelength of the sound wave. Class III denotes those where the feedback distance of a sound wave is an appreciable fraction of the wavelength of the sound. The former is sometimes called intermediate-field or quasi-compressible feedback, and the latter is called far-field or acoustic feedback.

One example of Class II feedback is the edge tone noise of a jet flow. Instabilities from a sheared jet hit an object downstream and generate a vortex. Due to this vortex, a sound wave of a certain frequency occurs, which returns to the place where the instabilities are generated and contributes to the maintenance of the instabilities. However, a sound wave can propagate into a resonator, such as a nearby pipe or cavity. If the frequency of the sound wave is the same as the resonant frequency of the resonator, feedback is repeated. This additional reflecting structure distinguishes Class III from the others. A typical example is an air-reed instrument.

A characteristic of a compressible feedback mechanism is frequency lock-in. This is a phenomenon where the whistling frequency increases as the speed of the flow increases, and then the whistling frequency becomes fixed at a specific frequency even if the speed of the flow increases. The frequency lock-in does not occur in Classes I and II. The narrow-gap whistling of a side mirror may be related to Class II or III, given that a complex internal flow structure is involved.

Fig. 2 presents typical flow configurations of instability-induced excitation with possible acoustic resonators [7]. In Fig. 2a, the basic excitation mechanisms are categorized as mixing layers, jets, wakes, swirling flows, jet tones, and cavities. Fig. 2b shows acoustic resonators that can be coupled with the mechanisms.

The internal flow configuration through a side mirror’s narrow-gap is complex and three-dimensional. A cavity is located between the inlet and outlet gaps. Based on the geometric characteristics of the narrow gap, the flow excitations of the jets, the jet tones, and cavities coupled with possible acoustic resonators are expected to be related to the whistling sound due to the narrow-gap flow of the side mirror. To the best of our knowledge, there has been no study about the whistling sound due to the narrow-gap flow of a side mirror. Thus, a literature review was performed on the preceding studies on jets, jet tones, and cavities coupled with acoustic resonators.

Vaik et al. [[8], [9]] studied an edge tone configuration consisting of a planar free jet that impinges on a wedge-shaped object. They carried out a parametric study by varying the mean exit velocity of the jet and the nozzle-to-wedge distance for both top-hat and parabolic-jet velocity profiles. They used this to provide a prediction formula for the edge tone frequency. Takahashi et al. [10] investigated two- and three-dimensional edge tone noise using aerodynamic sound theory. They numerically visualized the whistling sound using hydrodynamic variables and aerodynamic sound sources that were derived by Lighthill [[11], [12]] and rearranged by Howe [13].

Miyamoto et el. [14] studied acoustic vibrations of two- and three-dimensional flue pipes of an organ. The flow configuration was similar to that of Takahashi et al. [10] but included an additional resonator geometry. The frequency lock-in was observed at the pipe’s modal frequencies. Mohany et al. [15] and Hong et al. [16] investigated the acoustic resonance due to the wake flow of a cylinder in a duct. They found the frequency lock-in mechanism between the vortex shedding frequency and duct acoustic mode. Hong et al. identified two kinds of lock-in regions: synchronous and the duct-mode dominant regions.

After Rossiter [17] studied the tonal noise of cavity features, the feedback mechanism of cavity noise has been a classical problem in related research fields. Although the cavity acts as a resonator, Class II feedback can occur between the upstream cavity edge where the instability from the shear layer begins, as well as the downstream cavity edge where an acoustic wave is generated by collision with the upstream vortex. Schoder et al. [18] analysed the shear layer of a generic deep cavity with an overhanging lip. They identified the interaction between the boundary layer of shear flow shedding from the leading edge and acoustic wave back-propagating from the trailing edge of the cavity shape. The peak frequency was shown to be inversely proportional to the boundary layer thickness.

Yang et al. [19] analysed the feedback mechanism and described that the characteristic steps of feedback were vortex generation, shedding vortex, aerodynamic noise generation, vortex regeneration, re-shedding, and aerodynamic noise regeneration. Weipeng [20] described and visualized a feedback-loop cycle with a phase-averaged analysis of an open cavity simulation under supersonic conditions. A feedback acoustic wave was shown to be generated by Mach wave reflection at the trailing edge.

