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Technical Paper

The Effects of Unsteady On-Road Flow Conditions on Cabin Noise

2010-04-12
2010-01-0289
At higher speeds aerodynamic noise tends to dominate the overall noise inside the passenger compartment. Large-scale turbulent conditions experienced on the road can generate different noise characteristics from those under steady-state conditions experienced in an acoustic wind tunnel. The objective of this research is to assess the relationship between on-road flow conditions and the sound pressure level in the cabin. This research, covering links between the unsteady airflow around the vehicle and aeroacoustic effects, is a natural progression from previous aerodynamic studies. On-road testing was undertaken using a current production vehicle equipped with a mobile data logging system. Testing was carried out on major roads at typical highway speeds, where wind noise is very significant. Of particular interest are high-yaw conditions, which can lead to a blustering phenomenon.
Technical Paper

An Estimation of the Unsteady Aerodynamic Loads on a Road Vehicle in Windy Conditions

2004-03-08
2004-01-1310
Road vehicles operate almost entirely in the unsteady conditions created by the natural wind and the wakes of other vehicles. This is a time dependent and turbulent environment, which differs noticeably from the conditions simulated in the wind tunnel. Using a quasi-steady analysis the aerodynamic characteristics, as determined from wind tunnel tests, are used to derive the unsteady aerodynamic loads experienced by a typical road vehicle subjected to a random wind input. For this paper the wind energy spectrum is of the von Karman type and isotropic turbulence is assumed. The effects of vehicle speed, wind speed and wind direction on lift and side force spectra are presented.
Technical Paper

A Novel Test Rig for the Aerodynamic Development of a Door Mirror

2006-04-03
2006-01-0340
Door mirrors have a small but measurable contribution to the overall aerodynamic drag of a road vehicle. Typically for passenger cars and SUVs this is in the range 2.5–5%. It can be difficult to refine the shape of door mirrors as the improvements are, sometimes, too small to measure with any accuracy. A test rig has been developed which allows a full size door mirror to be tested in a model wind tunnel facility, which has better balance resolution, where the mirror is mounted to a partial vehicle body. This also results in a faster and cheaper method to develop shapes for door mirrors. The rig is described and the initial correlation tests presented. The limitations of the rig and some further applications are discussed.
Technical Paper

The Effect of Backlight Aspect Ratio on Vortex and Base Drag for a Simple Car-Like Shape

2008-04-14
2008-01-0737
The vortex structure in the wake of a car creates drag. In the case of a simple wing this drag component is well defined as a function of lift, but for road vehicles the relationship is more complex. The backlight surface has been shown to be a significant source of vortex drag and in this paper the influence of backlight aspect ratio on both vortex and base drag is investigated. The vortex drag factor is found to be independent of aspect ratio, while the base drag component is shown to be dependent on the ratio of base to frontal area.
Technical Paper

The Influence of Aerodynamic Lift on High Speed Stability

1999-03-01
1999-01-0651
The effect of aerodynamic lift on both straight line stability and lane change manoeuvrability of several small and medium sized European passenger cars has been determined from subjective track tests. The straight line and manoeuvring performance degrades with increasing lift and decreasing pitching moment. Increasing speed exacerbates the problem.
Technical Paper

The Influence of Ground Condition on the Flow Around a Wheel Located Within a Wheelhouse Cavity

1999-03-01
1999-01-0806
A 3D Navier-Stokes CFD model of a wheel located within a wheelhouse cavity has been produced. Both a stationary wheel on a fixed ground and a rotating wheel on a moving ground were considered. Extensive comparisons with the results of a wind tunnel investigation based on the same geometry are presented. These consist of three force coefficients and pressures on the internal faces of the cavity. Comparison with the experimental results gave encouraging agreement. It was found that the rotating wheel produced more drag than the stationary wheel whilst shroud drag decreased when the groundplane was moving compared to when it was stationary.
Technical Paper

Transient Aerodynamic Characteristics of Simple Vehicle Shapes by the Measurement of Surface Pressures

2000-03-06
2000-01-0876
Transient force and surface pressure data has been measured on a range of simple geometric shapes in order to gain an understanding of the complex time dependent and separated flow around a vehicle when subjected to a crosswind. The experiments were carried out using the Cranfield University model crosswind facility. It is found that the leeward face is the dominant area of transient activity. Maximum and minimum peak yawing moments at gust entry and exit are compared
Technical Paper

