Christoffer Landström

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.

Investigation of the Influence of Tyre Geometry on the Aerodynamics of Passenger Cars

It is well known that wheels are responsible for a significant amount of the total aerodynamic drag of passenger vehicles. Tyres, and mostly rims, have been the subject of research in the automotive industry for the past years, but their effect and interaction with each other and with the car exterior is still not completely understood. This paper focuses on the use of CFD to study the effects of tyre geometry (tyre profile and tyre tread) on road vehicle aerodynamics. Whenever possible, results of the numerical computations are compared with experiments. More than sixty configurations were simulated. These simulations combined different tyre profiles, treads, rim designs and spoke orientation on two car types: a sedan and a sports wagon. Two tyre geometries were obtained directly from the tyre manufacturer, while a third geometry was obtained from our database and represents a generic tyre which covers different profiles of a given tyre size. All geometries were deformed based on measured wind tunnel data under a defined load and rotating conditions of 100 kph. Results have shown that the main grooves consistently lead to a decrease of both drag and lift. The edge pattern however, did not show a clear trend for drag and lift with respect to the different configurations studied, although its influence was always more noticeable on the sports wagon. The larger profile of the generic tyre resulted in higher drag and lift values with relation to the tyres obtained from the manufacturer. For a given rim, a drag difference was observed between two tyre geometries with same profile but different tyre tread. These findings lead to the conclusion that tyre profile, as well as tyre pattern, are important to consider. These results were confirmed by wind tunnel tests. In conclusion, the work of aerodynamic optimization of rims cannot be separated from the tyre itself.

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.

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.

Investigation of Wheel Ventilation-Drag using a Modular Wheel Design Concept

Passenger car fuel consumption is a constant concern for automotive companies and the contribution to fuel consumption from aerodynamics is well known. Several studies have been published on the aerodynamics of wheels. One area of wheel aerodynamics discussed in some of these earlier works is the so-called ventilation resistance. This study investigates ventilation resistance on a number of 17 inch rims, in the Volvo Cars Aerodynamic Wind Tunnel. The ventilation resistance was measured using a custom–built suspension with a tractive force measurement system installed in the Wheel Drive Units (WDUs). The study aims at identifying wheel design factors that have significant effect on the ventilation resistance for the investigated wheel size. The results show that it was possible to measure similar power requirements to rotate the wheels as was found in previous works. The magnitude of the measured ventilation resistance confirms the conclusion that this effect should be taken into account when designing a wheel. It was found that some of the rim design factors have greater influences on the ventilation resistance than others. It was also shown that one of the investigated rims had lower ventilation resistance than measured for the fully-covered wheel configuration.

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.

Influence of Moving Ground Conditions on the Cooling Flows of Road Vehicles

To reduce fuel consumption and hence carbon dioxide emissions from road vehicles it is a necessity to reduce the amount of energy they spend during operation. One of the biggest potentials lay in the aerodynamics and in recent years road vehicles have become increasingly streamlined. However, improvements are still possible but, it requires more sophisticated simulation and experimental equipment to realize them. The introduction of moving ground systems in wind tunnels is believed to be an essential improvement to the experimental equipment. This paper explains the differences that arise in the flow field around a modern passenger car when it is being tested in stationary opposed to moving ground conditions. The study shows that moving ground conditions are essential to accurately predict the drag of the vehicle. Furthermore the moving ground condition also has a noticeable effect on the predicted massflow through the engine cooling air inlet.

Investigation of Aerodynamic Wheel Designs on a Passenger Car at Different Cooling Air Flow Conditions

Passenger cars represent the largest part of all means of personal transportation today. Thus, it is important to work towards reduced energy consumption of cars if a sustainable mobility is to be achieved. This involves many aspects of vehicle engineering; one of them being aerodynamics. This study focuses on aerodynamic drag and the contributions from the wheels at different cooling air flow configurations. Wheels and wheel housings are important for the overall aerodynamic drag on passenger cars. It has been shown that as much as 25 % of the aerodynamic drag originates from these components. Therefore, it is desirable to understand the flow structures related to the wheels and wheel housings, and how they interact with other important flow regions. This paper presents an investigation of the effects of wheel designs on aerodynamic drag at different cooling air flow configurations on a sedan type passenger car. Comparisons between numerical simulations and wind tunnel measurements are made for some of the configurations as well. Several additional wheel configurations were investigated numerically to further investigate the flow structures at the front and rear wheels. The numerical results show that the effects of radial wheel covering varied noticeably with cooling air flow configuration. In two of the configurations this resulted in a net drag increase with closed cooling air inlets. The best configuration with closed cooling air inlets generated an overall drag reduction of 29 drag counts compared with the numerical baseline with open cooling air inlets. In addition to the obvious drag reduction of closing the cooling air inlets, the main reasons for the additional decrease was limiting the drag increase at the front stagnation region and positive interference effects along the underbody and vehicle base.