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Technical Paper

Numerical Simulation of the Transient Heat-Up of a Passenger Vehicle during a Trailer Towing Uphill Drive

In the digital prototype development process of a new Mercedes-Benz, thermal protection is an important task that has to be fulfilled. In the early stages of development, numerical methods are used to detect thermal hotspots in order to protect temperature sensitive parts. These methods involve transient full Vehicle Thermal Management (VTM) simulations to predict dynamic vehicle heat-up during critical load cases. In order to simulate thermal control mechanisms, a coupled 1D to 3D thermal vehicle model is built in which the coolant and oil circuit of the engine, as well as the exhaust flow are captured in detail. When performing a transient 3D VTM analysis, the conduction and radiation phenomena are simulated using a transient structure model while the convective phenomena are co-simulated in a steady state fluid model. Both models are brought to interaction at predetermined points by an automatized coupling method.
Technical Paper

Model Scale Based Process for the Development of Aerodynamic Tire Characteristics

The geometric shape of the tires can have a large influence on the aerodynamic drag of a passenger car as it has been shown already in different publications like for example [1, 2, 3]. However, to optimize the shape of a tire, nowadays quite some effort is needed in terms of wind tunnel time and costs for prototype tires. In this paper an approach to optimize the tire's shape in model scale is described, which can help to reduce both development time and costs. The first step in the development of this method was to verify that the aerodynamic effects of the tire geometry in model scale are comparable to full scale tests. This was achieved by measuring different production tires in full scale and also by measuring the quarter scale version of the same tires. The only difference between the original and the model scale tires was that the scaled tires were not deformable. The results show that the difference between two sets of tires is comparable in full scale and in quarter scale.
Journal Article

Unsteady Aerodynamic Properties of a Vehicle Model and their Effect on Driver and Vehicle under Side Wind Conditions

In this paper the effect of aerodynamic modifications that influence the unsteady aerodynamic properties of a vehicle on the response of the closed loop system driver-vehicle under side wind conditions is investigated. In today's aerodynamic optimization the side wind sensitivity of a vehicle is determined from steady state values measured in the wind tunnel. There, the vehicle is rotated with respect to the wind tunnel flow to create an angle of attack. In this approach however, the gustiness that is inherent in natural wind is not reproduced. Further, unsteady forces and moments acting on the vehicle are not measured due to the limited dynamic response of the commonly used wind tunnel balances. Therefore, a new method is introduced, overcoming the shortcomings of the current steady state approach. The method consists of the reproduction of the properties of natural stochastic crosswind that are essential for the determination of the side wind sensitivity of a vehicle.
Technical Paper

CFD Approach to Evaluate Wind-Tunnel and Model Setup Effects on Aerodynamic Drag and Lift for Detailed Vehicles

Previous work by the authors showed the development of an aerodynamic CFD model using the Lattice Boltzmann Method for simulating vehicles inside the IVK Model-Scale Wind-Tunnel test-section. In both experiment and simulation, alternate configurations of the wind-tunnel geometry were studied to change the pressure distribution in the wind-tunnel test section, inducing a reduction in aerodynamic drag due to interference between the wind-tunnel geometry and the pressure on the surface of the vehicle. The wind-tunnel pressure distribution was modified by adding so-called “stagnation bodies” inside the collector to create blockage and to increase the pressure in the rear portion of the test section. The primary purpose of previous work was to provide a validated CFD approach for modeling wind-tunnel interference effects, so that these effects can be understood and accounted for when designing vehicles.
Journal Article

Integrated Numerical and Experimental Approach to Determine the Cooling Air Mass Flow in Different Vehicle Development Stages

This paper presents an integrated numerical and experimental approach to take best possible advantage of the common development tools at hand (1D, CFD and wind tunnel) to determine the cooling air mass flow at the different vehicle development stages. 1D tools can be used early in development when neither 3D data nor wind tunnel models with detailed underhood flow are available. A problem that has to be resolved is the dependency on input data. In particular, the pressure coefficients on the outer surface (i.e. at the air inlet and outlet region) and the pressure loss data of single components are of great importance since the amount of cooling air flow is directly linked to these variables. The pressure coefficients at the air inlet and outlet are not only a function of vehicle configuration but also of driving velocity and fan operation. Both, static and total pressure coefficient, yield different advantages and disadvantages and can therefore both be used as boundary conditions.
Journal Article

