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Journal Article

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

2011-04-12
2011-01-0154
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

Underhood Temperature Analysis in Case of Natural Convection

2005-05-10
2005-01-2045
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

Tool Support for Analyzing and Optimization Methods in Early Brake System Sizing Phases

2000-03-06
2000-01-0442
The manufacturers of passenger cars increasingly assign development and production of complete subsystems to the supplying industry. A brake system supplier has to give predictions about system quality and performance long time before the first prototypical system is built or even before the supplier gets the order for system development. Nowadays, the usage of computer-aided system design and simulation is essential for that task. This article presents a tool designed to support the development process. A special focus will be on how to define quality. A formal definition of quality is provided, illustrated and motivated by two examples.
Technical Paper

Thermal Simulation within the Brake System Design Process

2002-10-06
2002-01-2587
During the acquisition phase brake system supplier have to make predictions on a system's thermal behavior based on very few reliable parameters. Increasing system knowledge requires the usage of different calculation models along with the progress of the project. Adaptive modeling is used in order to integrate test results from first prototypes or benchmark vehicles. Since changes in the brake force distribution have a great impact on the simulation results fading conditions of the linings have to be integrated as well. The principle of co-simulation is used in order to use the actual brake force distribution of the system.
Technical Paper

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

2016-04-05
2016-01-1581
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

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

2003-03-03
2003-01-0429
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

The Influence of Rotating Wheels on Total Road Load

2007-04-16
2007-01-1047
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

The Influence of Ground Simulation and Wheel Rotation on Aerodynamic Drag Optimization - Potential for Reducing Fuel Consumption

1996-02-01
960672
In automobile development, wind tunnel measurements are used to optimize fuel consumption and the vehicle's road behavior. The classic measuring technique is based on a stationary vehicle set up in the wind tunnel with stationary wheels. Relative movement between vehicle and road surface is therefore ignored. In more recent studies, measurements have been taken with improved ground simulation. For example, a belt is used instead of the stationary wind tunnel floor and the car wheels rotate. Ground simulation using a belt and rotating wheels generally leads to a reduction in flow angularity at the front wheels, in the same way as blocking the cooling air flow, whereby, as a matter of fact, the aerodynamic drag is reduced. Analogous air flow angle correlations can be established for the effect of underfloor panels.
Journal Article

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

2009-04-20
2009-01-0001
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.
Journal Article

The Effect of Center Belt Roughness on Vehicle Aerodynamics

2009-04-20
2009-01-0776
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

Subjective Perception and Evaluation of Driving Dynamics in the Virtual Test Drive

2017-03-28
2017-01-1564
In addition to the analysis of human driving behavior or the development of new advanced driver assistance systems, the high simulation quality of today’s driving simulators enables investigations of selected topics pertaining to driving dynamics. With high reproducibility and fast generation of vehicle variants the subjective evaluation process leads to a better system understanding in the early development stages. The transfer of the original on-road test run to the virtual reality of the driving simulator includes the full flexibility of the vehicle model, the maneuver and the test track, which allows new possibilities of investigation. With the opportunity of a realistic whole-vehicle simulation provided by the Stuttgart Driving Simulator new analysis of the human’s thresholds of perception are carried out.
Technical Paper

Some Basic Investigations into the Principles of Ground Simulation Techniques in Automotive Wind Tunnels

1989-02-01
890369
With the help of theoretical considerations it is shown that the flow between a car and the ground is of boundary layer type, as long as there is no recirculation. Thus, boundary layer theory can be applied to evaluate the order of magnitude of typical effects like displacement and momentum-loss thicknesses of ground-plane boundary layer. If the boundary layer in a wind tunnel with stationary ground is to be controlled, either by distributed suction or by tangential blowing, to simulate on-road conditions, boundary layer theory can provide the orders of magnitude of modifications that have to be applied. Experiments with a ground-effect quarter-scale car with ground simulation by distributed suction and moving belt showed coincidence with theoretical predictions concerning the required suction rate, if integral coefficients (eg CD, CL) of both flow cases are matched.
Technical Paper

