Flow Control: Passive, Active, and Reactive

The ability to actively or passively manipulate a flow field to bring about a desired change is of immense technological and economical importance. This volume provides a thorough, up-to-date treatment of the basics of flow control and control practices that can be used to produce desired effects. The author explores the frontiers of flow control strategies, especially as applied to turbulent flows. Intended for engineering and physics students, researchers, and practitioners, Flow Control brings together in a single source a wealth of state-of-the-art information on this very active field.

Mohamed Gad-el-Hak is a Professor of Aerospace and Mechanical Engineering at the University of Notre Dame.

Preface

xiii

Nomenclature

xv

Introduction

1

(9)

Prologue

1

(1)

What Is Flow Control?

1

(2)

Five Eras of Flow Control

3

(1)

Has the Field Crested?

4

(6)

Governing Equations

10

(15)

Prologue

10

(1)

The Fundamental Equations

10

(4)

Conservation of Mass

11

(1)

Newton's Second Law

12

(1)

Conservation of Energy

13

(1)

Closing the Equations

14

(3)

Compressibility

17

(3)

Equations of Motion at the Wall

20

(1)

Turbulent Flows

21

(1)

The Karman Integral Equation

22

(3)

Unifying Principles

25

(9)

Prologue

25

(1)

Control Goals and Their Interrelation

25

(2)

Classification Schemes

27

(3)

Free-Shear and Wall-Bounded Flows

30

(1)

Regimes of Reynolds and Mach Numbers

31

(2)

Convective and Absolute Instabilities

33

(1)

Coherent Structures

34

(24)

Prologue

35

(1)

The Changing Paradigms of Turbulence

35

(7)

Visualization

36

(2)

First Principles

38

(1)

Statistical View of Turbulence

38

(1)

Reemergence of Coherent Structures

39

(1)

The Latest Tools

40

(2)

What Is a Coherent Structure?

42

(1)

Free-Shear Flows

42

(2)

Wall-Bounded Flows

44

(14)

Overview

46

(7)

Open Issues

53

(5)

Reynolds Number Effects

58

(46)

Prologue

57

(2)

The Last Conundrum

59

(2)

The Issues

61

(2)

Field versus Laboratory Flows

61

(1)

Reynolds Number

62

(1)

Flow Regimes

63

(6)

Viscous Region

64

(2)

Constant-Reynolds-Stress Region

66

(2)

Outer Layer

68

(1)

Comparison with Other Shear Flows

69

(4)

Mean Flow

73

(8)

The Rise and Fall of the Log-Law

74

(3)

Alternatives to the Logarithmic Profile

77

(2)

The Illusory Asymptotic State

79

(1)

Is Self-Preservation Ever Achieved?

80

(1)

Higher-Order Statistics

81

(9)

Root-Mean-Square Velocity Fluctuations

82

(3)

Reynolds Stress

85

(5)

Coherent Structures

90

(7)

Bursting Period

90

(2)

High Reynolds Number

92

(2)

Small Structures in Outer Layer

94

(1)

Inner Structures

94

(3)

Flow Control

97

(4)

Introductory Remarks

97

(1)

Riblets

97

(1)

Recovery Response

98

(3)

Control of High-Reynolds-Number Flows

101

(1)

Summary of Reynolds Number Effects

101

(3)

Transition Control

104

(16)

Prologue

104

(1)

Free-Shear versus Wall-Bounded Flows

104

(1)

Routes to Transition

105

(3)

Linear Stability Theory

108

(2)

Critical Re and Transition

109

(1)

Stability Modifiers

110

(8)

Wall Motion

111

(1)

Suction

112

(2)

Shaping

114

(2)

Wall Heating and Cooling

116

(2)

Wave Cancellation

118

(2)

Compliant Coatings

120

(30)

Prologue

120

(1)

Introduction

120

(2)

Compliant Coatings prior to 1985

122

(3)

Free-Surface Waves

125

(2)

System Instabilities

127

(2)

The Kramer Controversy

129

(2)

Transitional Flows

131

(10)

Linear Stability Theory

131

(2)

Coating Optimization

133

(6)

Practical Examples

139

(1)

The Dolphin's Secret

140

(1)

Turbulent Wall-Bounded Flows

141

(4)

