Aerodynamics of Road Vehicles From Fluid Mechanics to Vehicle Engineering Edited by Wolf-Heinrich Hucho Contributors Syed R. Ahmed Hans-Joachim Emmelmann Klaus-Dieter Emmenthal Helmut Flegl Werner Gengenbach Hans Götz Wolf-Heinrich Hucho Dietrich Hummel Görgün A. Necati Raimund Piatek Michael Rauser Butterworth-Heinemann London Boston Singapore Sydney Toronto Wellington (^ PART OF REED INTERNATIONAL P.L.C. All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing it in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright owner except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 33-34 Alfred Place, London, England WC1E 7DP. Applications for the copyright owner's written permission to reproduce any part of this publication should be addressed to the Publishers. Warning: The doing of an unauthorised act in relation to a copyright work may result in both a civil claim for damages and criminal prosecution. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be re-sold in the UK below the net price given by the Publishers in their current price list. English edition first published 1987 Reprinted 1990 Originally published under the title Aerodynamik des Automobils by Vogel-Verlag, Würzburg, West Germany. © Vogel-Verlag, Würzburg, 1981 English edition © Butterworth-Heinemann Ltd, 1987 British Library Cataloguing in Publication Data Aerodynamics of road vehicles : from fluid mechanics to vehicle engineering. 1. Motor vehicles — Aerodynamics I. Hucho, Wolf-Heinrich II. Aerodynamik des Automobils. English 629.04'9 TL245 ISBN 0-408-01422-9 Library of Congress Cataloging-in-Publication Data Aerodynamik des Automobils. English. Aerodynamics of road vehicles. Translation of: Aerodynamik des Automobils. Bibliography: p. Includes index. 1. Motor vehicles—Aerodynamics. I. Hucho, Wolf-Heinrich. TL245.A4713 1986 629.2 86-13693 ISBN 0-408-01422-9 Typeset by Scribe Design, Gillingham, Kent Printed in Great Britain at the University Press, Cambridge Preface The performance, handling and comfort of an automobile are significantly affected by its aerodynamic properties. A low drag is a decisive prerequisite for good fuel economy. Increasing fuel prices and stringent legal regulations ensure that this long-established relationship becomes more widely acknowledged. But the other aspects of vehicle aerodynamics are no less important for the quality of an automobile: side wind stability, wind noise, soiling of the body, the lights and the windows, cooling of the engine, the gear box and the brakes, and finally heating and ventilating of the passenger compartment all depend on the flow field around and through the vehicle. Vehicle aerodynamics is still an empirical science, if not an art. Whereas other technical disciplines such as aeronautics, naval architecture and turbomachinery are governed by well-established theoretical and ex- perimental methods of fluid mechanics, no consistent design procedures are yet available for road vehicles. The complexity of the flow field around a car, which is characterized by separation, must be blamed for this lack, and this means that the vehicle aerodynamicist must refer to a large amount of detail resulting from earlier development work. His success depends on his ability to transfer these results to his own problem and to combine results originating from many different earlier developments to a consistent solution. It is the intention of the present book to introduce the vehicle engineer to this approach. His interest is focused on three aspects: • the fundamental of fluid mechanics as related to vehicle aerodynamics; • the essential experimental results, presented as ground rules of fluid mechanics and brought to general validity wherever possible; • design strategies, showing how many existing single results can be combined to provide general solutions. The aerodynamics of passenger cars, commercial vehicles, sports cars and race cars is dealt with in detail. Not only the external flow field is covered; the problems of the several internal flow systems are treated as well. Because the external and the internal flow fields are interrelated, both have to be considered at the same time. The related test techniques are described in detail, emphasizing the correlation between the wind tunnel, which is the main tool of the vehicle aerodynamicist, and the road, Preface which is the real world for the car in a customer's hands. A chapter on numerical methods concludes the book. Although theoretical models are still of limited evidential value they are more and more used for guiding and supporting, rather than replacing, wind tunnel tests. The first German edition of this book was originally based on a course given by the authors at the 'Haus der Technik', Essen, Germany, under the aegis of Dr H. Hahn. This English version is a completely revised second edition. It is intended for vehicle engineers in industry and research, at universities and in administrative