Simone Sebben

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.

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.

Investigation of Wheel Aerodynamic Resistance of Passenger Cars

There are a number of numerical and experimental studies of the aerodynamic performance of wheels that have been published. They show that wheels and wheel-housing flows are responsible for a substantial part of the total aerodynamic drag on passenger vehicles. Previous investigations have also shown that aerodynamic resistance moment acting on rotating wheels, sometimes referred to as ventilation resistance or ventilation torque is a significant contributor to the total aerodynamic resistance of the vehicle; therefore it should not be neglected when designing the wheel-housing area. This work presents a numerical study of the wheel ventilation resistance moment and factors that affect it, using computational fluid dynamics (CFD). It is demonstrated how pressure and shear forces acting on different rotating parts of the wheel affect the ventilation torque. It is also shown how a simple change of rim design can lead to a significant decrease in power consumption of the vehicle. A way of introducing ventilation torque into the driving resistance equation is discussed. The results show that it is possible to assess ventilation resistance moment using CFD. It is demonstrated that brake discs have almost negligible ventilation torque, while the contribution of rims and tyres may vary depending on the rim design used and the velocity of the vehicle. It is also shown that for the designs investigated the equivalent ventilation force has a second order dependency on the vehicle velocity which allows an introduction of a ventilation resistance coefficient CD(vent) that is independent of velocity.

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.

Study of different tyre simulation methods and effects on passenger car aerodynamics

Wheel aerodynamics has long been identified as a major contributor to the aerodynamic forces acting on vehicles. Lately, tyre design has been taking more and more attention as it shows potential of drag reduction. This paper looks into different methods of simulating tyre rotations in CFD and their effects on the aerodynamic forces acting on the vehicle. A new methodology is tested by combining rotating wall and moving reference frame boundary conditions and compared to sliding mesh. Two fully detailed production tyres are investigated using the various methods to show the possibility of tyre design analysis using CFD.

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.

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.

Numerical implementation of Detached Eddy Simulation on a passenger vehicle and some experimental correlation

This study presents an implementation of Delayed Detached Eddy Simulation on a full-scale passenger vehicle for three configurations with the use of commercial software Harpoon (mesher) and Ansys Fluent (solver). The methodology aims to simulate the flow accurately around complex geometries at relevantly high Re-numbers for use in industrial applications, within an acceptable computational time. Geometric differences between the three configurations ensure significant drag changes that have a strong effect on the wake formation behind the vehicle. Therefore this paper focuses on the analysis of the base wake region. At first, the paper evaluates the performance of the DDES, where it verifies the different operating conditions of the flow around the vehicle with respect to the DDES-definition. In a second step the numerical results are correlated with force measurements and time-averaged flow-field investigations, conducted in the Volvo Cars Aerodynamic wind tunnel. The comparison confirms a good agreement between the experiments and the simulations. The resolved flow scales obtained by DDES give a further insight into differences in the wake flow characteristics between the configurations related to their contribution to drag.

Tyre Pattern Features and their Effects on Passenger Vehicle Drag

In light of the drive for energy efficiency and low CO 2 emissions, extensive research is performed to reduce vehicle aerodynamic drag. The wheels are relatively shielded from the main flow compared to the exterior of the passenger car, however, they are typically responsible for around 25% of the overall vehicle drag. This contribution is large as the wheels and tyres protrude into the flow and change the flow structure around the vehicle underbody. Given that the tyre is the first part of the wheel to get in contact with the oncoming flow, its shape and features have a significant impact on the flow pattern that develops. This study aims at identifying the general effects of two main tyre features, the longitudinal rain grooves and lateral pattern grooves, using both CFD and wind tunnel tests. This is performed by cutting generic representations of these details into identical slick tyres. Combinations of the two resulted in four physical tyre patterns that are tested on both a production and a closed rim. The test setup is reproduced in CFD taking the tyre deformation under loading into account. Due to the tyres deformation, Moving Reference Frame - grooves (MRFg) was used to model rotation, while the rim spokes were modelled with the sliding mesh approach. The results indicate that the rain grooves play a significant role in reducing drag when introduced on a slick tyre both in test and simulations, while the results from adding lateral grooves were less consistent dependent on the rim-tyre combination. The interaction between the longitudinal and lateral grooves could be observed on the overall vehicle drag. In general, CFD is able to predict the drag changes for different tyre patterns with good accuracy for the open rim, however the closed rim case proved to be more challenging.

Numerical Analysis of Aerodynamic Impact on Passenger Vehicles during Cornering

Governmental regulations and increased consumer awareness of the negative effects of green-house gases has led the automotive industry to massive invest in the energy efficiency of its fleet. One way towards accomplishing reduced fuel consumption is minimizing the drag of vehicles by improving its aerodynamics. Fuel consumption is measured by standardized driving cycles which do not consider aerodynamic losses during cornering. It is uncertain whether cornering has a significant impact on the drag, and the present study intends to investigate this numerically, using a generic vehicle model called the DrivAer. The model is considered in two different configurations: the notchback and the squareback. Cornering in various radiuses is modelled using a Moving Reference Frame approach which provides the correct flow conditions when simulating a stationary vehicle where the wind and ground are moving instead. Simulations are also performed for straight ahead driving conditions to provide data for comparison to a cornering vehicle. Results indicate that the drag increases when the cornering radius is small. This implies a higher fuel consumption than the standardized driving cycles suggest using straight-ahead drag coefficients. The detailed underbody of the DrivAer model is not symmetrical which, for large turning radiuses, results in a decrease of drag for left turns, while turning right results in an increase of drag. Cornering affects the squareback and the notchback similarly, although the squareback experiences a slightly higher drag throughout the cases investigated.