Numerical Comparison of Rolling Road Systems

The entire automotive industry is moving towards lower CO₂ emissions and higher energy efficiency. Especially for higher driving speeds this can be achieved by minimizing aerodynamic drag. Additionally, aerodynamic downforce is essential to maintain or even improve the handling performance of a vehicle. In order to optimize the vehicle's aerodynamic efficiency in wind tunnel tests, the boundary conditions of a vehicle driving on a road must be simulated properly. Particularly for optimizing the underbody region of a vehicle, ground simulation is an important issue in every wind tunnel. Today rolling road systems featuring one or more moving belts on the wind tunnel floor are a standard tool to simulate the complex boundary condition of a vehicle driving on the road. But generally the technical effort to measure aerodynamic forces accurately increases with improvement of the aerodynamic ground simulation.
Technical Paper

Challenges and Opportunities of Numerically Simulating the Idle Load Case for Vehicle Thermal Management

Collective life-cycle data is needed when developing components like elastomer suspension mounts. Life-time prediction is only possible using thermal load frequency distributions. In addition to current extreme load cases, the Idle Load Case is examined at Mercedes-Benz Car Group as a collective load case for Vehicle Thermal Management (VTM) numerical simulations in early development stages. It combines validation opportunities for HVAC, cooling and transmission requirements in hot-country-type ambient conditions. Experiments in climatic wind tunnels and coupled 3D CFD and heat transfer simulations of the Idle Load Case have been performed. Measurements show steady conditions at the end of the load case. Decoupling of the torque converter, changes in ambient temperature and the technical implementation of a wind barrier for still air conditions exhibit influence on component-level results. Solar load, however, does not significantly change the examined component temperatures.
Technical Paper

CFD Validation Study for a Sedan Scale Model in an Open Jet Wind Tunnel

Aerodynamic simulations using CFD is now a standard tool in the automotive industry, and is becoming more and more integrated in the aerodynamic design process of new vehicles. This process is distinguished by parallel development with wind tunnel experiments and CFD simulation results, which demands comparable results to be generated by the two development tools. As wind tunnel effects are not simulated in most industrial applications of CFD, the comparison with experimental results normally show differences partly due to wind tunnel effects and ground simulation effects. Therefore a deeper understanding of wind tunnel effects and methods to fully reproduce experimental values with CFD is necessary. In this paper, an extensive validation study with a detailed scale notchback model inside an open jet wind tunnel is presented. This study includes experimental data from the real wind tunnel as well as CFD simulation results with and without wind tunnel effects.
Technical Paper

CFD Investigations of Wind Tunnel Interference Effects

Wind tunnel interference effects are still considered to be negligible - or at least undesired - in automotive aerodynamics. Consequently, up to now there is no standard correction method which is used in everyday wind tunnel testing although a lot of research has been done in recent years. In most full-vehicle CFD simulations, wind tunnel interference effects are not simulated. The flow about the car is computed under idealized conditions. The wind tunnel is designed to simulate these conditions but fails to do so to some degree due to its limited size. Therefore a comparison of blockage-free CFD results and wind tunnel measurements is deficient. Hence CFD simulations including wind tunnel interference effects should be favored in the future for validation purposes. Furthermore, CFD offers new possibilities to investigate individual contributions to wind tunnel interference effects and therefore could help to increase the understanding of the flow in the wind tunnel.
Technical Paper

The Influence of Rotating Wheels on Total Road Load

Within in the scope of a road load investigation project at FKFS, the influence of rotating wheels on the road load of a passenger car was investigated. For this purpose an approach was developed to measure the ventilation resistance of a spinning wheel. This approach enables a comparison of different wheel sizes and rim designs. Together with aerodynamic drag measurements in the wind tunnel it is possible to evaluate different wheel configurations with respect to their contribution to the road load. The measuring approach and results of performed measurements are shown in this paper.
Technical Paper

Investigations in a Cooling Air Flow System under the Influence of Road Simulation

