Progress in Vehicle Aerodynamics and Thermal Management

During race track operating conditions, the vehicle is constantly accelerating and braking from high to very low velocities. This generates a lot of heat that needs to be absorbed by the brakes. Sufficient cooling is required to prevent the brakes from overheating. When brakes exceed their critical temperature, they can lose grip and start fading. Brakes can lose quite some heat through radiation and conduction to their surroundings, but most of the heat can be released through cooling airflow convection. Improving the cooling airflow to the brake discs can significantly lower the brake disc temperature during the race track duty cycle. An efficient design for convective cooling will avoid large drag penalties or significant brake disc weight increase. With simulation, the brake disc and brake system design can be optimized more efficiently, to allow more cooling airflow, by visualizing the flow and it can be used in early design stages. The 3D CFD simulation method is coupled to a radiation/conduction tool to include radiation, conduction and convection effects. It can predict the brake system temperature over time during the race track duty cycle. The results have been compared against experimental data and several design variants have been tested to improve the design.

In cold and snowy regions roads are often covered with snow during the winter time. On these roads snow particles can be rolled up by vehicles and these particles can penetrate into the air intake system of the engine. As a result an engine power drop can occur. This paper discusses three different possibilities to evaluate the risk of such an engine power drop. One possibility is given by measurements on a natural testing area. The second possibility consists of measurements in a climatic wind tunnel and the third possibility is given by numerical simulations. As examples for the experimental testing a natural testing area in Arjeplog (Sweden) and two climatic wind tunnels in and near Stuttgart will be explained in more detail. Information for the snow particle generation, the snow particle size and the snow mass flux will be provided. Additionally to this experimental testing a method for the virtual testing will be presented. This method is based on an Eulerian/Lagrangian approach within a commercial CFD-software. The simulation method allows an approximate calculation of the snow amount, which penetrates into the air filter body.

In the past prototypes were built and measurements were carried out to evaluate the snow amount, which penetrates into the air filter body. These measurements were possible at a later stage of the vehicle development process. We suggest to introduce the simulation method in an earlier stage of the development process. Thereby the air intake system can reach a high level of maturity before first prototypes are built.

Since the introduction of the generic aerodynamic research model DrivAer, an increasing amount of aerodynamic research and aerodynamic CAE method development activities have been based on this simplified generic car body. Due to the OpenSource nature of the model it has not only been used by academia but also by several automotive OEMs and CAE software developers. The DrivAer model has delivered high quality experimental data to permit validation of existing aerodynamic CAE capabilities and to accelerate the development of new more sophisticated numerical methods.

Vehicle aerodynamic performance is significantly influenced by the airflow through the engine bay. The current, closed cooling version of the DrivAer does not enable an assessment of the influence of the cooling airflow on the vehicle’s aerodynamic characteristics.

A new open cooling version of the DrivAer model is proposed to further expand the usability of the overall DrivAer concept. Beyond an extended usage in vehicle aerodynamics the layout of the model will allow for investigations related to powertrain cooling, heat protection, brake cooling and wind noise.

This paper focuses on the conceptual layout of the Open Cooling DrivAer model and will explain the instrumentation concept of the physical test model. Furthermore initial wind tunnel test data of the baseline configuration will be presented.

A Range Rover SUV with the cooling apertures open and closed had a range of force, surface pressure and flow-field measurements recorded in the FKFS Aero-acoustic wind tunnel. With modern automotive aerodynamics requiring low drag in a range of operating conditions, the primary motivation was to give insight into the interacting flow mechanisms which lead to differing aerodynamic behaviours depending on the vehicle configuration. Cooling drag, the drag difference between the vehicle with open and closed cooling apertures, is used as a metric to demonstrate a set of complex flow interactions occurring on this vehicle between the front and rear wheel wakes and the base wake.

The gaps at the tailgate of passenger cars can play an important role in the generation of wind noise. Depending on the particular geometry, flow separation and vortices may occur that cause pressure fluctuations perceived by the passengers. The underlying aeroacoustic effects can be hard to predict and extensive testing is required to eliminate disturbing wind noise.

