Lasse Malmkjaer Christoffersen

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.

Interference between Engine Bay Flow and External Aerodynamics of Road Vehicles

This study focus on the aerodynamic influence of the engine bay packaging, with special emphasis on the density of packaging and its effect on cooling and exterior flow. For the study, numerical and experimental methods where combined to exploit the advantages of each method. The geometry used for the study was a model of Volvo S60 sedan type passenger car, carrying a detailed representation of the cooling package, engine bay and underbody area. In the study it was found that there is an influence on the exterior aerodynamics of the vehicle with respect to the packaging of the engine bay. Furthermore, it is shown that by evacuating a large amount of the cooling air through the wheel houses a reduction in drag can be achieved.

Development of a Model Scale Heat Exchanger for Wind Tunnel Models of Road Vehicles

During the development of the aerodynamic properties of fore coming road vehicles down scaled models are often used in the initial phase. However, if scale models are to be utilised even further in the aerodynamic development they have to include geometrical representatives of most of the components found in the real vehicle. As the cooling package is one of the biggest single generators of aerodynamic drag the heat exchangers are essential to include in a wind tunnel model. However, due mainly to limitations in manufacturing techniques it is complicated to make a down scaled heat exchanger and instead functional dummy heat exchangers have to be developed for scaled wind tunnel models. In this work a Computational Fluid Dynamics (CFD) code has been used to show that it is important that the simplified heat exchanger model has to be of comparable size to that of the full scale unit. Based on the findings of the CFD simulations a physical model of a heat exchanger was developed and presented in this article. The model heat exchanger features realistic properties with respect to flow resistance and flow guiding.

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.

Improving the Cooling Airflow of an Open Wheeled Race Car

In this case study the cooling airflow of an existing open-wheeled racecar has been improved with the use of Computational Fluid Dynamics. The race team in context had at several occasions experienced overheating of their racecar and was looking for ways to improve the cooling performance without changing the bodywork radically. As the car is used for autocross events on tight and twisty courses it spends most of a lap in yawed condition. Therefore, a novel approach was taken to model these yawed conditions with the numerical method. The simulation was based on the fully detailed race car. Through the study it was possible to locate problem areas, and hence, give indications to where the bodywork should be modified. With subtle changes to the bodywork the cooling performance of the car was significantly improved and the drag kept at the same level.

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.

Wheel Strut Interference

To fine tune the aerodynamic properties of road going vehicles the flow along the underbody is of outmost importance. Hence moving ground facilities has been introduced. In full-width mono-belt facilities the test vehicle is most often suspended over the ground plane by a sting system. To mount the wheels, two possibilities exist. The designer of the wind tunnel model is faced with the choice to have the wheels on or off the model. The first option gives the most satisfying boundary conditions from a fluid dynamics point of view, however, the designer will face the problem of creating a frictionless chassis suspension system for the model. The ”wheels off” type does not require such complex system but unfortunately creates a non-realistic flow due to the presence of so called wheel struts that carries the wheels. In the present work a numerical study of the flow around a ”wheels off” model has been conducted. The study was performed to quantify how the presence of the wheel struts would affect the flow field and hence how this influence flow related measurements made on the model. The work has been done using a commercial CFD-code, running a standard k-epsilon model on a tet-dominated mesh, counting roughly 6.5 million cells. As expected the flow field in the vicinity of the wheels is affected due to the extra vorticity introduced by the struts. The modified pressure field alters the drag and of further significance is the influence from the wheel struts on the underbody flow field. It is shown that care must be taken in studies of underbody and later underhood flows, using a ”wheels-off” configuration.

Wing-Diffuser Interaction on a Sports Car

Amongst the aerodynamic devices often found on race cars, the diffuser is one of the most important items. The diffuser can work both to reduce drag and also to increase downforce. It has been shown in previously published studies, that the efficiency of the diffuser is a function of the diffuser angle, ground clearance and most importantly, the base pressure. The base pressure of a car is defined by the shape of the car and in particular the shape at the rear end, including the rear wheels. Furthermore, on most race cars, a wing is mounted at the rear end. Since the rear wheels and wing will influence the base pressure it is believed that, for a modern race car, there could be a strong interaction between these items and the diffuser.

Investigation of the cooling and underbody flow field on a detailed model scale passenger car. Part 1 - Cooling flows and Reynolds number effects

To decrease the lead time and hence cost for an aerodynamic development programme for a new car model, extended use of down scaled wind tunnel models is one possibility. This however, requires that the models carry many of the exterior, engine bay and underbody details that are present on the full scale counterpart. An important issue in scale testing is to understand the Reynolds number effects that will be present and with an increased detail level of the model it is believed that there will be other Reynolds number effects than on the simplified models that historically have been used. Therefore; the aim of this work is to explain some of the Reynolds number effects that are developed on a detailed model of a sedan type passenger car. The analysis is based on experimental data with both load and pressure measurements as the primary source carried out on a 30% model of a Volvo S60. In the experiment both moving ground and rotating wheels were utilized With the Reynolds number ranging from 8.8e5 to 3.5e6 it was possible to notice interesting Reynolds number effects. The results show that there is a significant Reynolds number dependency of the scale model. However, it is also shown that the Reynolds number dependency is dependent on the detail level of the model.

INVESTIGATION OF THE COOLING AND UNDERBODY FLOW FIELD ON A DETAILED SCALE MODEL PASSENGER CAR - PART 2: EFFECT OF GROUND SIMULATION

Future demands on passenger cars consist to a large extend of making them more energy efficient. Reducing the driving resistance by reducing the aerodynamic drag will be one important part in reducing fuel consumption. In most cases during passenger car development, early experimental investigations are performed in scale model wind tunnels. Considering that such models inevitably suffer from Reynolds number effects it is important to understand how this affects the test results. Investigations of the aerodynamics of a detailed scale model Volvo S60 have been performed in the aerodynamic wind tunnel at Chalmers University of Technology. The investigation aimed at increasing the understanding of how the flow field in scale model testing is affected by ground simulation and different cooling air flow configurations at different Reynolds numbers. A full width moving ground system was used in the experiments. Pressure taps were distributed between the cooling air inlets, the underbody and the vehicle base. An internal six component balance was used to measure global forces and moments. By combining the results from the measurements it was possible to increase the understanding of some of the local flow features. Results showed significant Reynolds number effects both with stationary ground as well as moving ground and rotating wheels. Global aerodynamic drag as well as front and rear axle lift was found to be affected.