This paper presents some recent results concerning the generation and minimization of cooling air drag, achieved in an integrated approach of numerical and experimental investigations. The baseline configuration of a production cars' cooling air flow system is analyzed. The analysis of the created drag shows, that most of the force changes due to the cooling air flow appear in the front region of the vehicle. However, the forces generated by heat exchangers are only a small share of the total changes. Additional drag is generated for example by the front wheels and by the components of the underhood compartment. The investigation of the influence of the vehicle rear end shape on the aerodynamics of the cooling air flow system shows, that two similar cars with different rear end shapes (notchback and squareback) can feature different cooling air drag values.
Journal Article

The Effect of Center Belt Roughness on Vehicle Aerodynamics

Recently built or refurbished wind tunnel facilities show a trend towards a detailed simulation of road conditions. Therefore, these wind tunnel facilities are equipped with boundary layer conditioning systems and a rolling road consisting of one or several belts in order to simulate the rotation of the wheels and the relative motion between the vehicle underfloor and the road. Belts are either realized in rubber or steel. Steel belts offer the possibility to be coated with rubber to protect the belt itself. This coating additionally offers the possibility to attain a certain roughness to represent the road surface. This paper presents measurements of the roughness of the steel belt systems installed in the IVK Model Scale and Aero-Acoustic Full Scale Wind Tunnel in comparison to road surfaces. Additionally, the influence of roughness on the aerodynamic coefficients drag and lift is presented and discussed for the SAE reference body with different rear end configurations.
Journal Article

The Effect of High Turbulence Intensities on Surface Pressure Fluctuations and Wake Structures of a Vehicle Model

The unsteady environment road vehicles are exposed to is subject of many investigations that are currently made. Yet, the approaching flow is only one aspect of unsteady forces acting on the vehicle. Unsteady wake structures also lead to time-varying surface pressures and consequently fluctuating forces even in steady and low turbulent flows. However, little is known about the influence of realistic flow conditions, i.e. as found on road, on the unsteady surface pressures and wake structures of a vehicle. Therefore, to derive a deeper understanding of the unsteady aerodynamic properties of a vehicle this paper presents results of measurements conducted on a vehicle body both in smooth and turbulent flow conditions in the IVK model scale wind tunnel. Unsteady surface pressure measurements in the area where separation occurs and the base of the vehicle were made together with time accurate total pressure measurements in the wake.
Technical Paper

Further Investigations on Gradient Effects

In automotive wind tunnels with modern road simulation installations boundary layer pre-suction is a widely-used technique for boundary layer control. The consequence of boundary layer pre-suction is an additional pressure gradient in front of the model. In order to investigate the effects of the additional pressure gradient on drag, experiments were conducted with two different models (scale 1:5) in the IVK Model Wind Tunnel. In these experiments the suction velocity of the boundary layer pre-suction served as a parameter to change the static pressure gradient along the test section and was for this purpose adjusted higher and lower than the standard suction velocity. It is shown that the total drag increment due to boundary layer pre-suction consists of at least two parts: the ground simulation increment and the static pressure gradient increment. The ground simulation increment is due to a decrease in the boundary layer thickness and the resulting modified flow beneath the model.
Technical Paper

Underhood Temperature Analysis in Case of Natural Convection

This paper describes a method to simulate underhood temperature distributions in passenger cars. A simplified engine compartment simulation test rig is used to perform measurements with well known boundary conditions to validate the simulation strategy. The measurement setup corresponds to idle without working fan. The aim of this setup is to validate cases with strong natural convection, e.g. thermal soaking. A coupled steady-state CFD run and thermal analysis is undertaken to simulate the temperature distribution in the test rig. Convective heat transfer coefficients and air temperatures are calculated in StarCD™. The radiative and conductive heat transfer is considered in a RadTherm™ analysis. The strong coupling of flow field and wall temperature in buoyancy driven flows requires an iterative process. Calculated temperatures are compared to measured results in order to validate the simulation method as far as possible. Some of the results are reported in this paper.
Technical Paper

The New 5-Belt Road Simulation System of the IVK Wind Tunnels - Design and First Results

