This paper presents an experimental method for measuring transmission paths from the exterior to the interior of a passenger vehicle using a reciprocal approach: A production vehicle was placed in a semi-anechoic environment; artificial noise sources were placed at the location of the occupant’s ear(s) inside the vehicle and beamforming arrays with a total of more than 300 microphones were used to observe apparent noise sources on the vehicle exterior resulting from transmission paths. This makes it possible to quickly measure transmission paths over the whole vehicle body. One of the motivations for this work is the monitoring of sealing quality on production vehicles. Artificial seal breaches were introduced on the vehicle and a number of excitation signals were assessed to develop a method to detect and localise leakage noise sources. The investigation and methodology demonstrate the potential for the use of reciprocal beamforming to detect acoustic transmission paths with a view to identifying errors that would impact a vehicle’s wind noise performance, for instance in a manufacturing environment. In addition, it also shows benefits in the use of a similar approach for detecting non-leakage transmission weaknesses, which has applications including the comparison of the isolation performance of various vehicles to external noises.

The reduction in wind noise is increasingly important to vehicle designers as overall vehicle refinement increases. Customers often fit accessories such as roof bars to vehicles, with the aerodynamic interaction of these components generating aeroacoustic noise sources. These are often tonal in nature and of particular annoyance to occupants. Sensors for automated driving fitted to future vehicles may also have a similar detrimental effect on vehicle refinement. Therefore, careful design of such components is important to minimise dissatisfaction. This paper presents the combined application of acoustic beamforming in a full-scale aeroacoustic wind tunnel and the use of a Lattice Boltzmann Method CFD code to characterise the aeroacoustic performance of a roof bar design when fitted to a production vehicle. The use of large microphone arrays within a wind tunnel allows the visualisation of noise sources generated by the flow over a vehicle, with correlation techniques utilised to identify those sources most relevant to occupants. The use of a validated CFD code allows further visualisation of the noise sources virtually, with additional benefits when used in advance of prototype parts being available. A series-production vehicle and roof bar paring were assessed, before deliberately inducing several aeroacoustic error states that may occur during roof bar development. These were analysed using visualisation techniques in both wind tunnel and CFD simulation. A range of aeroacoustic sources were identified, including tonal sources due to vortex shedding and acoustic resonance, as well as broadband sources from the interaction of the roof bars and mountings with flow structures formed over the vehicle. The paper concludes by highlighting key considerations when designing low-noise roof bars.

The ability to assess noise transmitted through seals to cabin interiors early in the design process is very important for automotive manufacturers. When a seal design is inadequate, the noise transmitted can dominate the interior noise, making the wind noise performance of the vehicle unacceptable. This can cause launch delays, increasing costs and risking loss of sales. Designing seals using conventional experimental processes is challenging, since the location and strength of flow noise sources are not known when the seal design is planned. Making changes to the seal system after the tooling stage is expensive for manufacturers as tooling and redesign costs can be considerable. Deliberate overdesign by adding multiple layers of seals in a wide range of locations also can reduce profit by unnecessarily raising part and manufacturing costs. Consequently, there is a strong motivation to use reliable computational capabilities to predict interior noise transmitted through seals early in the design process to address these challenges, designing seals right first time. The current study presents a computational process that can be used to predict interior noise transmitted through seals early in the design process. This computational approach uses a Lattice Boltzmann method (LBM) based computational fluid dynamics (CFD) solver to predict the transient flow field and exterior noise sources. A statistical energy analysis (SEA) solver was used to transmit noise from these sources into the cabin through glass panels and seals. Experiments were performed to quantify noise transmitted through glass panels, window seals and door seals, allowing validation of the computational predictions. Detailed flow analysis was performed to gain insight into the noise sources and the exterior loads on both the seals and glass panels. Accurate prediction of the seal noise and the insight provided by the flow analysis showed that this computational process can be used early in the vehicle development process to design efficient seals for improved wind noise performance.