departments. But it is also aimed at stylists and designers, students and professional writers in the car world. Detailed knowledge of fluid mechanics is not assumed. The chapter on the fundamentals of fluid mechanics provides the reader with the necessary details. This present English edition would not have come about were it not for the efforts of two true friends of the editor: Mr Gordon Taylor built the bridge to Butterworths and Dr Gino Sovran involved the publication department of the Society of Automotive Engineers (SAE), thus providing a sufficiently broad basis for the project. The editor is deeply indebted to both his friends. He also wishes to express his sincerest thanks to all who have contributed to this book: first of all, of course, to the authors for their readiness to carry the burden of preparing the manuscripts; thanks as well to the secretaries and draughtsladies for typing the manuscripts and for drawing the figures; thanks to the companies of the authors for having given them permission to contribute to the book. The editor expresses his warmest thanks to his wife, Irmgard, for her untiring assistance during the preparation of the extensive material and to his former secretary, Mrs Hildegard Backes, for typing and editing the final manuscript and for continuing to do so even when the editor was in the course of changing his employer. Finally, thanks are owed to the publishers: to Vogel-Verlag, Würzburg, for granting the licence, to Butterworths for good and patient cooperation and the SAE publications department for sharing the project. Wolf-Heinrich Hucho Schwalbach am Taunus, Federal Republic of Germany December 1986 Contributors Dr.-Ing. Syed R. Ahmed is divisional head in the Institute of Design Aerodynamics, German Aeronautical and Space Research Establishment (DLR), Braunschweig, Federal Republic of Germany. He received a Dipl.-Ing. Degree in 1964, followed in 1970 by a Dr.-Ing. degree in fluid mechanics from the Technical University at Braunschweig. Since 1975 he has been actively engaged in theoretical and experimental study of vehicle aerodynamics. Dipl.-Ing. Hans-Joachim Emmelman studied mechanical and aircraft engineering at the Technical University, Darmstadt, Federal Republic of Germany. From 1970 to 1978 he was test engineer and assistant department head in the climatic wind tunnel of Volkswagen AG. Since 1979 he has been an assistant staff engineer, aerodynamics, at Adam Opel AG. Dr.-Ing. Klaus-Dieter Emmenthal studied mechanical engineering at the Technical University, Braunschweig. He then worked for nine years at the German Aerospace Authority as research engineer. Since 1970 he has been with Volkswagen AG and is currently department manager responsible for engine and accessories research. He obtained his doctorate at the Technical University, Aachen, Federal Republic of Germany. Dipl.-Ing. Helmut Flegl studied mechanical engineering at the Technical University, Munich. In 1966 he began work at the Dr.-Ing.h.c. F. Porsche AG. As test engineer he was responsible for the design of many successful racing cars. He is currently director of research at Porsche's R&D Centre, Weissach, Federal Republic of Germany. Dr.-Ing. Werner Gengenbach studied mechanical engineering at the Technical University, Karlsruhe, from 1954 to 1959. He obtained his doctorate in 1967 for his thesis on 'Behaviour of car tyres on dry and especially wet pavement'. Since 1971 he has worked for Audi AG, Ingolstadt, Federal Republic of Germany, first as manager for testing heating, air conditioning and cooling systems; and since 1980 as manager of quality analysis. Dipl.-Ing. Hans Götz is manager of body development at Daimler-Benz in Sindelfingen, Federal Republic of Germany. He studied mechanical Contributors engineering at the Technical University, Stuttgart. After a period as a research engineer on air conditioning, he joined Daimler-Benz in 1961. He has been involved in aerodynamics, safety, vibration and acoustic technologies. Dr.-Ing. Wolf-Heinrich Hucho studied mechanical engineering at the Technical University, Braunschweig. From 1961 to 1968 he was assistant to Professor Schlichting. For 11 years he worked for Volkswagen AG, first as head of the wind tunnel department, later as departmental manager, engine research and fluid dynamics. Since 1979 he has held positions as director of Research & Development and general manager in the German automobile supply industry. Prof. Dr.-Ing. Dietrich Hummel, Institute for Fluid Mechanics of the Technical University, Braunschweig, Federal Republic of Germany, studied mechanical engineering at the Technical Universities in Stuttgart and Braunschweig. As assistant to Professor Schlichting in 1968 he obtained his doctorate, and in 1972 he qualified as lecturer in fluid mechanics and aircraft aerodynamics. His special interests are separated flows and bird flight. Dr.