In 2001 the FKFS (Research Institute of Automotive Engineering and Vehicle Engines, Stuttgart) took into operation state-of-the-art 5-belt systems for road simulation in the 22.45m2-IVK automotive wind tunnel and in the 1.65m2-IVK model wind tunnel. In these systems, a narrow belt running between the vehicles' wheels is fitted with 4 balance-mounted wheel rotation drives and a vehicle restraint system. The FKFS opted for MTS steel belt technology due to its small size, low power requirements and excellent tracking stability. Due to air bearings below the belt, the flat-belt wheel rotation units in the full-scale wind tunnel permit aerodynamic force measurements at full wheel load (8 kN) up to 70 m/s. In combination with the hydrostatic suspension of the units, integrated longitudinal force transducers permit realistic measurements of the wheels' rolling resistance. In the model wind tunnel FKFS wheel rotation units with Poly-V belts are used with small wheel loads up to 80 m/s.
Technical Paper

Flow around an Isolated Wheel - Experimental and Numerical Comparison of Two CFD Codes

This paper presents velocity and pressure measurements obtained around an isolated wheel in a rotating and stationary configuration. The flow field was investigated using LDA and a total pressure probe in the model scale wind tunnel at IVK/FKFS. Drag and lift were determined for both configurations as well as for the wheel support only. These results were used as a reference for comparing numerical results obtained from two different CFD codes used in the automotive industry, namely STAR-CD™ and PowerFLOW™. The comparison gives a good overall agreement between the experimental and the simulated data. Both CFD codes show good correlation of the integral forces. The influence of the wheel rotation on drag and lift coefficients is predicted well. All mean flow structures which can be found in the planes measured with LDA can be recognized in the numerical results of both codes. Only small local differences remain, which can be attributed to the different CFD codes.
Technical Paper

A New Approach to Predicting Component Temperature Collectives for Vehicle Thermal Management

There is a growing need for life-cycle data – so-called collectives – when developing components like elastomer engine mounts. Current standardized extreme load cases are not sufficient for establishing such collectives. Supplementing the use of endurance testing data, a prediction methodology for component temperature collectives utilizing existing 3D CFD simulation models is presented. The method uses support points to approximate the full collective. Each support point is defined by a component temperature and a position on the time axis of the collective. Since it is the only currently available source for component temperature data, endurance testing data is used to develop the new method. The component temperature range in this data set is divided in temperature bands. Groups of driving states are determined which are each representative of an individual band. Each of the resulting four driving state spaces is condensed into a substitute load case.
Technical Paper

The New Interchangeable Three-belt System in the IVK Full-Scale Wind Tunnel of University of Stuttgart: Design and First Results

With its recent wind tunnel upgrade, FKFS installed the first interchangeable three-belt / five-belt-system (FKFS first®) in a full scale automotive wind tunnel. With the five-belt system, which today is a state-of-the-art ground simulation technique, the system is ideally suited for production vehicle development work. The five-belt system offers high flexibility, quick access to the underfloor and vehicle fixation, and setting the vehicle’s ride height by the restraint device. The first results of the five-belt system have already been published in SAE 2015-01-1557 [1]. The three-belt system on the other hand, offers a much more sophisticated ground simulation technique which is necessary especially for sports and racing cars. For such vehicles with very low ground clearances, it is important to have a more accurate ground simulation, in order to capture the same aerodynamic modes of action and response as on the road.
Technical Paper

Comparison of Different Ground Simulation Techniques for Use in Automotive Wind Tunnels

The range of applicability and the physical restrictions for the use of ground-simulation techniques in automotive wind tunnels are elucidated. The techniques considered are the moving-belt technique, as well as boundary layer control techniques like tangential blowing and distributed normal suction for use in wind tunnels with stationary ground boards. Attention has to be paid to the question of whether the flow to be simulated is of boundary layer or Couette type. In the case of boundary layer flow, interaction of the ground-floor boundary layer with the inviscid flow in the gap between a vehicle and the road can be fully simulated by introducing a negative transpiration velocity along the stationary ground plane. In practise however, angularity effects on the external flow result from mismatched control parameters. Very small relative ground clearances give rise to the formation of a Couette flow between the road and the vehicle.