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Technical Paper

Analysis of a Vehicle’s Longitudinal Response to an Unsteady Wind Environment

In a preceding paper an on-road investigation of the longitudinal aerodynamic response of a vehicle to ambient wind was presented. That study resulted in a frequency-dependent response function with a distinctive maximum within the range of the natural frequency of the vehicle on its suspension system. This finding raised the question as to whether the horizontal response of a car’s deceleration to wind gusts is associated with or caused by the suspension’s natural frequency. The objective of the present work is an attempt to shed some light on this question by the investigation of both deceleration and pitch angle fluctuations in additional on-road experiments. Both vehicle velocity and total airspeed in the driving direction and the pitch angle are recorded by independent data acquisition systems during a set of coastdown experiments.
Technical Paper

On-Road Investigation of a Vehicle's Longitudinal Response to Wind Gusts

The objective of this work is an attempt to investigate the longitudinal aerodynamic response of a vehicle to ambient wind. The natural wind environment is usually unsteady and causes therefore slight oscillations of the vehicle's velocity around its cruising speed. The additional force superposed to the steady drag caused by the smooth oncoming air flow is generated by ambient turbulence. For the on-road investigation of the vehicle's longitudinal response to the ambient turbulent flow field both vehicle velocity and relative airspeed in driving direction are recorded by independent data acquisition systems during a set of coastdowns. The deceleration data and the vehicle's response to the airspeed fluctuations are derived by numerical filtering techniques and differentiation of the motorcar's velocity-time history. The study results in a frequency-dependent response function.
Technical Paper

Conservation of Energy Method for On-Road Aerodynamic Drag Investigation

A significant reduction of the aerodynamic drag can be achieved by the refinement of the vehicle underside. But the investigation of the aerodynamic effectiveness of underfloor panels in wind tunnels turns out to be a challenging task. For this reason on-road methods like coastdown analysis are frequently employed. Most of these advanced coastdown methods determine the coefficients of the total drag by fitting a mathematical model to the measured velocity data. This requires separate time consuming tests in the laboratory to measure the wheel and transmission losses. The objective of the presented work was the development of an analysis method, which enables a simpler and more practicable on-road determination of aerodynamic improvements. In this method the rolling resistance can be factored out by employing the conservation of energy as basic approach for the investigation of air drag differences.
Technical Paper

On-Road Aerodynamic Drag Analysis by Simultaneous Linear Inversion of the Equation of Motion

The objective of this work is an investigation of the aerodynamic improvements by e.g. the application of underfloor panels that might be masked by traditional, fixed-ground wind tunnel testing. Of prime importance is the simplicity and practicability of the developed analysis method, with particular emphasis on industrial application. A coastdown method with minimal instrumentation effort is chosen to determine drag coefficients on the road. Usually such experiments demand the measurement of vehicle velocity, airspeed, yaw angle and some additional environment characteristics like temperature and air pressure. However, in principle it is only required to measure the vehicle's speed-time history during the test, from which the road load results can be derived by mathematics of inversion. In this work several underfloor configurations of the test vehicle are investigated by only considering the motorcar's speed data retrieved from the control area network data bus during the coastdown.