-Ing. Görgün A. Necati studied mechanical engineering at the Technical University, Istanbul, Turkey. His postgraduate studies were at the Technical University, Hanover, Federal Republic of Germany. Since 1969 he has worked in the R&D section of Ford-Werke, Cologne, Federal Republic of Germany. Major activity fields: wind tunnel and road testing, dynamics, aerodynamics and acoustics of motor vehicles. Dipl.-Ing. Raimund Piatek studied mechanical engineering, especially fluid mechanics, at the University of Bochum, Federal Republic of Germany. Since 1978 he has been employed by Volkswagen AG as research engineer in the climatic wind tunnel department. Dipl.-Ing. Michael Rauser studied aeronautics and astronautics at the University of Stuttgart and is presently supervisor of aerodynamics, vehicle research, at the Dr.-Ing.h.c.F.Porsche AG, R&D Centre, Weissach, Federal Republic of Germany. Chapter 1 Introduction to automobile aerodynamics Wolf-Heinrich Hucho 1.1 Scope 1.1.1 Basic principles The flow processes to which a moving vehicle is subjected fall into three categories: • flow of air around the vehicle; • flow of air through the body; • flow processes within the machinery. The first two flow fields are closely related. For example, the flow of air through the engine compartment is directly dependent upon the flow field around the vehicle. Both fields must be considered together. On the other hand, the flow processes within the engine and transmission are not directly connected with the first two, and are not treated here. The external flow subjects the vehicle to forces and moments which greatly influence the vehicle's performance and directional stability. Until recently vehicle aerodynamics was concerned almost exclusively with these two effects, and has only lately focused on the need to keep the windows and lights free of dirt and accumulated rain water, to reduce wind noise, to Figure 1.1 Streamlines in the longitudinal midsection of a VW Golf I (Rabbit), photographed for a full-sized vehicle in the large climatic wind tunnel of the Volkswagen AG. The lines of smoke were introduced in the plane of the longitudinal centreline to show the flow pattern with symmetrical oncoming flow. This flow state exists only when there is no side wind 1 2 Introduction to automobile aerodynamics prevent windscreen wipers lifting, and to cool the engine oil sump and brakes, etc. From the flow pattern shown in Fig. 1.1 some significant flow processes can be discerned, for example flow separation at the rear of the vehicle. Although the streamlines follow the contour of the vehicle over long stretches, even in the area of sharp curves, the air flow separates at the rear edge of the roof, forming a large wake which can be observed (Fig. 1.2) by introducing smoke into the bubble behind the vehicle instead of in the adjacent external flow as in Fig. 1.1. Figure 1.2 Wake of a VW Golf I, photographed as in Fig. 1.1, smoke introduced into the wake The aerodynamic drag D, as well as the other force components and moments, increases with the square of the vehicle speed V: D~V2 (1.1) With a medium-size European car, aerodynamic drag accounts for nearly 80 per cent of the total road resistance at 100 km/h (62mile/h). There is therefore much scope for improving economy by reducing aerodynamic drag. For this reason drag remains the focal point of vehicle aerodynamics, whether the objective is speed or fuel economy. The complete expression for Eqn 1.1 is: D = cOA^- V2 (1.2) where cD is the non-dimensional drag coefficient; A is the projected frontal area of the vehicle (Fig. 1.3); and p is the density of the surrounding air. The drag D of a vehicle is therefore determined by its frontal area A, and by its shape, the aerodynamic quality of which is described by the drag coefficient cD. Generally the vehicle size, and hence frontal area, is determined by the design requirements, and efforts to reduce drag are concentrated on reducing the drag coefficient. The distance between the streamlines ahead of the car compared with those above the vehicle provide an indication of the lift (Fig. 1.1). Closely spaced streamlines mean high velocity and consequently low static pressure (see section 2.3.1). The pressure difference between the upper and lower Scope 3 Projection plane Frontal area Parallel light Figure 1.3 Definition of the frontal area A of a vehicle sides of the vehicle produces a resultant force, at right angles to the direction of motion, which is called lift. As a rule the lift is in the upward direction, i.e. it tends to lift the vehicle and therefore reduces effective wheel loads. It is coupled with a pitching moment, which differentially affects the wheel loads at the front and rear. Below 100 km/h (62 mile/h) lift and pitching moment have only a small effect upon the vehicle, even in a cross-wind. They do change the attitude of the car in relation to the road and therefore slightly affect the aerodynamic drag. The reduction of the wheel loads, however, is small in