Road Load Determination Based on Driving-Torque-Measurement

2003-03-03
2003-01-0933
This paper introduces a driving-torque measurement method for the determination of vehicle road load and its components. To increase the accuracy, the torque measurements are combined with rolling resistance measurements performed with a specially developed trailer. This method is a strictly experimental approach and does not use any mathematical models. The experimental techniques are described as well as the proceedings to compare test stand and road measurements. The results that are shown prove that this method is suitable for the investigation of single road load components. Furthermore, the comparison of different rolling resistance measurement devices shows the potential of the measurement trailer and the necessity to perform rolling resistance measurements on real road surfaces and not solely on test stands.
Technical Paper

Reduced Model of a Vehicle Cabin for Transient Thermal Simulation

2018-05-30
2018-37-0022
In the proposed work the transient thermal modeling of a vehicle cabin has been performed. Therefore, a reduced model has been developed based on a one-node discretization of the cabin air. The conduction in the solid parts is accounted for by a one-dimensional heat transfer approach, the radiation exchange between the surfaces is based on view factors adopted from a 3D reference and the convective heat transfer from the cabin surfaces to the cabin air is conducted with the help of heat transfer coefficients calculated in a 3D reference simulation. The cabin surface is discretized by planar wall elements, including the outer shell of the cabin and inner elements such as seats. Each wall element is composed of several homogeneous material layers with individual thicknesses. Investigations have been conducted on the temporal and spatial resolution of the layer structure of these wall elements, for the 3D model as well as for the reduced one.
Journal Article

Rating Mass-related Energy Demand for Vehicles with New Powertrain Concepts

2011-06-09
2011-37-0010
The combination of enhanced powertrains and adapted vehicle concepts can reduce the energy demand of vehicles significantly, especially when energy conversion efficiency rises and at the same time driving resistances decrease. In addition, new powertrain concepts are able to offer extra functionality due to a growing cross-linking with chassis and vehicle body. The design of highly linked vehicles and powertrain systems requires additional new development methods in order to answer interacting questions of driving dynamics and vehicle energy efficiency at an early stage of development. In the paper a database-based simulation platform is presented which was developed at the IVK of the University of Stuttgart in cooperation with the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS). The simulation platform is used as an example to discuss mass reducing developments for various powertrain concepts.
Technical Paper

On the Correction of Interference Effects in Open Jet Wind Tunnels

1996-02-01
960671
This paper deals with the correction for interference effects on the flow about bluff bodies in open-jet wind tunnels. Altogether, five different interference effects in open-jet tunnels are identified and described by physical models. Besides the classical jet-expansion correction which has been treated repeatedly by various authors throughout the last 60 years, the new correction method also includes the effect of jet deflection due to the proximity of a vehicle to the nozzle. Furthermore, far-field interference effects originating from the nozzle and the collector are described. For this purpose nozzle and collector effects are treated as solid- wall blockage phenomena, and with the aid of Biot-Savart principles the induced velocity at the model position is calculated. Finally, the static- pressure gradient in an empty test section generates a horizontal buoyancy force when a model is placed in a wind tunnel stream.
Technical Paper

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

2013-04-08
2013-01-0873
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.
Journal Article

Numerical Comparison of Rolling Road Systems

2011-06-09
2011-37-0017
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.
Journal Article

New Motion Cueing Algorithm for Improved Evaluation of Vehicle Dynamics on a Driving Simulator

2017-03-28
2017-01-1566
In recent years, driving simulators have become a valuable tool in the automotive design and testing process. Yet, in the field of vehicle dynamics, most decisions are still based on test drives in real cars. One reason for this situation can be found in the fact that many driving simulators do not allow the driver to evaluate the handling qualities of a simulated vehicle. In a driving simulator, the motion cueing algorithm tries to represent the vehicle motion within the constrained motion envelope of the motion platform. By nature, this process leads to so called false cues where the motion of the platform is not in phase or moving in a different direction with respect to the vehicle motion. In a driving simulator with classical filter-based motion cueing, false cues make it considerably more difficult for the driver to rate vehicle dynamics.
Technical Paper

Model Scale Based Process for the Development of Aerodynamic Tire Characteristics

2014-04-01
2014-01-0585
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.
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