Tim Walker

Detailed flow studies in close proximity of rotating wheels on a passenger car

Moving ground systems with rotating wheels have been used in wind tunnel tests during the last decades. Several studies on the effects of rotating wheels and the importance of wheel aerodynamics have been published. It is well known that both the local flow field and the global aerodynamic forces are affected by rotation of the wheels. Different studies indicate that the most significant effect from rotating the wheels is interference effects between the rear wheels and the underbody and vehicle base [1], [2]. A detailed flow field investigation around the wheels in close proximity to the vehicle has been performed on a passenger car in the Volvo Aerodynamic Wind Tunnel. Two omnidirectional 12-hole pressure probes were traversed in a number of planes close to the wheels. Effects of changing different parameters such as ground simulation and rim geometry were investigated. The local flow field has been scrutinised and related to the global aerodynamic properties of the vehicle. A clear dependency on ground simulation as well as a dependency on wheel geometry was found in the local flow field. All configurations showed lower global drag with moving ground activated. Generally a good correlation with numerical results was observed.

Influences of Different Front and Rear Wheel Designs on Aerodynamic Drag of a Sedan Type Passenger Car

Efforts towards ever more energy efficient passenger cars have become one of the largest challenges of the automotive industry. This involves numerous different fields of engineering, and every finished model is always a compromise between different requirements. Passenger car aerodynamics is no exception; the shape of the exterior is often dictated by styling, engine bay region by packaging issues etcetera. Wheel design is also a compromise between different requirements such as aerodynamic drag and brake cooling, but as the wheels and wheel housings are responsible for up to a quarter of the overall aerodynamic drag on a modern passenger car, it is not surprising that efforts are put towards improving the wheel aerodynamics. The actual force on the wheels is typically not a full quarter of the overall drag, but as the wheels strongly interact with several other key flow features such as cooling air flow, underbody flow and the base wake, the wheels have a large influence on the overall aerodynamic performance of the vehicle. This study investigates the potential of different wheel design parameters focusing on reduced aerodynamic drag. A correlation with experimental measurements on a full size vehicle is presented and several additional configurations are analyzed numerically using a standard automotive CFD approach. Furthermore, the potential of optimizing the front and rear wheels individually is investigated to some extend. Results show that closing most of the rear wheels results in local drag reductions along the rear end underbody, rear wheels and vehicle base. The fully covered rear wheel typically reduced base drag between 6-7 drag counts. Effects of covering the front wheels were more complex as both upstream and downstream flow regions were affected, and it was shown that for the vehicle investigated in this study a limited amount of outer radial covering of the wheel gave the largest drag reduction. The investigation of using different front and rear wheel designs showed that this concept have potential in reducing overall drag as it generated the largest drag reduction in this study of approximately 22 drag counts.

Aerodynamic Effects of Different Tire Models on a Sedan Type Passenger Car

Targets for reducing emissions and improving energy efficiency present the automotive industry with many challenges. Passenger cars are by far the most common means of personal transport in the developed part of the world, and energy consumption related to personal transportation is predicted to increase significantly in the coming decades. Improved aerodynamic performance of passenger cars will be one of many important areas which will occupy engineers and researchers for the foreseeable future. The significance of wheels and wheel housings is well known today, but the relative importance of the different components has still not been fully investigated. A number of investigations highlighting the importance of proper ground simulation have been published, and recently a number of studies on improved aerodynamic design of the wheel have been presented as well. This study is an investigation of aerodynamic influences of different tyres. Two different tyre models were investigated in combination with three different wheel designs using the Volvo Aerodynamic Wind Tunnel; including moving ground and rotating wheels. In addition to force measurements, flow field investigations were also performed using both surface pressure probes and 12- hole pressure probes. The tyre sizes investigated in this study were 215/50R17 and 215/55R16. An investigation of changes to the tyre geometry for 215/55/R16 tyres was also performed using two high speed cameras in the wind tunnel. Results show that different tyre types have a significant effect on not only aerodynamic drag, but also on lift to some extent. Drag differences between 5 – 10 drag counts were measured depending on wheel and vehicle configuration. It was also concluded that the drag difference between tyre types was dependent on wheel design. The flow field investigations showed noticeable changes to the front wheel wake structures as well as significant changes in the rear wheel and base wake structures. Investigations of the tyre deformations showed changes in wheel lift, as well as radial expansion and axial compression correlating with the observed drag changes.