In the present work, the acoustic characteristics of an exemplary sedan car are investigated with a focus on the rear end. Different sealing concepts for the tailgate gap are evaluated using wind tunnel measurements and computational fluid dynamics. The study incorporates interior-noise spectra and a deeper analysis of pressure fluctuations around the rear glass panels.

The results imply that the numerical method is suitable to predict the performance of different sealing concepts if small gaps and structures are sufficiently resolved. Furthermore, the simulation provides insight to physical mechanisms and allows the localization of sensitive areas.

In this paper the effects produced by the wheels on the bi-stable reflectional symmetry breaking (RSB) mode seen for the wake of a square-back geometry (Grandemange et al. [11]) are investigated considering a modified version of the Windsor body already studied in Perry et al.  [18]. The contribution of the wheels and their rotation to the changes in the base pressure distribution and the wake topology is characterised by means of pressure tappings and 2D-3C particle image velocimetry. Balance measurements are used to further characterise the changes in the strength of the RSB mode. For the pure square-back configuration, the results show a general increase of the base drag as a consequence of the strengthening of the suction over the lower portion of base, due to the formation of a pair of counter rotating vortices acting close to the bottom trailing edge. At the same time, the RSB mode is weakened, leading to a reduction in the fluctuations recorded for the lateral component of the aerodynamic force. The sensitivity of the RSB mode to small changes in the shape of the model’s trailing edges is characterised by looking at the effects produced by short tapers, with a slant angle of 12° and a chord equal to 4% of the model length, applied to either the horizontal or the vertical trailing edges. The results show that the RSB mode disappears when the effect of the wheels is paired to the upwash generated by the slanted surface (when applied to the bottom trailing edge), although it is still clearly visible when the tapers are applied to the side edges of the base, in contrast with the results reported by Pavia et al. [16] for the same geometry without wheels.

Automotive wind tunnels still see an increasing significance in the vehicle development process. Nevertheless, the flow topology in the test sections of open jet wind tunnels is not yet understood completely. A large source for transient structures is the shear layer developing between the high velocity nozzle flow and the calmly air in the plenum.

For a better understanding, the shear layer is investigated in the symmetry plane with a multi-hole probe and PIV measurements in this paper. The analysis of the recorded data shows fluctuations with a Strouhal number of 0.46 based on the nozzle hydraulic diameter. This value matches other researches of jet flow. However, the concept of vortices separating at the nozzle edge seems to be incomplete. The PIV measurements reveal a wave structure in the shear layer at the nozzle exit. These waves release larger vortices moving along the outer boundary of the shear layer. Mode decomposition (POD) shows changing intensities of these vortices for varying wind tunnel velocities. Different flow structures are detected when delta wings are mounted to the nozzle edges.

Mercedes-AMG continues to grow. The sports car and performance brand of Mercedes-Benz expanded the top end of its product range with the introduction of the new Mercedes-AMG GT R. Never before has Mercedes-AMG packed so much motorsport technology into a production vehicle than it has in the new AMG GT R. The challenge for the aerodynamic engineers was the development of a unique car with special requirements: distinctive proportions, clean coupe design, front-mid-twin-turbo V-8 engine rated at 430 kW/585 hp, singular package, light weight, transaxle, extreme cooling requirements and above all, the goal of being the fastest of its class on the world’s most demanding racetrack. The result is an intelligent, distinctive and innovative combination of aerodynamic features that fits the complex requirements of this vehicle. The interaction of the active aerodynamic features provides the right aerodynamic performance for each driving situation. This allows the combination of driving dynamics of a Mercedes-AMG GT3 race car with the everyday practicality of the Mercedes-AMG GT, assuring race circuit performance and low fuel consumption. The result is a lap time of 7:11 min (Journal Sport Auto 1/2017) at the Nordschleife. Benchmark.