Sun et al. [21] investigated the flow instabilities of a two-dimensional open cavity by varying the free stream Mach number from 0.1 to 1.6. A stability curve was provided over a wide range of Mach numbers and Reynolds numbers. Yamouni et al. [22] performed a global stability analysis over an open cavity. They showed that in the incompressible regime (Ma less than 0.3), the flow is subject to global instabilities due to Kelvin-Helmholtz instabilities in the shear layer, which are strengthened by a hydrodynamic pressure feedback. In the compressible regime (Ma > 0.3), all unstable global modes are continuously connected to the incompressible shear layer modes as the Mach number decreases.

Winkler [23] et al. investigated the acoustic resonance of a cooler(which was a kind of cavity) in a turbo engine. In resonance, the cavity mouth flow had a flapping motion with local vorticity ejection and a large separation bubble. The forcing of the cavity was shown to consist of cavity mode, spatial shear layer instabilities, and Helmholtz resonator.

This study investigated the generation mechanism of the narrow-gap whistling of a side mirror. The mechanism seems to be related to all of the described whistling sound-generation configurations (jet tone and cavity combined with a possible acoustic resonator). A more challenging problem is that both the external flow of a side mirror housing and the internal flow of its narrow-gap need to be accurately simulated at the same time to reproduce the whistling sound generation due to the narrow-gap flow.

A wind tunnel test was performed to confirm the flow conditions where a whistling sound is generated by a side mirror. The relative directions and speeds of the airflow were identified for the onset of whistling sounds with the corresponding whistling frequency. A numerical analysis of the entire external and internal flows of the side mirror was conducted using compressible Large Eddy Simulation (LES) techniques with high-resolution grids to capture the generation mechanism of whistling sounds, which is generally based on the feedback mechanism between the vortex and the sound waves. The numerical results were validated through a comparison of the predicted sound pressure spectrum with the measured ones. The main aerodynamic sources were identified using the vortex sound sources, and the generation mechanism of whistling sounds based on the feedback mechanism was also revealed by visualizing the standing wave pattern formed in the narrow-gap and coupled with the identified vortex sound sources.

Section snippets

Experimental setup

An experiment for whistling sound measurement was conducted to find the inflow conditions where the whistling sound is generated by the narrow-gap flow of a vehicle side mirror and to characterize the frequency spectrum of the generated whistling sound. Fig. 3 shows the experimental setup for the measurement of aerodynamic noise from airflow passing through a side mirror. A small wind tunnel was used to create air inflow for the side mirror. For recording the whistling sound, a microphone was

Numerical analysis

In this study, two types of calculations were conducted. A computational aeroacoustic simulation based on a compressible LES technique was conducted for simultaneous simulation of both the flow and acoustic fields. To identify the flow structures responsible for the whistling sound generation, a vortex sound source was used as the aerodynamic sound source index. Acoustic modal analysis was then performed on the interior domain of the narrow-gap passage to find the possible feedback mechanism of

Flow analysis results

The boundary layer needs to be simulated with sufficient resolution for reliable computation of the internal flow of the narrow-gap. Fig. 11a and 11b show the computed y+ distributions over the outer surface of the side mirror, and Fig. 11c and 11d show those of the lower and upper surfaces of the narrow-gap at t = 0.125 s. It can be seen that y+ is less than 1 for most surface regions. These results confirm that the LES with the given grids resolves the internal and external boundary flows of

Conclusion

This study investigated the generation mechanism of whistling in a narrow gap of a side mirror using high-resolution LES techniques. An experiment was performed to find the inflow wind conditions for the onset of whistling sound and the corresponding spectral characteristics. It was found that the whistling sound began to be generated when the inflow wind speed reached 20 m/s, and the whistling frequency increased from 3626 Hz to 4342 Hz as the inflow velocity increased from 20 to 26 m/s. Then,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. 2020R1F1A1066701).

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