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Understanding Aerodynamics: Arguing from the Real Physics

ISBN: 978-1-119-96751-4
576 pages
December 2012
Understanding Aerodynamics: Arguing from the Real Physics (1119967511) cover image
Much-needed, fresh approach that brings a greater insight into the physical understanding of aerodynamics

Based on the author’s decades of industrial experience with Boeing, this book helps students and practicing engineers to gain a greater physical understanding of aerodynamics. Relying on clear physical arguments and examples, Mclean provides a much-needed, fresh approach to this sometimes contentious subject without shying away from addressing "real" aerodynamic situations as opposed to the oversimplified ones frequently used for mathematical convenience. Motivated by the belief that engineering practice is enhanced in the long run by a robust understanding of the basics as well as real cause-and-effect relationships that lie behind the theory, he provides intuitive physical interpretations and explanations, debunking commonly-held misconceptions and misinterpretations, and building upon the contrasts provided by wrong explanations to strengthen understanding of the right ones.

  • Provides a refreshing view of aerodynamics that is based on the author’s decades of industrial experience yet is always tied to basic fundamentals.
  • Provides intuitive physical interpretations and explanations, debunking commonly-held misconceptions and misinterpretations
  • Offers new insights to some familiar topics, for example, what the Biot-Savart law really means and why it causes so much confusion, what “Reynolds number” and “incompressible flow” really mean, and a real physical explanation for how an airfoil produces lift.
  • Addresses "real" aerodynamic situations as opposed to the oversimplified ones frequently used for mathematical convenience, and omits mathematical details whenever the physical understanding can be conveyed without them.
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Foreword xi