With ever stricter emissions regulations and the upcoming certification procedure defined in WLTP, the importance of aerodynamics for OEMs is increasing. This paper presents the aerodynamics development of the fifth generation Land Rover Discovery and gives an insight into the recent migration towards the use of new tools and processes to ensure future vehicles’ compliance is achieved.

In the early development stages, numerical methods were exclusively used to optimise the main proportions of the vehicle, as well as to understand sensitivities and draw a road-map to attain the aerodynamics attribute targets. A full-scale test property (“Aerobuck”) was then built and tested in the fixed-ground MIRA Full Scale Wind Tunnel (FSWT) to optimise specific areas in order to reduce the drag coefficient. These tests were done in combination with an extensive use of CFD to enable a better understanding of the flow fields and mechanisms involved. Finally, the development of an aerodynamically optimised wheel enabled the lowest drag coefficient to date for a Land Rover SUV, C_D = 0.33.

Although certified in the MIRA FSWT, the validation process has also seen prototypes tested in the moving-ground FKFS Aeroacoustic Wind Tunnel (AAWT) benefiting from 5-belt ground simulation. A direct comparison between experimental and numerical results has also been made; they generally show good agreement between the two tools except for the prediction of so-called “cooling drag”.

The Polo is entering its 7th generation. The car is based on a new architecture concept using a new modular design. Both the car body and the platform are newly designed. The platform will be used by several brands of the Volkswagen group for a variety of vehicles.

The Polo has been designed to meet demanding emission requirements. One of the key contributors to achieving the requirements is improved aerodynamics in spite of a significant growth of vehicle frontal area. These improvements were applied to the entire fleet of vehicles, thus ensuring that future legislative targets will be met.

In order to significantly reduce the vehicle’s fuel consumption, a lot of attention was paid to the new design of the NEOPLAN TOURLINER. During the design phase, each detail was therefore looked at closely in order to determine how to implement the NEOPLAN design criteria, specific vehicle design and detailed design in a way that optimizes the drag value coefficient (c_D). Through optimization loops, we found a target-oriented design using CFD and implemented this meticulously in the area at the front of the bus, the front roof dome, the mirrors, the air-conditioning system cladding, the wheel arch panels, wheel trim, entire rear panel, as well as areas in the underbody. The results were then examined on a scale model in the wind tunnel of Pininfarina. A lot of effort was put to build a model which includes finest details, e.g. the entire underbody-structure and (motorized) rotating wheels. Eventually, the drag value was confirmed through test drives of initial vehicles. These are constant speed tests in which probe data of drive momentum, vehicle speed and wind velocity/direction are used to calculate the c_D value.

The c_D value was reduced by over 20% in comparison to the Tourliner, released in 2002, from 0.46 to 0.36, now. The vehicle therefore achieves overall fuel savings of 10% in comparison to the predecessor bus, reducing fuel consumption to an average of approx. 20 L/100 km.

This represents a high degree of innovation, as the improved consumption is in a standard, entry-level vehicle of the NEOPLAN brand. The standard Tourliner not only has a more attractive acquisition cost – it is also designed to reduce consumption. More for people and the environment!

The further reduction of the aerodynamic drag of today’s passenger cars requires the development of regions such as the vehicle underbody, the wheel housings and the tires.

The aerodynamics of modern day passenger cars are usually developed under constant and reproducible conditions in a wind tunnel. However, spontaneously occurring wind changes during on-road drive are of significant interest. Recently, BMW AG has been able to develop virtual methods enabling the investigation of such spontaneous on-road driving events. These methods have been used to assess driving in slipstream, cornering and the impact of naturally occurring wind gusts. During slipstream driving, a noticeable drag-reducing effect was found for a passenger vehicle behind a truck and behind different sized passenger cars. There was a significant effect on vehicle lift. Cornering conditions were examined by only changing the curvature of the streamlines in the flow field. Simulation results indicate a significant change on vehicle coefficients compared to straight line driving. Lastly, the flow in gusty wind conditions was investigated. Simulation results confirmed results from academic wind tunnel measurements. An increase in the aerodynamic yaw moment was found under transient oncoming flow conditions compared to the quasi-steady inflow. Overall, the results from the three investigated driving conditions have a significant relevance of the aerodynamic effects under real driving conditions.