Vehicle aeroacoustic performance has a major influence on customer perception and also has importance for safety and comfort. Wind noise performance was once differentiated by the quality of sealing. Today, achieving competitive wind noise performance also depends on minimising aeroacoustic noise sources generated by the vehicle form, and on attenuation in the noise pathway from sources on the exterior to the vehicle interior. The reduction in noise transmission, especially through glazed surfaces, will continue to play an important role in controlling cabin noise, with a particular emphasis on achieving attenuation efficiently in terms of component mass. The human brain is not only sensitive towards the level of steady broadband noise, but distinctive features such as tonality or modulation draw the attention of the vehicle occupant and impact negatively on perception. Complex indices are often required to define good wind noise performance. This includes the consideration of multiple frequency bands and effects of the range of yaw angles experienced on-road. A key to achieving future vehicle refinement is bringing together an understanding of unsteady onset flow conditions, their impact on cabin sound pressure level and modulation and, in turn, the impact of noise level and modulation on psychoacoustic perception.

For the automotive industry, the quality and level of the wind noise contribution has a growing importance and therefore should be addressed as early as possible in the development process. Each component of the vehicle is designed to meet its individual noise target to ensure the wind noise passenger comfort level inside the vehicle is met. Sunroof broadband noise is generated by the turbulent flow developed over the roof opening. A strong shear layer and vortices impacting on the trailing edge of the sunroof are typical mechanisms related to the noise production. Sunroof designs are tested to meet broadband noise targets. Experimentally testing designs and making changes to meet these design targets typically involves high cost prototypes, expensive wind tunnel sessions and potentially late design changes. To reduce the associated costs as well as development times, there is strong motivation for the use of a reliable numerical prediction capability early in the vehicle design process.Previous investigations have shown the possibility to use transient CFD/CAA simulations based on Lattice Boltzmann Methods to assess the wind noise performance of mirrors, wipers, underbody designs and buffeting performance of sunroofs and open side windows. This paper presents the use of this computational approach on two production vehicles to assess the broadband noise generated by a fully open sunroof. Computational predictions of mesh deflectors as well as yaw effect were validated against wind tunnel measurements. Also, detailed flow analysis was performed to understand the noise generation mechanisms and to explain the effect of the mesh deflector and flow yaw angle on the interior noise. Accurate prediction of the wind noise performance of the sunroof and the insight provided by the flow analysis proves that this computational approach can be used to make design decisions during the vehicle development process.

Car manufacturers put large efforts into reducing wind noise to improve the comfort level of their cars. Each component of the vehicle is designed to meet its individual noise target to ensure the wind noise passenger comfort level inside the vehicle is met. Sunroof designs are tested to meet low-frequency buffeting (also known as boom) targets and broadband noise targets for the fully open sunroof with deflector and for the sunroof in vent position. Experimentally testing designs and making changes to meet these design targets typically involves high cost prototypes, expensive wind tunnel sessions, and potentially late design changes. To reduce the associated costs as well as development times, there is strong motivation for the use of a reliable numerical prediction capability early in the vehicle design process.In the past, a computational approach based on a Lattice Boltzmann Method has been extensively validated for assessing the wind noise performance of mirrors, wipers, underbody designs and buffeting performance of sunroofs and open side windows. This paper presents the use of this computational approach on the Range Rover production vehicle to assess the sunroof buffeting performance with and without a mesh deflector added, which are commonly used to improve the buffeting performance. This approach was extended to assess the broadband noise generated by the deflector up and the sunroof at vent position. Computational predictions were validated against wind tunnel measurements for all these configurations. Also detailed flow analysis was performed to provide insight into the noise generation mechanisms. Accurate prediction of the wind noise performance of the sunroof and the insight provided by the flow analysis prove that this computational approach can be used to make design decisions during the vehicle development process.