Series Preface xiii

Preface xv

List of Symbols xix

1 Introduction to the Conceptual Landscape 1

2 From Elementary Particles to Aerodynamic Flows 5

3 Continuum Fluid Mechanics and the Navier-Stokes Equations 13

3.1 The Continuum Formulation and Its Range of Validity 13

3.2 Mathematical Formalism 16

3.3 Kinematics: Streamlines, Streaklines, Timelines, and Vorticity 18

3.3.1 Streamlines and Streaklines 18

3.3.2 Streamtubes, Stream Surfaces, and the Stream Function 19

3.3.3 Timelines 22

3.3.4 The Divergence of the Velocity and Green’s Theorem 23

3.3.5 Vorticity and Circulation 24

3.3.6 The Velocity Potential in Irrotational Flow 26

3.3.7 Concepts that Arise in Describing the Vorticity Field 26

3.3.8 Velocity Fields Associated with Concentrations of Vorticity 29

3.3.9 The Biot-Savart Law and the “Induction” Fallacy 31

3.4 The Equations of Motion and their Physical Meaning 33

3.4.1 Continuity of the Flow and Conservation of Mass 34

3.4.2 Forces on Fluid Parcels and Conservation of Momentum 35

3.4.3 Conservation of Energy 36

3.4.4 Constitutive Relations and Boundary Conditions 37

3.4.5 Mathematical Nature of the Equations 37

3.4.6 The Physics as Viewed in the Eulerian Frame 38

3.4.7 The Pseudo-Lagrangian Viewpoint 40

3.5 Cause and Effect, and the Problem of Prediction 40

3.6 The Effects of Viscosity 43

3.7 Turbulence, Reynolds Averaging, and Turbulence Modeling 48

3.8 Important Dynamical Relationships 55

3.8.1 Galilean Invariance, or Independence of Reference Frame 55

3.8.2 Circulation Preservation and the Persistence of Irrotationality 56

3.8.3 Behavior of Vortex Tubes in Inviscid and Viscous Flows 57

3.8.4 Bernoulli Equations and Stagnation Conditions 58

3.8.5 Crocco’s Theorem 60

3.9 Dynamic Similarity 60

3.9.1 Compressibility Effects and the Mach Number 63

3.9.2 Viscous Effects and the Reynolds Number 63

3.9.3 Scaling of Pressure Forces: the Dynamic Pressure 64

3.9.4 Consequences of Failing to Match All of the Requirements for Similarity 65

3.10 “Incompressible” Flow and Potential Flow 66

3.11 Compressible Flow and Shocks 70

3.11.1 Steady 1D Isentropic Flow Theory 71

3.11.2 Relations for Normal and Oblique Shock Waves 74

4 Boundary Layers 79

4.1 Physical Aspects of Boundary-Layer Flows 80

4.1.1 The Basic Sequence: Attachment, Transition, Separation 80

4.1.2 General Development of the Boundary-Layer Flowfield 82

4.1.3 Boundary-Layer Displacement Effect 90

4.1.4 Separation from a Smooth Wall 93

4.2 Boundary-Layer Theory 99

4.2.1 The Boundary-Layer Equations 100

4.2.2 Integrated Momentum Balance in a Boundary Layer 108

4.2.3 The Displacement Effect and Matching with the Outer Flow 110

4.2.4 The Vorticity “Budget” in a 2D Incompressible Boundary Layer 113

4.2.5 Situations That Violate the Assumptions of Boundary-Layer Theory 114

4.2.6 Summary of Lessons from Boundary-Layer Theory 117

4.3 Flat-Plate Boundary Layers and Other Simplified Cases 117

4.3.1 Flat-Plate Flow 117

4.3.2 2D Boundary-Layer Flows with Similarity 121

4.3.3 Axisymmetric Flow 123

4.3.4 Plane-of-Symmetry and Attachment-Line Boundary Layers 125

4.3.5 Simplifying the Effects of Sweep and Taper in 3D 128

4.4 Transition and Turbulence 130

4.4.1 Boundary-Layer Transition 131

4.4.2 Turbulent Boundary Layers 138

4.5 Control and Prevention of Flow Separation 150

4.5.1 Body Shaping and Pressure Distribution 150

4.5.2 Vortex Generators 150

4.5.3 Steady Tangential Blowing through a Slot 155

4.5.4 Active Unsteady Blowing 157

4.5.5 Suction 157

4.6 Heat Transfer and Compressibility 158

4.6.1 Heat Transfer, Compressibility, and the Boundary-Layer Temperature Field 158

4.6.2 The Thermal Energy Equation and the Prandtl Number 159

4.6.3 The Wall Temperature and Other Relations for an Adiabatic Wall 159

4.7 Effects of Surface Roughness 162

5 General Features of Flows around Bodies 163

5.1 The Obstacle Effect 164

5.2 Basic Topology of Flow Attachment and Separation 168

5.2.1 Attachment and Separation in 2D 169

5.2.2 Attachment and Separation in 3D 171

5.2.3 Streamline Topology on Surfaces and in Cross Sections 176

5.3 Wakes 186

5.4 Integrated Forces: Lift and Drag 189

6 Drag and Propulsion 191

6.1 Basic Physics and Flowfield Manifestations of Drag and Thrust 192

6.1.1 Basic Physical Effects of Viscosity 193

6.1.2 The Role of Turbulence 193

6.1.3 Direct and Indirect Contributions to the Drag Force on the Body 194

6.1.4 Determining Drag from the Flowfield: Application of Conservation Laws 196

6.1.5 Examples of Flowfield Manifestations of Drag in Simple 2D Flows 204

6.1.6 Pressure Drag of Streamlined and Bluff Bodies 207

6.1.7 Questionable Drag Categories: Parasite Drag, Base Drag, and Slot Drag 210

6.1.8 Effects of Distributed Surface Roughness on Turbulent Skin Friction 212

6.1.9 Interference Drag 222

6.1.10 Some Basic Physics of Propulsion 225

6.2 Drag Estimation 241

6.2.1 Empirical Correlations 242

6.2.2 Effects of Surface Roughness on Turbulent Skin Friction 243

6.2.3 CFD Prediction of Drag 250

6.3 Drag Reduction 250

6.3.1 Reducing Drag by Maintaining a Run of Laminar Flow 251

6.3.2 Reduction of Turbulent Skin Friction 251

7 Lift and Airfoils in 2D at Subsonic Speeds 259

7.1 Mathematical Prediction of Lift in 2D 260

7.2 Lift in Terms of Circulation and Bound Vorticity 265

7.2.1 The Classical Argument for the Origin of the Bound Vorticity 267

7.3 Physical Explanations of Lift in 2D 269

7.3.1 Past Explanations and their Strengths and Weaknesses 269

7.3.2 Desired Attributes of a More Satisfactory Explanation 284

7.3.3 A Basic Explanation of Lift on an Airfoil, Accessible to a Nontechnical Audience 286

7.3.4 More Physical Details on Lift in 2D, for the Technically Inclined 302

7.4 Airfoils 307

7.4.1 Pressure Distributions and Integrated Forces at Low Mach Numbers 307

