cranfield university obinna hyacinth ehirim aerodynamics and performance enhancement of a ...

326 Pages·2017·22.14 MB·English

CRANFIELD UNIVERSITY

OBINNA HYACINTH EHIRIM

AERODYNAMICS AND PERFORMANCE ENHANCEMENT OF A
GROUND-EFFECT DIFFUSER

CRANFIELD DEFENCE AND SECURITY
Centre for Defence Engineering
Aeromechanical Systems Group

PhD Thesis

Supervisors:
Professor Kevin Knowles
Dr. Alistair J. Saddington

October 2017
CRANFIELD UNIVERSITY

CRANFIELD DEFENCE AND SECURITY
Centre for Defence Engineering
Aeromechanical Systems Group

PhD Thesis

Academic Year 2017 - 2018

Obinna Hyacinth Ehirim

AERODYNAMICS AND PERFORMANCE ENHANCEMENT OF A
GROUND-EFFECT DIFFUSER

Supervisors: Professor Kevin Knowles and Dr. Alistair J. Saddington

© Cranfield University 2017. All rights reserved. No part of this
publication may be reproduced without the written permission of the
copyright owner.
ABSTRACT

This study involved experimental and equivalent computational
investigations into the automobile-type 3―D flow physics of a diffuser bluff
body in ground-effect and novel passive flow-control methods applied to the
diffuser flow to enhance the diffuser’s aerodynamic performance. The bluff
body used in this study is an Ahmed-like body employed in an inverted
position with the slanted section together with the addition of side plates
along both sides forming the ramped diffuser section.
The first part of the study confirmed reported observations from
previous studies that the downforce generated by the diffuser in proximity
to a ground plane is influenced by the peak suction at the diffuser inlet and
subsequent static pressure-recovery towards the diffuser exit. Also, when
the bluff body ride height is gradually reduced from high to low, the diffuser
flow as indicated by its force curve and surface flow features undergoes four
distinct flow regimes (types A to D). The types A and B regimes are
reasonably symmetrical, made up of two low-pressure core longitudinal
vortices travelling along both sides of the diffuser length and they increase
downforce and drag with reducing ride height. However, below the ride
heights of the type B regime, types C and D regimes are asymmetrical
because of the breakdown of one vortex; consequently a significant loss in
downforce and drag occurs.
The second part of the study involved the use ― near the diffuser exit ―
of a convex bump on the diffuser ramp surface and an inverted wing
between the diffuser side plates as passive flow control devices. The
modification of the diffuser geometry with these devices employed
individually or in combination, induced a second-stage pressure-drop and
recovery near the diffuser exit. This behaviour was due to the radial
pressure gradient induced on the diffuser flow by the suction surface
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curvature of the passive devices. As a result of this aerodynamic
phenomenon, the diffuser generated across the flow regimes additional
downforce, and a marginal increase in drag due to the profile drag induced
by the devices.

Keywords: ground-effect aerodynamics, diffuser, passive flow control,
inverted wing, convex bump, downforce, drag

ii
To my parents
for always supporting me

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ACKNOWLEDGEMENTS

Firstly, I would like to thank my supervisors, Professor Kevin
Knowles and Dr. Alistair Saddington for the guidance and support I
received over the course of this research work. Due to their supervision, I
have gained an in-depth insight of the subject matter.
I would also like to acknowledge the financial support to undertake
this research project from the Niger Delta Development Commission
(NDDC) of Nigeria and from Cranfield University. Without such monetary
assistance, it would have been impossible for me to embark on this PhD
research work.
Thanks also goes to Mike, Brian and Karl at the workshop who
helped me build the test model for the wind tunnel experiments. Likewise, I
appreciate the assistance of Dr. Mark Finnis and Professor Nicholas Lawson
with the use of the wind tunnel and the laser Doppler velocimetry
equipment respectively.
I acknowledge the cordiality all through my time working on this
research from fellow PhD students’ ― Joao, Rahim, Luke and David. I would
also like to thank Rachel, Iain, Lauren, and other staff members of the
Cranfield Barrington Library for providing a friendly atmosphere during my
part-time hours working at the library.
Finally, I am grateful for the support and encouragement I received
from my family and friends here in the United Kingdom, the United States,
and Nigeria; particularly from my parents in Nigeria, and my buddy ― the
late Chinweike Okegbe, PhD.
All thanks to God.

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TABLE OF CONTENTS

Abstract................................................................................................................................................ i
Acknowledgements ........................................................................................................................ iv
List of Figures .................................................................................................................................. ix
List of Tables ................................................................................................................................ xxiv
Nomenclature……………………………………………………………………………………………………………........xxv