A vehicle driving on the road experiences unsteady flow conditions which are not generally reproduced in the development environment.This paper investigates the potential importance of this difference to aeroacoustics and hence to occupant perception and proposes a methodology to enable better ranking of designs by taking account of wind noise modulation.Two approaches of reproducing the effects of unsteady wind on aeroacoustics were investigated: an active wind tunnel Turbulence Generation System (TGS) and a quasi-steady approach based on measurements at a series of fixed yaw angles.A number of tools were used to investigate the onset flow and its impacts, including roofmounted probe, acoustic heads and surface microphones.External noise measurements help to reveal the response of separate exterior noise sources to yaw.The noise experienced by the driver or passenger ear facing the side-glass is dominated by increased sound pressure levels when the adjacent side-glass is the leeward side of the vehicle with some non-linear effects as leeward yaw produces first accelerated flow and then separation.In part because of non-linearity in response to yaw, a challenging parameter for a wind tunnel simulation of dynamic yaw is achieving a wide enough variation in yaw angle and this work suggests that considering an appropriate range of yaw angles is more important than capturing the dynamics.In terms of passenger perception, the most important effect of a time-varying onset flow was demonstrated to be the modulation of wind noise rather than the increase in time-averaged cabin noise.For the case considered, at 130 km/h, the impact of wind-noise modulation was found to be equivalent to an extra 1-2 dBA in terms of passenger perception, while the increment in time-averaged cabin noise would be only 0.2 dBA.This work seeks to move forward the understanding of how differences between the development and on-road environment can impact wind noise and to point the way to being able to better assess The Effects of Unsteady Flow Conditions on Vehicle in Cabin and External Noise Generation

The reduction of wind noise of vehicle is desirable, particularly in the premium vehicle sector, where customers are sensitive to the levels of background noise in the cabin, leading to discomfort and a reduction in the perceived quality of the vehicle. It is therefore important that any wind noise issues are identified and resolved as early as possible in the vehicle development process. When running aero-acoustic wind tunnel measurements, it is important to get as much information as possible from each test, since facility time is generally expensive. Typical sound pressure measurements using binaural heads at each passenger seat position allow a good comparison of the subjective noise level between different vehicle configurations, but they do not provide direct insight to the source of the measured sound and therefore where to focus vehicle development. A spherical beamforming array is a useful tool for localizing sound sources in the vehicle. With a short measurement time, the tool allows the localization of the key acoustic sources for different frequencies. Different geometry configurations can be tested to direct the test engineers to the best configuration, thus improving the efficiency of measurements. This paper shows the results from wind tunnel measurements with the spherical beamforming array for an SUV and convertible sports car. A number of geometry modifications were assessed to highlight the possible range of measurements. This will be explained in more detail and illustrated by means of examples.

Research has shown that lack of ground effect simulation in the wind tunnel could result in different flow conditions from those experienced by the vehicle on the road, influencing aerodynamic noise inside the cabin. The focus of this work is the influence of moving ground and rotating wheels on both aeroacoustic and aerodynamic measures for a typical European luxury saloon. The acoustic influence due to moving ground and rotating wheels including both flow effects and the background noise contribution of the associated equipment is investigated. Results demonstrate a significant underbody influence in cabin noise up to 5 dBA and confirm the feasibility of using a moving ground plane for aeroacoustic measurements, in contrast to moving ground and rotating wheels which can mask acoustic changes.

On-road, a vehicle experiences unsteady flow conditions due to turbulence in the natural wind, moving through the unsteady wakes of other road vehicles and travelling through the stationary wakes generated by roadside obstacles. Separated flow structures in the sideglass region of a vehicle are particularly sensitive to unsteadiness in the onset flow. These regions are also areas where strong aeroacoustic effects can exist, in a region close to the passengers of a vehicle. The resulting aeroacoustic response to unsteadiness can lead to fluctuations and modulation at frequencies that a passenger is particularly sensitive towards. Results presented by this paper combine on-road measurement campaigns using instrumented vehicles in a range of different wind environments and aeroacoustic wind tunnel tests. A new cabin noise simulation technique was developed to predict the time-varying wind noise in a vehicle using the cabin noise measured in the steady environment of the wind tunnel, and a record of the unsteady onset conditions on the road, considering each third-octave band individually. The simulated cabin noise predicted using this quasi-steady technique was compared against direct on-road cabin noise measurements recorded under the same flow conditions to assess the response of the vehicle to oncoming flow unsteadiness. The technique predicted the modulation of the wind noise under unsteady on-road conditions with good fidelity. This is because the cabin noise response to oncoming flow unsteadiness remained generally quasi-steady up to fluctuation frequencies of approximately 2 to 5 Hz, with fluctuations at higher scales having a progressively smaller impact, and because most of the onset flow fluctuation energy on the road occurs at frequencies below this threshold. The relative impact of the baseline level of cabin noise and the sensitivity of the cabin noise to changes in yaw angle were assessed in terms of occupant perception and this highlighted the importance of modulation. This can provide guidance when assessing the on-road wind noise performance of vehicle geometry modifications and of different vehicles.

