AERODYNAMIC DRAG MECHANISMS

OF BLUFF BODIES AND ROAD VEHICLES

PUBLISHED SYMPOSIA Held at the

General Motors Research Laboratories Warren, Michigan

Friction and Wear, 1959 Robert Davies, Editor

Internal Stresses and Fatigue in Metals, 1959 Gerald M. Rassweiler and William L. Grube, Editors

Theory of Traffic Flow, 1961 Robert Herman, Editor

Rolling Cont-act Phenomena, 1962 Joseph B. Bidwell, Editor

. Adhesion and Cohesion, 1962 Philip Weiss, Editor

Cavitation in Real Liquids, 1964 Robert Davies, Editor

Liquids: Structure, Properties, Solid Interactions, 1965 Thomas J. Hughel, Editor

Approximation of Functions, 1965 Henry L. Garabedian, Editor

Fluid Mechanics of Internal Flow, 1967 Gino Sovran, Editor

Ferroelectricity, 1967 Edward F. Weller, Editor

Interface Conversion for Polymer Coatings, 1968 Philip Weiss and G. Dale Cheever, Editors

Associative Information Techniques, 1971 Edwin L. J acks, Editor

Chemical Reactions in the Urban Atmosphere, 1971 Charles S. Tuesday, Editor

The Physics of Opto-Electronic Materials, 1971 Walter A. Albers, Jr., Editor

Emissions From Continuous Combustion Systems, 1972 Walter Comelius and William G. Agnew, Editors

Human Impact Response, Measurement and Simulation, 1973 William F. King and Harold J. Mertz, Editors

The Physics of Tire Traction, Theory and Experiment, 1974 Donald F. Hays and Allan L. Browne, Editors

The Catalytic Chemistry of Nitrogen Oxides, 1975 Richard ,L. Klimisch and J ohn G. Larson, Editors

Future Automotive Fuels - Prospects, Performance, Perspective, 1977 J oseph M. Colucci and Nicholas E. Gallopoulos, Editors

Aerodynamic Drag Mechanisms ofBluff Bodies and Road Vehicles, 1978 Gino Sovran, Thomas Morel and William T. Mason, J r., Editors

AERODYNAMIC DRAG MECHANISMS OF BLUFF BODIES

AND ROAD VEHICLES

Edited by GINO SOVRAN, THOMAS MOREL and WILLIAM T. MASON, Jr.

General Motors Research Laboratories

PLENUM PRESS • NEW YORK - LONDON • 1978

Library of Congress Cataloging in Publication Data

Symposium on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehic1es, General Motors Research Laboratories, 1976. Aerodynamic drag mechanisms of bluff bodies and road vehic1es.

"Proceedings of the Symposium on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles held at the General Motors Research Laboratories, Warren, Michi­gan, September 27-28, 1976."

Inc1udes bibliographical references and index. 1. Motor vehic1es-Aerodynamics-Congresses. 2. Drag (Aerodynamics)-Congresses.

1. Sovran, Gino. Il. Morel, Thomas. Ill. Mason, William T. IV. General Motors Cor­poration. Research Laboratories. V. Tide. TL245.S95 1976 629.2'31 77-16545 ISBN 978-1-4684-8436-6 ISBN 978-1-4684-8434-2 (eBook) DOI 10.1007/978-1-4684-8434-2

Proceedings of the Symposium on Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehic1es held at the General Motors Research Laboratories, Warren, Michigan,

September 27-28, 1976

© 1978 Plenum Press, New York Softcover reprint of the hardcover Ist edition 1978 A Division of Plenum Publishing Corporation 227 West 17th Street, New York, N.Y. 10011

All righ ts reserved

No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means electronic, mechanical, photocopying, microfilming,

recording, or otherwise, without written permission from the Publisher

v

PREFACE

These Proceedings contain the papers and oral discussions presented at the Symposium on AERODYNAMIC DRAG MECHANISMS of Bluff Bodies and Road Vehides held at the General Motors Research Laboratories in Warren, Michigan, on September 27 and 28, 1976. This international, invitational Symposium was the twentieth in an annual series, each one having been in a different technical discipline. The Symposia provide a forum for areas of science and technology that are of timely interest to the Research Laboratories as weIl as the technical community at large, and in which personnel of the Laboratories are actively involved. The Symposia furnish an opportunity for the exchange of ideas and current knowledge between participating research specialists from educational, industrial arid governmental institutions and serve to stimulate future research activity.

