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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 aerodynamic drag, fuel consumption and hence CO2 emissions, of a road vehicle depend strongly on its flow structures and the pressure drag generated. The rear end flow which is an area of complex three-dimensional flow structures, contributes to the wake development and the overall aerodynamic performance of the vehicle. This paper seeks to provide improved insight into this flow region to better inform future drag reduction strategies. Using experimental and numerical techniques, two vehicle shapes have been studied; a 30% scale model of a Volvo S60 representing a 2003MY vehicle and a full scale 2010MY S60. First the surface topology of the rear end (rear window and trunk deck) of both configurations is analysed, using paint to visualise the skin friction pattern. By means of critical points, the pattern is characterized and changes are identified studying the location and type of the occurring singularities. The flow field away from the surface is then analysed using PIV measurements and CFD for the scale model and CFD simulations for the full scale vehicle. The flow field is investigated regarding its singular points in cross-planes and the correlation between the patterns for the two geometries is analysed. Furthermore, it is discussed how the occurring structures can be described in more generalized terms to be able to compare different vehicle geometries regarding their flow field properties. The results show the extent to which detailed flow structures on similar but distinct vehicles are comparable; as well as providing insight into the complex 3D wake flow.

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 roof-mounted 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.

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.

Road vehicle models are tested in a wind tunnel with a moving ground when an accurate simulation of the under-floor flow is required, for instance for the aerodynamic development of the underbody. The use of a moving ground necessitates that the model and wheels be supported using one or more struts. These struts can cause complex interference flow fields including three-dimensional separations in the area surrounding them. This work aims to quantify the magnitude of these interference effects for a saloon vehicle and to begin to explain the physics of the flow. Models were mounted over a stationary ground plane using an under-floor balance to measure aerodynamic forces and moments on the model. Each model is tested with and without mock struts, which do not actually support the model. Comparisons are made between results from configurations with and without the mock struts in order to quantify their aerodynamic effects. Tests performed on a typical four-door sedan show a slight increase in drag and greater increase in lift due to interference by an overhead support strut. Wheel support stings were found to produce an increase in lift. The compilation of both the overhead strut and wheel stings cause a combined effect on both lift and drag.

The unsteady wake of the Ahmed model has been investigated experimentally using time-accurate 5-hole and hot-wire probes and computationally using Exa PowerFLOW. The backlight angle was 25° which was chosen to be slightlybelow the critical angle observed computationally. The CFD results are similar to experimental results for backlight angles just below the critical angle (eg: 27.5°, 30°). Unsteady flow-field reconstruction of the experimental results and the CFD results both revealed a quasi-two-dimensional vortex!shedding type structure from the bottom of the model base. This results in a semi-periodic build upland collapse of the near wake and produces a symmetric oscillation in the strength and position of the rear pillar vortices. The period of this phenomenon corresponds to a Strouhal number of approximately 0.5 based on free-stream velocity and the square root of the model frontal area.

The current generation of sports racing cars such as those competing under the Le Mans “LM”P and “LM”GTP regulations are particularly sensitive to the pitch of the vehicle. This is a consequence of the low ground clearances that must be adopted to maximise the benefits that can be gained from ground effect and of the very large floor plan area of these cars. To achieve optimum cornering and straight line performance the suspension characteristics are often tuned to the aerodynamic forces in order to reduce the pitch and hence the drag of the vehicle at high speeds whilst retaining relatively high downforce when cornering. A series of accidents at the 1999 Le Mans 24-hour race have highlighted the potential instability of these vehicles which resulted in the catastrophic ‘take-off’ of one of the “LM”GTP cars during the race and others during qualifying and the pre-race ‘warm-up’. The data presented here have been extracted from a detailed experimental study of a typical “LM”GTP car under design and off-design pitch conditions including extreme cases of nose-up pitching moment to assess the onset of instability i.e rotation leading to take-off. Additional data are presented to demonstrate the influence of possible regulation changes upon these parameters.