Historically vehicle aerodynamic development has focused on testing under idealised conditions; maintaining measurement repeatability and precision in the assessment of design changes. However, the on-road environment is far from ideal: natural wind is unsteady, roadside obstacles provide additional flow disturbance, as does the presence of other vehicles. On-road measurements indicate that turbulence with amplitudes up to 10% of vehicle speed and dominant length scales spanning typical vehicle sizes (1-10 m) occurs frequently.These non-uniform flow conditions may change vehicle aerodynamic behaviour by interfering with separated turbulent flow structures and increasing local turbulence levels. Incremental improvements made to drag and lift during vehicle development may also be affected by this non-ideal flow environment.On-road measurements show that the shape of the observed turbulence spectrum can be generalised, enabling the definition of representative wind conditions.Here, unsteady Lattice-Boltzmann Method (LBM) simulations are used to evaluate the modification of the aerodynamics of a fast-back saloon by realistic on-road flow conditions. The turbulent conditions are added as fluctuations to the freestream flow, based on a prescribed turbulence spectrum, integral length scale, and turbulence intensity. The impact of these flow conditions on the separated flow structures and the incremental effects of design changes are evaluated. Further, the characteristics of anisotropic turbulence in the unsteady flow field around the vehicle are compared to show how freestream turbulence modifies the local turbulent field.This approach enables the evaluation of design decisions against a broader range of operating conditions than can be achieved in traditional wind-tunnel testing.

A vehicle on the road encounters an unsteady flow due to turbulence in the natural wind, unsteady wakes of other vehicles and as a result of traversing through the stationary wakes of roadside obstacles.Unsteady effects occurring in the sideglass region of a vehicle are particularly relevant to wind noise.This is a region close to the driver and dominated by separated flow structures from the A-pillar and door mirrors, which are sensitive to unsteadiness in the onset flow.Since the sideglass region is of particular aeroacoustic importance, the paper seeks to determine what impact these unsteady effects have on the sources of aeroacoustic noise as measured inside the passenger compartment, in addition to the flow structures in this region.Data presented were obtained during on-road measurement campaigns using two instrumented vehicles, as well as from aeroacoustic wind tunnel tests.Conventional admittance functions relating oncoming flow yaw angle to cabin noise response are generally not suitable due to the non-linear steady state characteristics obtained in the wind tunnel, i.e. the cabin noise does not vary with yaw angle in a linear fashion under steady-state conditions.Therefore two alternative approaches were used based on instantaneous conditions to determine a quasi-steady predicted cabin noise time-history.These techniques demonstrated that the cabin noise response to oncoming flow unsteadiness remained generally quasi-steady up to fluctuation frequencies of approximately 2 to 5 Hz, where above this smaller flow scales have a progressively smaller impact on cabin noise fluctuations.Therefore, with a measurement of both the cabin noise in the steady environment of the wind tunnel and the unsteady onset flow conditions, the fluctuations (and thus the modulation) of the wind noise under these unsteady conditions is able to be predicted.