Experimental and Numerical Investigations of the Base Wake on an SUV

With the increase in fuel prices and the increasingly strict environmental legislations regarding CO2 - emissions, reduction of the total energy consumption of our society becomes more important. Passenger vehicles are partly responsible for this consumption due to their strong presence in the daily life of most people. Therefore reducing the impact of cars on the environment can assist in decreasing the overall energy consumption. Even though several fields have an impact on a passenger cars performance, this paper will focus on the aerodynamic part and more specifically, the wake behind a vehicle. By definition a car is a bluff body on which the air resistance is for the most part driven by pressure drag. This is caused by the wake these bodies create. Therefore analyzing the wake characteristics behind a vehicle is crucial if one would like to reduce drag. With the recent upgrade of wind tunnels with a moving belt system, the opportunity has emerged to investigate the flow field in the wake behind vehicles, matching closer the real on-road driving conditions. This study investigates experimentally and numerically the wake behind a passenger car of an SUV-type. Three configurations with a significant change in CD have been chosen for the analysis. Their wake shape together with their respective closure points have been analyzed using three planes, namely one x-plane, one y-plane and one z-plane. Results have shown that the numerical simulations correlate well with the experiments in wake shape and wake behavior. However in the chosen configurations they underestimate the wake length. A distinct interference of the traversing unit presence can be noted in the experimental results.

Fundamentals, Basic Principles in Road Vehicle Aerodynamics and Design

The objective of this chapter is to briefly explain some of the basic principles of fluid mechanics applied to road vehicle aerodynamics. It introduces the main concepts of drag, lift, boundary-layer, wake separation, and so on. It also describes the importance and relevance of aerodynamics in the development process of new car models. Aerodynamics has a significant influence on other road vehicle properties such as performance, handling, contamination, and comfort. These aspects are highlighted in the chapter. An overview of a typical aerodynamic development process for high volume production cars is given. It explains the need for attribute balancing, engineering compromises, as well as other considerations such as costs and manufacturing. Interaction effects of different areas of the body are described in a general way in this chapter. Finally, we discuss some future styling trends, and the need of reducing cooling air, increasing under body paneling, and methods for flow control.

Effect of rear-end extensions on the aerodynamic forces of an SUV

Under a global impulse for less man-made emissions, the automotive manufacturers search for innovative methods to reduce the fuel consumption and hence the CO2-emissions. Aerodynamics has great potential to aid the emission reduction since aerodynamic drag is an important parameter in the overall driving resistance force. As vehicles are considered bluff bodies, the main drag source is pressure drag, caused by the difference between front and rear pressure. Therefore increasing the base pressure is a key parameter to reduce the aerodynamic drag. From previous research on small-scale and full-scale vehicles, rear-end extensions are known to have a positive effect on the base pressure, enhancing pressure recovery and reducing the wake area. This paper investigates the effect of several parameters of these extensions on the forces, on the surface pressures of an SUV in the Volvo Cars Aerodynamic Wind Tunnel and compares them with numerical results. To decrease the dependency of other effects within the engine bay and underbody, the SUV has been investigated in a closed-cooling configuration with upper and lower grille closed and with a smoothened underbody. These results might change if the study would be conducted with a less smooth underbody and in an open-cooling configuration. Extensions with different shapes and dimensions have been placed around the perimeter of the base exterior. The chosen design philosophy of the extensions allowed for different combinations with variable inclination angles depending on their position along the base perimeter, multiple extension lengths and shapes to be investigated. The results show that the extension shape is an important factor in reducing the aerodynamic drag. Significant drag reductions could be obtained while maintaining the vehicles rear lift within acceptable levels for stability with a kicker attached to the extension. The investigation shows the reduction with a kicker holds for up to 7.5° yaw angles. With a beneficial shape, the extension length can be significantly reduced. The reduced drag is visible in the wake by a more concentric wake.