relation to the static wheel load and the directional stability is hardly affected by lift. This does not apply to high-speed sports cars, where spoilers are often added to counteract the effects of lift. With racing cars, wings ensure that Figure 1.4 Negative lift wings on a Formula 1 racing car 4 Introduction to automobile aerodynamics the-wheel loads increase with speed (Fig. 1.4). How such negative lift wings are tuned in specific cases is described in Chapter 7. With cross-winds the air flow around the vehicle is asymmetric to the longitudinal centre plane. The shape of the car must be such that the additional forces and moments remain so small that the directional stability is not greatly affected (see Chapter 5). First, the need to react to a cross-wind of varying intensity and direction is inconvenient, as the driver must continually apply steering corrections. Secondly, in very rare cases there is the danger of total loss of control; this can only be countered by suitable aerodynamic design. However, it is also important to prevent drivers from being surprised by side-wind gusts, and being unable to react quickly enough. Better design of roads and their surroundings can help to overcome this problem. Soiling of the rear of the vehicle can be studied from the wake flow as shown in Fig. 1.2; details are discussed in Chapter 6. Dust or dirty water is whirled up by the wheels, and dust particles and water droplets distributed throughout the entire wake region by turbulent mixing, and deposited on the rear of the vehicle. Since the flow pattern at the rear has a significant influence upon the aerodynamic drag, soiling of the rear cannot be considered in isolation. Figure 1.1 shows how the external flow field relates to flow processes inside the vehicle. The flow into the radiator (see Chapter 9) is determined by the flow pattern in front of the vehicle. It can be seen that the stagnation point is at the level of the bumper, and that the air flow is oblique to the openings above and below the bumper (not visible in Fig. 1.1). The grill should be designed to direct this air to the radiator, which is generally vertical, while keeping the pressure loss as low as possible. The flow is attached in the region of the concave space formed by the engine hood and the windscreen. Here there is a pressure build-up, which, as described in Chapter 10, can be utilized for driving air through the heating and ventilation system. On most vehicles the fresh air inlet opening is positioned in the middle of this area. However, at this point the pressure is dependent upon the driving speed, which results in an increase of the fresh air flow as speed increases, making maintenance of steady conditions in the passenger compartment quite difficult. If the inlet openings for the fresh air are moved to points on the body which are at ambient pressure, it is possible to separate the external and internal flow fields, at least while the oncoming flow is symmetrical (no side wind). The fresh air fan, which must be correspondingly larger, then provides a flow which is independent of the driving speed (though only when the exit vents in the body are located in areas of ambient pressure as well). The most important internal flow fields are the air flow through the radiator and engine compartment, and the heater or ventilation flow through the passenger compartment. Some types of vehicles—such as racing cars—have separate flow ducts for the oil cooler, brake cooling, and the combustion air for the engine (see Chapter 7). The engine cooling system has the task of removing a heat flux Q, which is of approximately the same magnitude as the useful engine power P: Q~P (1.3) Scope 5 As vehicle design has developed, the requirements for cooling air have increased considerably. Since a larger cooling air flow is required for water cooling than for air cooling, these requirements must be related to the type of cooling (see Chapter 9 for details): 1. Engine power has increased continuously over the years, making necessary greater volumes of cooling air. 2. Following the demands of styling and aerodynamics, the front end of cars has become flatter over the years. The openings available for entry of the cooling air have become smaller as a result (Fig. 1.5). Moreover, the earlier large coherent inlet area has been broken up into individual sub-areas. 3. As a result of compact design, less space is available in the engine compartment for the radiator and cooling air duct. 4. In the interests of safety the body has continuously been reinforced at the front end ('hard edge'), so that the flow is impeded by wide bumpers and cross-members. 1950 1955 1960 1965 1970 1975 Y e a r — ► Figure 1.5 Cooling air inlet area in relation to installed engine power, shown as a function of time, after K.