Aerodynamic Drag of a Compact SUV as Measured On-Road and in the Wind Tunnel

2002-03-04
2002-01-0529
Growing concerns about the environmental impact of road vehicles will lead to a reduction in the aerodynamic drag for all passenger cars. This includes Sport Utility Vehicles (SUVs) and light trucks which have relatively high drag coefficients and large frontal area. The wind tunnel remains the tool of choice for the vehicle aerodynamicist, but it is important that the benefits obtained in the wind tunnel reflect improvements to the vehicle on the road. Coastdown measurements obtained using a Land Rover Freelander, in various configurations, have been made to determine aerodynamic drag and these have been compared with wind tunnel data for the same vehicle. Repeatability of the coastdown data, the effects of drag variation near to zero yaw and asymmetry in the drag-yaw data on the results from coastdown testing are assessed. Alternative blockage corrections for the wind tunnel measurements are examined.
Technical Paper

The Effect of Free Stream Turbulence on A-pillar Airflow

2009-04-20
2009-01-0003
Various studies have shown that the level of wind noise experienced inside cars on the road in unsteady conditions can be substantially different from that measured in wind tunnel tests conducted using a low turbulence facility. In this paper a simple geometric body representing the cabin of a passenger car has been used to investigate the effects of free stream turbulence, (FST), on the A-pillar vortex flowfield and the side glass pressure distribution. Beneath the A-pillar vortex, both mean and dynamic pressures are increased by FST. The unsteady pressure can be associated with wind noise and the flow visualization shows the peak unsteadiness is related to the separation of the secondary vortex.
Journal Article

Links between Notchback Geometry, Aerodynamic Drag, Flow Asymmetry and Unsteady Wake Structure

2011-04-12
2011-01-0166
The rear end geometry of road vehicles has a significant impact on aerodynamic drag and hence on energy consumption. Notchback (sedan) geometries can produce a particularly complex flow structure which can include substantial flow asymmetry. However, the interrelation between rear end geometry, flow asymmetry and aerodynamic drag has lacked previous published systematic investigation. This work examines notchback flows using a family of 16 parametric idealized models. A range of techniques are employed including surface flow visualization, force measurement, multi-hole probe measurements in the wake, PIV over the backlight and trunk deck and CFD. It is shown that, for the range of notchback geometries investigated here, a simple offset applied to the effective backlight angle can collapse the drag coefficient onto the drag vs backlight angle curve of fastback geometries.
Journal Article

The Effects of Unsteady On-Road Flow Conditions on Cabin Noise: Spectral and Geometric Dependence

2011-04-12
2011-01-0159
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.
Journal Article

The Bandwidth of Transient Yaw Effects on Vehicle Aerodynamics

2011-04-12
2011-01-0160
A vehicle on the road encounters an unsteady flow due to turbulence in the natural wind, the unsteady wakes from other vehicles and as a result of traversing through the stationary wakes of road side obstacles. There is increasing concern about potential differences in aerodynamic behaviour measured in steady flow wind tunnel conditions and that which occurs for vehicles on the road. It is possible to introduce turbulence into the wind tunnel environment (e.g. by developing active turbulence generators) but on-road turbulence is wide ranging in terms of both its intensity and frequency and it would be beneficial to better understand what aspects of the turbulence are of greatest importance to the aerodynamic performance of vehicles. There has been significant recent work on the characterisation of turbulent airflow relevant to road vehicles. The simulation of this time-varying airflow is now becoming possible in wind tunnels and in CFD.
Journal Article

The Decay of Bluff Body Wakes

2011-04-12
2011-01-0178
Vehicles on the road operate in the turbulent flow field resulting from the combined effects of the natural wind and the wakes of other vehicles. While substantial data exists on the properties of the natural wind, much less information is available for the wake properties of road vehicles. The wake information available for road vehicle shapes is mainly restricted to the near wake region, but to understand the vehicle operating environment it is the wake downstream of this region which is of interest. To determine the range of this area of interest requires some knowledge of the decay of the wake properties. From wind tunnel studies using small simple bluff bodies in free stream and in ground proximity the principle wake properties, velocity deficit and peak turbulence intensity have been measured. The maximum velocity deficit is shown to approximately decay with x-2/3, where x is the distance downstream, while turbulence intensity decays at a slightly slower rate.
Journal Article

Cross Winds and Transients: Reality, Simulation and Effects

2011-04-12
2011-01-0172
This paper provides a published counterpart to the address of the same title at the 2010 SAE World Congress. 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 road side obstacles. This last term is of greatest significance. Various works related to the characterization, simulation and effects of on-road turbulence are compared together on the turbulence spectrum to highlight differences and similarities. The different works involve different geometries and different approaches to simulating cross wind transients but together these works provide guidance on the most important aspects of the unsteadiness. On-road transients include a range of length scales spanning several orders of magnitude but the most important scales are in the in the 2-20 vehicle length range.
Technical Paper