The interaction of underhood flow with external aerodynamics is part of the main topics for the development of automotive aerodynamics. In particular, the location and design of cooling air exits as well as the mass flow through the radiator package are considered to be important factors. The DrivAer is an open-source, aerodynamic reference model, which has a realistic overall geometry compared to rather simple, generic shapes such as the SAE body. The model is flexible in use as it provides different rear end and wheel designs. The open-grille upgrade of the DrivAer enhances the potential of the body towards a reference model for more detailed, open cooling investigations.

In the present study, the 40% scale Mock-Up DrivAer model of the Technical University of Munich was upgraded with underhood flow for wind tunnel tests with moving ground simulation. The wind tunnel measurements covered different underhood flow configurations including variations for the pressure drop of the radiator package and different locations for cooling air exits – both for different rear end shapes of the model. Additionally, the effect of underhood flow on the aerodynamics of both the notchback and the estateback rear end were calculated for six different radiator packages using DDES simulations with OpenFOAM®.

The new Porsche wind tunnel was built in three years from 2011 and 2014. In the meantime, Porsche decided to build two reference cars, so as to be able to compare the pressure distribution on the vehicle surface, both on the road and in the wind tunnel.

The concept of having a reference car to compare results on the road and in the wind tunnel is not new. In the case of Porsche, the choice of the vehicles was guided by two requirements, i.e. to have a car shape typical of the Porsche family and to produce different blockage effects.

The cars were equipped with the best available instrumentation, which was carefully calibrated in order to insure accuracy and repeatability.

Multiple road tests and measurements in various state of the art wind tunnels, with different moving ground simulation systems and different geometries, were then carried out.

As a result of the post processing of these data, it has been possible to recognize interference effects in the various wind tunnels.

Porsche has then used these vehicles in the calibration phase of the new wind tunnel, so as to reach a flow quality, in terms of time-averaged values, that is as close as possible to conditions experienced on the road.

Furthermore, thanks to the interchangeable moving ground system in the new Porsche wind tunnel, the reference cars were able to show clearly, for the same wind tunnel interferences, the differences in the pressure distribution caused by the two different moving ground systems.

In addition, thanks to the high-quality 5 belt system, it was possible to measure the c_{D_vent} (ventilation drag), necessary to improve the prediction of Fx in coast-down tests. A comparison between wind tunnel and coast-down results was carried out and will be shown.

Bad weather conditions such as rain and snow affect driving comfort and safety. An unobstructed view onto the surrounding traffic is indispensable. Water whirled up by preceding vehicles can soil the vehicle windows. Depending on the resulting soiling pattern, this can impair the visibility. Therefore, the origin of the vehicle side glass soiling and its influencing factors were investigated in a wind tunnel, resulting in the identification of different soiling phenomena. A crucial parameter is the surface condition as a function of the velocity. Additionally, a new evaluation method is presented which detects and classifies the different types of water accumulations and illustrates the local soiling frequency. It provides information about the composition of soiling patterns and thus can support to during the development process for side glass soiling.

This paper introduces an innovative pressure measurement system for unsteady flow measurement. Unsteady flow is currently one of the hot topics of road vehicles aerodynamics and an inexpensive, simple-to-use tool for its investigation has long been missing. Conventional pressure measurement systems with centralized pressure scanners and long tubings do not satisfy the needs of unsteady phenomena. An approach of miniature MEMS pressure sensors is used instead, whereby the sensors are installed directly on the surface of the body under investigation. This concept not only enables the possibility of unsteady flow measurement but also offers a fast and reliable method of pressure field investigation. Finally, the measurement system was aimed at the specific needs of road vehicle productive testing and provides an effective tool capable of rapid experiment preparation, reliable measurement and simple post-processing.

The newly developed system was tested and validated in different experiments in a specialized test stand and in model scale wind tunnel measurements.