-D. Emmenthal The cooling air must be routed in such a manner that the velocity of the air in front of the radiator is as uniform as possible, thus ensuring optimum radiator efficiency. In addition, the aerodynamic drag of the car is considerably increased as a result of the loss of momentum in the cooling air duct. This increase in drag can be kept small with suitable measures (see section 4.3.2.12). If the ram air flow is not sufficient for cooling, a fan must be added; radiator and fan must be matched to produce an economical system so that the smallest possible amount of power is required to drive the fan. The air flowing through the passenger compartment must perform three groups of tasks (see Chapter 10): 1. Sufficient ventilation must be assured. All contaminants in the form of gases, vapours and dust must be expelled from the passenger compartment. Simultaneously, this provides for replacement of the oxygen consumed through breathing. 2. A comfortable internal climate must be produced and assured for a wide range of variation in the external conditions. For winter operation 6 Introduction to automobile aerodynamics a high-performance heater must be provided. In summer comfort must be ensured by the circulation of fresh air. In extremely hot countries this alone is not sufficient and the air must be cooled with an air conditioner. 3. The internal flow must pass along the windows so that mist evaporates (demisting) and ice, which can form on both sides of the windows, melts (deicing). Particular requirements are placed on the dynamic characteristic of the flow system in the passenger compartment. For instance, the heater is expected to provide heat quickly after the engine is started. However, during cruise the internal climate should be independent of the vehicle speed, the operating state of the engine and the external climate. The flow should produce as little noise as possible; wind noises must be avoided and the fan noise minimized. The openings in the body, with which the internal flow is coupled with the external flow, must be designed so that water cannot enter even under extreme conditions (e.g. in a car wash). The objectives of the aerodynamic design work outlined above are influenced by the type of vehicle under consideration. For instance, during the aerodynamic design of a passenger car, the main consideration is drag. On a high-speed minibus or van, reduction of sensitivity to cross-winds may be the primary goal. Various solutions are available depending upon the type of vehicle. On a racing car the objective will be to improve the traction of the tyres, using negative aerodynamic lift regardless of styling; the wings at the front and back have even become characteristic of modern racing cars. On the other hand minimizing the drag of a passenger vehicle must be accomplished with less conspicuous methods which conform to current styles. 1.1.2 Working methods Parallels exist between the aerodynamics of automobiles and aircraft. The primary objectives are very similar: good driving or flying characteristics (longitudinal dynamics); low aerodynamic drag; balance of forces and moments in both axes perpendicular to the direction of forward motion to ensure good driving or flight stability (transverse stability). Further processing of the measured aerodynamic data in the equations of motion also indicates similarities. In spite of this, motor vehicle aerodynamics differs in significant respects from aircraft aerodynamics. For example, aircraft aerodynamics are permeated to a great extent by theory.1 The aerodynamic design of an aircraft nowadays derives initially from theoretical, i.e. numerical, considerations, followed by experimental work on small-scale models in wind tunnels and finally in flight tests with a prototype. However, with motor vehicles most of the aerodynamic development work is done experimentally. In principle two different approaches are followed. Until recently, work started with a model (full scale or small scale) designed by the styling department. Aerodynamic development was mainly fine tuning, maintaining the styling as little changed as possible (detail optimization). Nowadays work often starts with a low drag body which is developed into a car in the wind tunnel in conjunction with the stylist (see section 4.4). The Scope 7 smaller dimensions of the motor vehicle offer the advantage of wind tunnel testing of full-scale models or even ready-to-drive prototypes. There are primarily two reasons why the procedure differs from that of aircraft design. In contrast to an aircraft, the design of a vehicle is not dictated wholly by aerodynamics. Style, performance, handling, safety, comfort and, of course, production engineering are all important considerations. Increased fuel prices have, however, led to greater emphasis upon aerodynamics. Repeated attempts have been made to apply the results of aircraft aerodynamics to motor vehicles and significant achievements have been made in the solution to individual problems. However, a comprehensive theory of motor vehicle aerodynamics does not yet