A Wind Tunnel Study Correlating the Aerodynamic Effect of Cooling Flows for Full and Reduced Scale Models of a Passenger Car

In the early stages of an aerodynamic development programme of a road vehicle it is common to use wind tunnel scale models. The obvious reasons for using scale models are that they are less costly to build and model scale wind tunnels are relatively inexpensive to operate. It is therefore desirable for model scale testing to be utilized even more than it is today. This however, requires that the scale models are highly detailed and that the results correlate with those of the full size vehicle. This paper presents a correlation study that was carried out in the Chalmers and Volvo Car Aerodynamic Wind Tunnels. The aim of the study was to investigate how successfully a correlation of the cooling air flow between a detailed scale model and a real full size vehicle could be achieved. Results show limited correlation on absolute global aerodynamic loads, but relative good correlation in drag and lift increments. Furthermore, changes in local surface pressure on the underbody and base are in good agreement.

Effects of Ground Simulation on the Aerodynamic Coefficients of a Production Car in Yaw Conditions

Automotive wind tunnel testing is a key element in the development of the aerodynamics of road vehicles. Continuous advancements are made in order to decrease the differences between actual on-road conditions and wind tunnel test properties and the importance of ground simulation with relative motion of the ground and rotating wheels has been the topic of several studies. This work presents a study on the effect of active ground simulation, using moving ground and rotating wheels, on the aerodynamic coefficients on a passenger car in yawed conditions. Most of the published studies on the effects of ground simulation cover only zero yaw conditions and only a few earlier investigations covering ground simulation during yaw were found in the existing literature and all considered simplified models. To further investigate this, a study on a full size sedan type vehicle of production status was performed in the Volvo Aerodynamic Wind Tunnel. The effect of active ground simulation in yawed wind conditions was investigated for different wheel and cooling air inlet configurations. Results show that aerodynamic drag increased more during yaw when using active ground simulation compared to stationary ground conditions. For some of the configurations this resulted in yaw ranges where active ground simulation produced higher aerodynamic drag than with stationary ground. This was mainly connected to the development of the front wheel wakes and the deflection of the windward wheel wake along the underbody during yaw. Aerodynamic lift generally decreased but the front and rear lift balance changed somewhat during yaw for some configurations. Local surface pressure measurements were used to identify important flow field changes and the integrated surface pressures showed a qualitative agreement with the measured drag and lift.

Wake and Unsteady Surface-Pressure Measurements on an SUV with Rear-End Extensions

Previous research on both small-scale and full-scale vehicles shows that base extensions are an effective method to increase the base pressure, enhancing pressure recovery and reducing the wake size. These extensions decrease drag at zero yaw, but show an even larger improvement at small yaw angles. In this paper, rear extensions are investigated on an SUV in the Volvo Cars Aerodynamic Wind Tunnel with focus on the wake flow and on the unsteady behavior of the surface pressures near the base perimeter. To increase the effect of the extensions on the wake flow, the investigated configurations have a closed upper- and lower grille (closed-cooling) and the underbody has been smoothed with additional panels. This paper aims to analyze differences in flow characteristics on the wake of an SUV at 0° and 2.5° yaw, caused by different sets of extensions attached to the base perimeter. Extensions with several lengths are investigated with and without a kick. Unsteady pressure sensors attached to the base perimeter of the vehicle are used. The fluctuations are examined for the reference vehicle and the vehicle with extensions for different freestream velocities and under a yaw angle ranging from 0° to 7.5° yaw. The current investigation indicates that the drag reduction obtained at 2.5° yaw with a “kick” mounted at the rear edge of the extensions have an effect on the size of the wake and enhance the pressure recovery behind the vehicle. The addition of the kick alters the wake distribution and changes the corresponding flow pattern with the largest effect under yaw conditions. For the unsteady surface pressure measurements, a technique based on Empirical Mode Decomposition (EMD) is applied that results in instantaneous frequencies of the signal. It gives the option of signal reconstruction limited to the frequency area of interest. A Proper Orthogonal Decomposition (POD) on the signals is also conducted, showing large variance in the area below the catwalk for the first two modes. To enhance the correlation, this paper investigates the combination of both mode decompositions, where the POD is applied to EMD-filtered signals.

Investigation of vehicle ride height and wheel position influence on the aerodynamic forces of ground vehicles

To prevent test vehicles from movement during experiments in modernaerodynamic wind tunnels, fastening struts are typically used for a rigid connection between the model and the force balance underneath the wind tunnel floor. A weakness of this experimental set-up is that such struts limit the vertical movement of the vehicle. By analysing experimental data from the Volvo Cars wind tunnel and corresponding CFD simulations the differences in measurements using struts with and without vertical displacement have been analysed and compared. The model used was a Volvo S60.