Influence of Windshield Wiper Movement on Vehicle Aerodynamics
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Influence of Windshield Wiper Movement on Vehicle Aerodynamics
AUTHORS
Daniel Demel, M. Sc. is PhD Student in the Department of Aerodynamics and Multiphase Flows at BMW AG in Munich (Germany).
In the course of increasing virtualization, the use of flow simulations in the early stages of vehicle development at BMW has increased significantly. These calculations provide a detailed insight into the flow field and can reveal potentials for optimization. In this work, a method for including windshield wiper movement in CFD simulations is presented and the influence of the transient full vehicle flow through the wipers is investigated.
INTRODUCTION
Peter Kostorz, M. Eng. is PhD Student in the Department of Aerodynamics with Focus on Unsteady Flows at BMW AG in Munich (Germany).
Honorary Prof. Dr.-Ing. Thomas Schütz is Professor of the Institute of Fluid Mechanics and Aerodynamics at TU Darmstadt (Germany) and Product Line Manager of S Series Motorcycles at BMW AG in Berlin (Germany).
Numerical flow simulations are nowadays fully integrated in the development process and enable early analysis of flow processes on the virtual vehicle. The confidence in single-phase simulations is supported by a variety of validations [1, 2]. Due to the complexity of a simu lation with moving grids, windshield wipers are primarily designed by expe riments on prototypes, with the focus on cleaning performance and avoiding undesirable effects like for example wi per chattering [3]. In the present work, the influence of windshield wiper movement on the transient full vehicle flow is investigated using the modal analysis on the carried out simulations. For this purpose, the basics of the evaluation techniques as well as the simulation setup are described in the first part and the results are shown in the second part. BASICS OF EVALUATION TECHNIQUES
Prof. Dr.-Ing. habil. Sven Grundmann is Head of the Institute of Fluid Mechanics at Universität Rostock (Germany).
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The drag force of a vehicle results from a pressure and a friction component. For comparison purposes, this value is expressed in the form of the dimensionless drag coefficient. For dedimensioning
the frontal area A projected in the longitudinal direction of the vehicle and the dynamic pressure of the free air flow ½ ρ u ∞2 is used according to Eq. 1: Eq. 1
FD
c D = _______ 2 ½ ρ A u∞
Fourier analysis can be used to analyze the dynamics of the drag and the modal coefficients. With the help of the discrete Fourier transformation a sampled signal can be decomposed into a finite series of single-frequency oscillations. This transformation is the time-discrete, further developed equivalent to the Fourier transformation, with which a function can be split into multiple sine and cosine functions of varying strength. Every harmonic oscillation has a time constant frequency. The energetic part of an oscillation in the original function is quantified by its spectral power density in the power spectrum [4]. With the aid of the modal decompo si
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