exist. The computation of the air flow around aircraft is simplified by the fact that the flow fields around the individual components such as the wing, Figure 1.6 Flow around a passenger car (schematic) 8 Introduction to automobile aerodynamics fuselage and tail unit can be handled separately. The interaction between the components can also be assessed theoretically. Since the air flow is generally 'attached', the calculation can be accomplished in two steps. First the non-viscous flow field is determined; then the effect of viscosity is calculated from 'boundary layer' theory. The theoretical methods upon which this procedure is based have been developed continuously and have been expanded to include other requirements such as those resulting from higher flying speeds (Mach-number effects). The flow field around a car cannot be treated in the same way, for two reasons. From Figs 1.1 and 1.2 it is clear that the flow past a car is strongly governed by separation. Figure 1.6 provides further information on the type and location of separation. The effect of viscosity is no longer confined to comparatively small zones close to the surface of the body (boundary layer). Furthermore, with a car it is not possible to distinguish several more or less independent flow fields. The flow field around a car body has to be treated as a whole. Chapter 13 summarizes the present state of numerical methods in car aerodynamics. These methods may be used to guide the work in the wind tunnel. However, much of the aerodynamic design of a car is to prevent, or to tune, separation. The only way to do this is through experimentation. 1.1.3 Related fields There are also useful parallels to related fields illustrated in Fig. 1.7, for example in the aerodynamics of buildings: • flow around bluff bodies • flow fields governed by separation • ground influence and ground boundary layer • interference between buildings • wind tunnel testing techniques. Figure 1.7 Fields related to automobile aerodynamics Scope 9 Building aerodynamics addresses a number of similar objectives: • determination of the effective air forces on the building as a whole • calculation of the air forces upon parts such as roofs, facades and windows • influencing the surrounding flow field for protection of pedestrians • matching of the surrounding flow and the internal flow (climate, chimney draught). Useful reference material includes Hoerner1 2 (wind forces on build- ings), Ackeret1 3 (significant problems of building aerodynamics, based on clear examples), Sachs 1A (presentation of the current state of knowledge), and construction aerodynamics in condensed form by Houghton and Carruthers.15 The flow field surrounding a train is very similar to that surrounding a road vehicle. The primary difference results from coupling of individual cars into long trains, which produces a very long body in comparison to its height and width. Special relationships result when trains meet one another, due to the small gap between the tracks, as well as when driving into tunnels and driving through very narrow tunnels. The primary development goals for railway aerodynamics are: • low aerodynamic drag • reduction of the pressure peaks when trains meet one another, and when driving into a tunnel • reduction of the influence of side winds • matching internal and external flow for purposes of cooling and ventilation. In contrast to the development of road vehicles, for which the trend to higher driving speeds has virtually vanished with the exception of racing cars, speeds are still being increased in the railway sector. For this reason aerodynamics is becoming increasingly significant in this branch of transportation technology. Some early data on the resistance of trains is given by Hoerner.14 A comprehensive survey on train aerodynamics including many references has been presented by Peters.16 Further information has been provided by Gawthorpe.1 7 The problems encoun- tered with high-speed trains, particularly in driving through tunnels have been given by Neppert and Sanderson1 δ and by Steinheuer.1 9 The flow field around a ship above the water line is also a focus of increasing attention. The aerodynamic drag of a water-displacing ship is small in comparison to its water resistance, but not so for fast hydroplanes, hydrofoils and hovercraft. The aerodynamics of a surface ship include the lateral force in addition to the resistance, which is of particular concern for ships with high superstructures, such as ferries, when docking. On the other hand, the flow of air around the funnel is a prime concern for passenger ships. The aerodynamics of the sail have many problems in common with wings. As for trains, naval architects depend upon individual publications, there being no comprehensive work on this subject. Data on the aerodynamic drag are given by Hoerner.12 Of the numerous works on the funnel air flow, those from Thieme1 10 are worthy of mention. Gould1 n