7.4.2 Profile Drag and the Drag Polar 316

7.4.3 Maximum Lift and Boundary-Layer Separation on Single-Element Airfoils 319

7.4.4 Multielement Airfoils and the Slot Effect 329

7.4.5 Cascades 335

7.4.6 Low-Drag Airfoils with Laminar Flow 338

7.4.7 Low-Reynolds-Number Airfoils 341

7.4.8 Airfoils in Transonic Flow 342

7.4.9 Airfoils in Ground Effect 350

7.4.10 Airfoil Design 352

7.4.11 Issues that Arise in Defining Airfoil Shapes 354

8 Lift and Wings in 3D at Subsonic Speeds 359

8.1 The Flowfield around a 3D Wing 359

8.1.1 General Characteristics of the Velocity Field 359

8.1.2 The Vortex Wake 362

8.1.3 The Pressure Field around a 3D Wing 371

8.1.4 Explanations for the Flowfield 371

8.1.5 Vortex Shedding from Edges Other Than the Trailing Edge 375

8.2 Distribution of Lift on a 3D Wing 376

8.2.1 Basic and Additional Spanloads 376

8.2.2 Linearized Lifting-Surface Theory 379

8.2.3 Lifting-Line Theory 380

8.2.4 3D Lift in Ground Effect 382

8.2.5 Maximum Lift, as Limited by 3D Effects 384

8.3 Induced Drag 385

8.3.1 Basic Scaling of Induced Drag 385

8.3.2 Induced Drag from a Farfield Momentum Balance 386

8.3.3 Induced Drag in Terms of Kinetic Energy and an Idealized Rolled-Up Vortex Wake 389

8.3.4 Induced Drag from the Loading on the Wing Itself: Trefftz-Plane Theory 391

8.3.5 Ideal (Minimum) Induced-Drag Theory 394

8.3.6 Span-Efficiency Factors 396

8.3.7 The Induced-Drag Polar 397

8.3.8 The Sin-Series Spanloads 398

8.3.9 The Reduction of Induced Drag in Ground Effect 401

8.3.10 The Effect of a Fuselage on Induced Drag 402

8.3.11 Effects of a Canard or Aft Tail on Induced Drag 404

8.3.12 Biplane Drag 409

8.4 Wingtip Devices 411

8.4.1 Myths Regarding the Vortex Wake, and Some Questionable Ideas for Wingtip Devices 411

8.4.2 The Facts of Life Regarding Induced Drag and Induced-Drag Reduction 414

8.4.3 Milestones in the Development of Theory and Practice 420

8.4.4 Wingtip Device Concepts 422

8.4.5 Effectiveness of Various Device Configurations 423

8.5 Manifestations of Lift in the Atmosphere at Large 427

8.5.1 The Net Vertical Momentum Imparted to the Atmosphere 427

8.5.2 The Pressure Far above and below the Airplane 429

8.5.3 Downwash in the Trefftz Plane and Other Momentum-Conservation Issues 431

8.5.4 Sears’s Incorrect Analysis of the Integrated Pressure Far Downstream 435

8.5.5 The Real Flowfield Far Downstream of the Airplane 436

8.6 Effects of Wing Sweep 444

8.6.1 Simple Sweep Theory 444

8.6.2 Boundary Layers on Swept Wings 449

8.6.3 Shock/Boundary-Layer Interaction on Swept Wings 464

8.6.4 Laminar-to-Turbulent Transition on Swept Wings 465

8.6.5 Relating a Swept, Tapered Wing to a 2D Airfoil 468

8.6.6 Tailoring of the Inboard Part of a Swept Wing 469

9 Theoretical Idealizations Revisited 471

9.1 Approximations Grouped According to how the Equations were Modified 471

9.1.1 Reduced Temporal and/or Spatial Resolution 472

9.1.2 Simplified Theories Based on Neglecting Something Small 472

9.1.3 Reductions in Dimensions 472

9.1.4 Simplified Theories Based on Ad hoc Flow Models 472

9.1.5 Qualitative Anomalies and Other Consequences of Approximations 481

9.2 Some Tools of MFD (Mental Fluid Dynamics) 482

9.2.1 Simple Conceptual Models for Thinking about Velocity Fields 482

9.2.2 Thinking about Viscous and Shock Drag 485

9.2.3 Thinking about Induced Drag 486

9.2.4 A Catalog of Fallacies 487

10 Modeling Aerodynamic Flows in Computational Fluid Dynamics 491

10.1 Basic Definitions 493

10.2 The Major Classes of CFD Codes and Their Applications 493

10.2.1 Navier-Stokes Methods 493

10.2.2 Coupled Viscous/Inviscid Methods 497

10.2.3 Inviscid Methods 498

10.2.4 Standalone Boundary-Layer Codes 501

10.3 Basic Characteristics of Numerical Solution Schemes 501

10.3.1 Discretization 501

10.3.2 Spatial Field Grids 502

10.3.3 Grid Resolution and Grid Convergence 506

10.3.4 Solving the Equations, and Iterative Convergence 507

10.4 Physical Modeling in CFD 508

10.4.1 Compressibility and Shocks 508

10.4.2 Viscous Effects and Turbulence 510

10.4.3 Separated Shear Layers and Vortex Wakes 511

10.4.4 The Farfield 513

10.4.5 Predicting Drag 514

10.4.6 Propulsion Effects 515

10.5 CFD Validation? 515

10.6 Integrated Forces and the Components of Drag 516

10.7 Solution Visualization 517

10.8 Things a User Should Know about a CFD Code before Running it 524

References 527

Index 539

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“As someone who has been involved with aerodynamics for more years than I care to remember, I have rarely come across a book that is so readable and that provides so many (to me a least) genuinely new insights into the subject and its applications.   This book should be high on the wish list of any practising aerodynamicist, whether in industry or academia.”  (Aeronautical Journal, 1 August 2013)

“This is a sophisticated book for people immersed in the study of fluid dynamics and aerodynamics; it will give them in-depth knowledge of both the physical phenomena and the mathematical equations that are used to describe and predict these phenomena. Summing Up: Recommended. Graduate students in aerospace engineering, researchers/faculty, and aircraft design professionals.”  (Choice, 1 July 2013)

“Based on the author’s decades of industrial experience with Boeing, this book helps students and practicing engineers to gain a greater physical understanding of aerodynamics. Relying on clear physical arguments and examples, Mcleanprovides a much-needed, fresh approach to this sometimes contentious subject without shying away from addressing "real" aerodynamic situations as opposed to the oversimplified ones frequently used for mathematical convenience.”  (Expofairs.com, 11 March 2013)

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