Thesis Overview ........................................................................................................................ xxviii
1 Introduction ................................................................................................................................ 1
1.1 Research Background ..................................................................................................... 1
1.1.1 Ground-effect aerodynamics .................................................................................. 1
1.1.2 Internal flow diffusers .............................................................................................. 2
1.1.3 The ground-effect diffuser ...................................................................................... 4
1.2 The Ground-effect Diffuser and Downforce Production ............................................. 7
1.3 Aerodynamic Performance Definition of the Ground-effect Diffuser ...................... 10
2 Literature Review .................................................................................................................... 15
2.1 Experimental Studies ..................................................................................................... 16
2.1.1 Longitudinal vortices ............................................................................................. 16
2.1.2 Diffuser length and area ratio ............................................................................... 20
2.1.3 Diffuser ride height ............................................................................................... 23
2.1.4 Diffuser angle........................................................................................................ 26
2.2 Computational Studies ................................................................................................... 30
2.2.1 Steady-state numerical simulations ...................................................................... 30
2.2.2 Transient numerical simulations ........................................................................... 32
2.3 Ground-effect Diffuser Performance Enhancement .................................................. 34
2.3.1 Active and passive flow control ............................................................................ 36
2.3.2 Coandă effect ....................................................................................................... 41
2.3.3 Flow-curvature effect ............................................................................................ 43
2.4 Aims and Objectives of the Current Work................................................................... 44
3 Research Methods and Set-up ............................................................................................ 47
3.1 Experimental Set-up ....................................................................................................... 47
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3.1.1 Wind tunnel facility and test operating conditions ................................................ 47
3.1.2 Wind tunnel test models ....................................................................................... 49
3.1.3 Test model installation and ride height positioning .............................................. 54
3.2 Experimental Techniques .............................................................................................. 55
3.2.1 Force measurements ............................................................................................ 55
3.2.2 Surface pressure measurements ......................................................................... 56
3.2.3 Surface flow visualisation ..................................................................................... 58
3.2.4 Laser Doppler velocimetry measurements ........................................................... 60
3.3 Computational Set-up and Techniques ....................................................................... 65
3.3.1 Computational domain and grid............................................................................ 65
3.3.2 Boundary conditions ............................................................................................. 69
3.3.3 Numerical methods ............................................................................................... 69
3.3.4 Governing equations ............................................................................................ 71
3.3.5 Simulation procedure ............................................................................................ 77
4 Plane Diffuser (Baseline): Forces, Pressures and Flow Characteristics .............. 84
4.1 Diffuser Flow Behaviour and Flow Regime Types .................................................... 84
4.1.1 Force enhancement (flow regime A) .................................................................... 90
4.1.2 Maximum force (flow regime B) ............................................................................ 92
4.1.3 Force reduction (flow regime C) .........................................................................100
4.1.4 Low force (flow regime D) ...................................................................................101
4.2 Further Measurements in Freestream .......................................................................102
4.2.1 Fixed and moving ground conditions ..................................................................103
4.2.2 Flow hysteresis ...................................................................................................105
4.2.3 Reynolds number sensitivity ...............................................................................106
4.2.4 Pitching moment and pitch angle sensitivity .......................................................109
4.2.5 Effects of transition fixing ....................................................................................113
4.2.6 Base surface pressures ......................................................................................115
4.3 Baseline Downforce and Drag (CFD vs. Experiments) ..........................................118
4.4 Surface Pressure (CFD vs. Experiment) ..................................................................121
4.5 Surface Flow Features (CFD vs. Experiment) .........................................................129
4.6 Further Discussion .......................................................................................................137
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4.7 Summary ........................................................................................................................140
5 Plane Diffuser (Baseline): Edge Vortex Behaviour and Velocity Profiles .............141
5.1 Diffuser Edge Vortex Development ...........................................................................141
5.1.1 Vortex behaviour in flow regime A ......................................................................147
5.1.2 Vortex behaviour in flow regime B ......................................................................155
5.1.3 Vortex behaviour in flow regime C .....................................................................160
5.1.4 Vortex behaviour in flow regime D .....................................................................164
5.2 Velocity Profile Development ......................................................................................168
5.2.1 Underbody boundary layer profiles ....................................................................172
5.2.2 Velocity profiles of diffuser near-wake region .....................................................176
5.2.3 Flow structures of diffuser near-wake region .....................................................184
5.3 Further Discussion .......................................................................................................188
5.4 Summary ........................................................................................................................190
6 Flow Physics of Diffuser with Passive Flow Control ..................................................192
6.1 Flow Control Methods: Introduction ...........................................................................192
6.1.1 Convex bump......................................................................................................193
6.1.2 Inverted wing in diffuser ......................................................................................198
6.1.3 Combination of convex bump and inverted wing................................................203
6.2 Flow Characteristics: Baseline and Passive Flow Control Methods .....................205
6.2.1 Diffuser body downforce and drag measurements ............................................206
6.2.2 Reynolds number sensitivity ...............................................................................210
6.2.3 Aerodynamic efficiency (lift-to-drag ratio) ...........................................................212
6.2.4 Underbody and diffuser surface pressure measurements .................................214
6.2.5 Diffuser ramp surface flow features ....................................................................220
6.3 Flow Characteristics: CFD vs. Experiments .............................................................227
6.3.1 Force measurements (downforce and drag) ......................................................227
6.3.2 Surface pressure measurements .......................................................................229
6.3.3 Surface flow features ..........................................................................................237
6.3.4 Skin friction drag .................................................................................................244
6.4 Velocity Profile Measurements ...................................................................................250
6.4.1 Flow structures of diffuser near-wake region .....................................................250
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6.4.2 Velocity profiles of diffuser near-wake region .....................................................251
6.4.3 Circulation in diffuser near-wake region .............................................................257
6.5 Further Discussion .......................................................................................................259
6.6 Summary ........................................................................................................................261
7 Conclusions and Future Work Recommendations......................................................263
7.1 Conclusions ...................................................................................................................263
7.2 Recommendations for Future Work ...........................................................................268
References .....................................................................................................................................271
Appendices ....................................................................................................................................281
Appendix A ― Measurements Uncertainty and Repeatability Analysis .........................281
Appendix B ― Wind Tunnel Model Geometry ...................................................................286
Appendix C ― Experimental and Numerical Test Conditions .........................................289
Appendix D ― Experimental Downforce and Drag Measurements ................................293
Appendix E ― Publications Prepared from this Thesis ....................................................294

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