The present world-wide energy situation makes it highly desirable to reduce the force required to move road vehicles through the atmosphere. A significant amount of the total energy consumed for transportation is expended in overcoming the aerodynamic resistance to motion of these vehicles. Reductions in this aerodynamic drag can therefore have a large impact on ground transportation energy requirements.

Although aerodynamic development work on road vehides has been performed for many years, it has not been widely reported or accompanied by much basic research. Certainly, the research has not been on a scale of the comprehensive efforts in the aeronautical field. This is partly attributable to the complexity of the flows, but has been more the consequence of a less pressing need to know.

The primary objective of the Symposium was to explore the basic mechanisms by which drag is generated on bluff bodies. The configurations and flow fields of interest shared the foIlowing major characteristics: three-dimensional bodies (symmetric and

vi PREFACE

asymmetric) with flow fields containing numerous regions of quasi-two-dimensional and three-dimensional flow separation both in front as weH as in the rear, with discrete vortices in the wake, and a lift-to-drag ratio of about unity. The schematic representation, shown below, of some of the key flow features thought to exist around automobiles, excluding those of the rotating wheels and of the underbody, was drawn to visually convey these characteristics to prospective contributors of the technical program.

For some of the authors not previously involved with road-vehicle aerodynamics, and therefore working outside their field of direct experience and knowledge, this constituted a principal exposure to the type of flow field of interest and to its most significant features. To whatever degree the schematic is inaccurate or unrepresenta­tive, the responsibility is that of the Symposium Chairmen. Another purpose of the sketch was the portrayal of general geometrical characteristics relevant to road vehicles, of which the most significant are: complexity of body shape due to functional constraints, planview aspect ratio less than 0.5, length-to-diameter ratio of about 2. Ground clearance about 10-15% of body "diameter", motion relative to the ground, and an engine-compartment cavity with through flow are other distinctive features. In addition, in the case of tractor-trailer trucks, there is the problem of two-body configurations.

The program of the Symposium was structured to produce a Proceedings that constituted the present state of understanding of bluff body fluid mechanics relevant to road vehicles, defined the most critical unknown areas, and delineated promising avenues for future research. The situation that has existed, and largely still exists, is one where the major expertise in fluid mechanics resides in people having litde knowledge of, or interest in, the flow fields generated by road vehicles. As a result, those actively involved in vehicle aerodynamics have not generally had the advantage of inputs from these experts while exploring the lower limits of aerodynamic drag.

PREFACE vii

Increased communication hetween these two groups was a goal of the Symposium, and we hope that at least some success is reflected in the contents of these Proceedings.

Active and intensive discussion of the technical presentations was vigorously promoted. Also, all attendees were invited to hring prepared discussions. These were cleared hy the particular session Chairperson and the organizers for appropriateness and length. Although the discussions were recorded, verbatim transcripts were not necessarily used. As required, discussions were edited to aid brevity and clarity. Some reordering of the material was done in several discussion periods so as to effectively group all comments on a given subject. We gratefully acknowledge the assistance of a numlier of the attendees whose on-the-spot evaluations were very helpful in the discussion editing process.

A significant highlight of the symposium not recorded in the Proceedings was the banquet presentation by MR. BOBBY ALLISON. His ac count of personal experiences with the aerodynamics involved in stock-car racing was fascinating, and effectively cut through the varied background of the Symposium participants. He "told it like it is" on the track, and his descriptions of "drafting" and "sling-shotting" challenge translation to basic aerodynamic terms.