A vehicle on the road encounters an unsteady flow due to turbulence in the natural wind, due to the unsteady wakes of other vehicles and as a result of traversing through the stationary wakes of roadside obstacles. There is increasing concern about potential differences between the steady flow conditions used for development and the transient conditions that occur on the road. This paper seeks to determine if measurements made under steady state conditions can be used to predict the aerodynamic behaviour of a vehicle on road in a gusty environment. The project has included measurements in two full size wind tunnels, including using the Pininfarina TGS, steady-state and transient inlet simulations in Exa Powerflow, and a campaign of testing on-road and on-track. The particular focus of this paper is on steady wind tunnel measurements and on-road tests, representing the most established development environment and the environment experienced by the customer, respectively. Measurements of the surface pressure on the front sideglass were used for comparisons as this area exhibits a complex flow which is highly sensitive to yaw angle and which is also an important region, for wind noise considerations in particular. It was found that, if the transient on-road environment is known then steady-state wind tunnel measurements can be used to predict accurately the transient surface pressures, provided the methodology is sufficiently rigorous. Admittance or transfer function techniques are commonly used to compare transient and steady-state results and the limitations of these methods are shown here when the spectra of self-excited and externally imposed unsteadiness overlap. A new method is introduced to obtain a “true” transfer or admittance function, unconfused by the presence of self-excited unsteadiness. The aerodynamic admittance was found to be close to unity up to a frequency of 2-10 Hz and it then drops progressively.

The in-cabin sound pressure level response of a vehicle in yawed wind conditions can differ significantly between the smooth flow conditions of the aeroacoustic wind tunnel and the higher turbulence, transient flow conditions experienced on the road. Previous research has shown that under low turbulence conditions there is close agreement between the variation with yaw of in-cabin sound pressure level on the road and in the wind tunnel. However, under transient conditions, sound pressure levels on the road were found to show a smaller increase due to yaw than predicted by the wind tunnel, specifically near the leeward sideglass region. The research presented here investigates the links between transient flow and aeroacoustics. The effect of small geometry changes upon the aeroacoustic response of the vehicle has been investigated. It was found that sideglass pressures showed close agreement at all turbulence levels while surface sound pressure levels also showed similar behaviour under a wide range of on-road flow conditions. While the overall sideglass sound pressure level changed under the various yaw conditions, the change in shape of the frequency spectrum was less significant. Geometry changes made to a base vehicle reduced the sensitivity of the in-cabin noise to on-road turbulence, showing that shape-change can modify sensitivity to on-road turbulence.

On-road, a vehicle experiences unsteady flow conditions due to turbulence in the natural wind, moving through the unsteady wakes of other road vehicles and travelling through the stationary wakes generated by roadside obstacles. There is increasing concern about potential differences between steady flow conditions that are typically used for development and the transient conditions that occur on-road. This work considers whether steady techniques are able to predict the unsteady results measured on-road, the impact of this unsteadiness on the noise perceived in the cabin and whether minor changes made to the geometry of the vehicle could affect this. Both external aerodynamic and acoustic measurements were taken using a full-size vehicle combined with measurements of the noise inside the cabin. Data collection took place on-road under a range of wind conditions to accurately measure the response of the vehicle to oncoming flow unsteadiness, with steady-state measurements taking place in full-scale aeroacoustic wind tunnels. Overall it was demonstrated that, using a variety of temporal and spectral approaches, steady techniques were able to predict unsteady on-road results well enough to assess cabin noise by correctly taking into account the varying on-road flow conditions. Aerodynamic admittance values remained less than unity in the sideglass region of the vehicle, with the exception of the the region nearest the A-pillar. The reducing unsteady energy at frequencies greater than 10 Hz, combined with the corresponding roll-off in admittance, implies that unsteady frequencies below 10 Hz affect the vehicle most, where the response remains quasi-steady. Quasi-steady cabin noise simulations allowed a subjective assessment of the predicted unsteady cabin noise, where the impact of cabin noise modulations were quantified and found to be important to perception. Minor geometry changes affected the sensitivity of cabin noise to changes in yaw angle, altering modulation and therefore having an important impact on the unsteady wind noise perceived on-road.