On the Optimisation of Road Vehicle Leading Edge Radius in Varying Levels of Freestream Turbulence

2006-04-03
2006-01-1029
It has been recognised that the ideal flow conditions that exist in the modern automotive wind tunnel do not accurately simulate the environment experienced by vehicles on the road. This paper investigates the effect of varying one flow parameter, freestream turbulence, and a single shape parameter, leading edge radius, on aerodynamic drag. The tests were carried out at model scale in the Loughborough University Wind Tunnel, using a very simple 2-box shape, and in the MIRA Full Scale Wind Tunnel using the MIRA squareback Reference Car. Turbulence intensities up to 5% were generated by grids and had a strong effect on transcritical Reynolds number and Reynolds sensitivity at both model scale and full scale. There was a good correlation between the results in both tunnels.
Technical Paper

The Influence of Ground Simulation on the Aerodynamics of a Simple Car Model

1997-02-24
970134
The aerodynamic development of cars is conventionally carried out in the wind tunnel with the working section floor representing the ground surface. While recognising that the boundary conditions are false it has generally been assumed that this basic ground simulation is adequate for road cars. This situation is currently being reappraised. In this investigation a simple 1/4 scale car model has been tested in the MIRA Model Wind Tunnel, using both moving and stationary belt to represent the ground surface. The body and wheel drag contributions were measured independently. The influence of rear body shape, front spoilers, cooling airflow, and underfloor roughness on the aerodynamic characteristics, for both ground conditions has been assessed.
Technical Paper

Assessing the Effects of Shear and Turbulence During the Dynamic Testing of the Crosswind Sensitivity of Road Vehicles

1997-02-24
970135
With increasing speeds and the anticipated reduction in weight of modern cars, the issue of crosswind sensitivity is becoming increasingly important. In a previous paper by the same authors, the normal method of testing such aerodynamic characteristics at model scale, using static models at yaw to the freestream, was compared with dynamic testing, in which the model is propelled across a ‘gust’ simulated by a wind tunnel. A direct comparison using a similar gust profile for both static and dynamic tests was made with the conclusion that the simple static test technique was underestimating the true transient loads. Further tests have been carried out, on a generic squareback (or estate) model, during which the effect of varying both the vertical velocity profile and the turbulence intensity within the gust was considered.
Technical Paper

Vortex Drag for a Simple Bluff Body at Incidence in Ground Proximity

2005-04-11
2005-01-0869
Aerodynamic drag is comprised of pressure drag and skin friction only. The drag component associated with lift forces is contained within these two terms. In the case of a simple wing this drag component, called induced drag, is reasonably well defined as a function of lift, but for road vehicles the relationship is more complex. In this paper the drag due to lift, which will be called vortex drag, is investigated for a simple car-like shape at incidence in proximity to the ground. The vortex drag is derived from the parabolic relationship between drag and lift. The effects of ground clearance are considered for both moving a stationary ground simulations. The results are compared with data for other simple bodies.
Technical Paper

Aerodynamic Drag Reduction for a Simple Bluff Body Using Base Bleed

2003-03-03
2003-01-0995
Wind tunnel tests have been conducted on a simple bluff body model, representing a car like shape, to investigate drag reduction opportunities from injecting low velocity air into the base region. This flow is known as base bleed. Most tests have been carried out using a square back shape. The effects of flow rate, porosity and porosity distribution over the base area have been investigated. In all cases drag is reduced with increasing bleed rate, but the optimum porosity is a function of bleed rate. A significant part of the drag reduction occurs without the bleed flow and arises from the presence of a cavity in the model. The effects of cavity size are examined for different base configurations. Some factors affecting implementation are considered.
Journal Article

CFD Investigation of the Effect of the Salient Flow Features in the Wake of a Generic Open-Wheel Race Car

2015-04-14
2015-01-1539
It is well known that in motorsport the wake from an upstream vehicle can be detrimental to the handling characteristics of a following vehicle, in particular in formulae with high levels of downforce. Previous investigations have been performed to characterize the wake from an open wheel race car and its effect on a following car, either through the use of multiple vehicles or purpose-built wake generators. This study investigates how the wake of an upstream race car impacts the aerodynamic performance of a following car in a close-following scenario. Wakes are imposed on the inlet of a CFD simulation and wake parameters (eg: velocity deficit, trailing vorticity) are directly manipulated to investigate their individual impacts on the following vehicle. The approach provides a useful alternative to the simulation of multi-vehicle cases but a better simulation could be achieved by including wake unsteadiness from the upstream vehicle.
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