We would like to express appreciation to the Management of hoth the Fluid Dynamics Research Department and the Research Laboratories for their expression of confidence in permitting complete freedom of expression with regard to the scope and detailed structure of the technical program. We also wish to recognize the very important contributions of our two technical advisers - PROFS. MARK V. MORKOVIN and ANATOL ROSHKO. Their technical knowledge, experience, wisdom and judgment provided large and invaluable inputs to the detailed program planning and execution.

The smooth functioning of the meeting and its pleasant atmosphere were principally the work of MR. KURT T. ANTONIUS of the Technical Information Department, who handled the many details of the physical arrangements in an excellent manner. Many thanks go to several of our colleagues in the Fluid Dynamics Research Department who worked as session aides and provided valuable assistance in many different ways. Finally, the overall Symposium experience could never have been translated into this final copy without the very able and extensive efforts of MR. DA VID N. HAVELOCK and his Graphic Arts Group of the Technical Information Department, all of whom have not yet learned how to say no.

GINOSOVRAN THOMAS MOREL

WILLIAM T. MASON, IR.

CONTENTS

Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Introduction G. Sovran, General Motors Research Laboratories 1

SESSION I

Chairperson: P.B.S. Lissaman, Aero Vironment, Incorporated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The Aerodynamic Drag of Cars Current Understanding, Unresolved Problems, and Future Prospects

W.-H. Hucho, Volkswagen werk AG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Discussion ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

The Drag Related Flow Field Characteristics of Trucks and Buses

W. T. Mason, Jr. and P. S. Beebe, General Motors Research Laboratories ...... . References Discussion ............................................ .

Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies

45 89 91

P. W. Bearman, Imperial College. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Discussion ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

ix

x CONTENTS

SESSION II

Chairperson, S. E. Widnall, Massachusetts Institute of Technology . . . . . . . . . . . . . . . . . . . . . . . . . .. 129

Some General Characteristics and Properties of Three-Dimensional Flow Fields

E. C. Maskell ............................................. 131 Discussion ............................................. 131

Mechanisms of Two and Three-Dimensional Base Drag D. J. Maull, Cambridge University ............................... 137

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 153 Discussion .•.................•......................... 154

Drag-Reducing Techniques for Axi-symmetric Bluff Bodies

W. A. Mair, Cambridge University ............................... 161 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 178 Discussion .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 179

SESSION III

Chairperson: A.M.O. Smith, University of California at Los Angeles. . . . . . . . . . . . . . . . . . . . . . . . . .. 189

The Effect of Base Slant on the Flow Pattern and Drag of Three-Dimensional Bodies

T. Morel, General Motors Research Laboratories ..................... 191 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 216 Discussion .................................... . . . . . . . .. 217

Recent Japanese Research on Three-Dimensional Bluff-Body Flows Relevant to Road-Vehicle Aerodynamics

H. Nakaguchi, University of Tokyo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 227 References ............................................ , 245 Discussion ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 246

Interaction Effects on the Drag of Bluff Bodies in Tandem

A. Roshko and K. Koenig, California Institute of Technology ........... 253 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Discussion ................ . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

CONTENTS

SESSION IV

Chairperson: M. V. Morkovin, Illinois Institute of Technology .... . . . . . . . . . . . . . . . . . . . . . . . . . .. 287

Numerical Modeling of Blunt-Body Flows -Problems and Prospects

M. T. Landahl, Massachusetts Institute of Technology. . . . . . . . . . . . . . . . . .. 289 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 301 Discussion ............................................. 302

Prospects for Numerical Simulation of Bluff-Body Aerodynamics

C. W. Hirt and J. D. Ramshaw, University of California Los Alarnos Scientific Laboratory .............. 313 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 348 Discussion ............................................. 350

General Discussion and Outlook for the Future Moderator: M. V. Morkovin,

Illinois Institute of Technology

Symposium Wrap-Up G. Sovran, General Motors Research Laboratories

357

369

Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 373

Subject Index ............................................. 379

XI