The Future

145

(3)

Parting Remarks

148

(2)

Separation Control

150

(39)

Prologue

150

(1)

Introduction

150

(3)

The Phenomenon of Separation

150

(1)

Separation Control

151

(1)

Method of Control

152

(1)

Outline

153

(1)

Steady and Unsteady Separation

153

(8)

Steady Separation

153

(1)

Unsteady Separation

154

(4)

Vortex-Induced Separation

158

(3)

Equations of Motion at the Wall

161

(2)

Equation for Turbulent Flows

161

(2)

Velocity Profile Modifiers

163

(5)

Shaping

163

(2)

Transpiration

165

(3)

Wall Heat Transfer

168

(1)

Moving-Wall and Time-Dependent Separations

168

(4)

Moving Walls

168

(2)

Time-Dependent Separation

170

(2)

Three-Dimensional Separation

172

(1)

Turbulators

172

(2)

Momentum Addition to Near-Wall Flow

174

(9)

Introductory Remarks

174

(2)

Passive Vortex Generators

176

(1)

Passive Turbulence Amplification

177

(1)

Active Wall-Region Momentum Addition

178

(5)

Separation Provocation

183

(2)

Delta Wings

183

(2)

Parting Remarks

185

(4)

Recapitulation

185

(1)

Computational Fluid Dynamics

186

(1)

Applications and Comparisons

187

(1)

Recommendations

188

(1)

Low-Reynolds-Number Aerodynamics

189

(16)

Prologue

189

(1)

Introduction

189

(2)

Low-Reynolds-Number Airfoils

191

(1)

Conditions for Bubble Formation

192

(2)

Bubble's Breakdown

194

(1)

Control Goals and Their Interrelation

195

(2)

Separation Control

197

(1)

Transition Control via Shaping

198

(4)

Turbulators

202

(1)

Parting Remarks

203

(2)

Drag Reduction

205

(24)

Prologue

205

(1)

Background

206

(3)

Drag Reduction

209

(1)

Skin-Friction Reduction

210

(2)

Reduction of Cf in Turbulent Flows

212

(11)

Reduction of Near-Wall Momentum

212

(2)

Introduction of Foreign Substance

214

(4)

Methods Involving Geometry

218

(4)

Recent Innovations

222

(1)

Relaminarization

223

(2)

Synergism

225

(3)

Parting Remarks

228

(1)

Mixing Enhancement

229

(21)

Prologue

229

(1)

Introduction

230

(1)

Mixing

231

(4)

Kinematical Description

233

(2)

Enhancement Strategies

235

(1)

Early Transition

235

(3)

Wall-Bounded Flows

236

(1)

Free-Shear Flows

237

(1)

Turbulence Enhancement

238

(2)

Turbulent Wall-Bounded Flows

239

(1)

Turbulent Free-Shear Flows

239

(1)

Heat Transfer

240

(4)

Chaotic Mixing

244

(6)

Noise Reduction

250

(19)

Prologue

250

(1)

Introduction

250

(2)

Fundamentals of Acoustics

252

(3)

Sources of Sound

255

(4)

Noise Control

259

(10)

Relation to Other Flow-Control Goals

259

(2)

Passive Noise Control

261

(2)

Compliant Coatings

263

(1)

Jet Noise

264

(2)

Turbomachinery Blades

266

(1)

Antisound

267

(2)

Microelectromechanical Systems

269

(49)

Prologue

269

(1)

Introduction

269

(3)

Flow Physics

272

(36)

Fluid Mechanics Issues

272

(1)

Fluid Modeling

273

(4)

Continuum Model

277

(3)

Compressibility

280

(2)

Boundary Conditions

282

(7)

Molecular-Based Models

289

(10)

Liquid Flows

299

(4)

Surface Phenomena

303

(5)

Typical Fluid Applications

308

(7)

Introductory Remarks

308

(1)

Turbulence Measurements

308

(2)

Micropumps

310

(2)

Microturbines

312

(1)

Microbearings

313

(2)

Parting Remarks

315

(3)

Frontiers of Flow Control

318

(41)

Prologue

318

(1)

Introduction

318

(2)

The Taming of the Shrew

318

(1)

Control of Turbulence

319

(1)

Outline

320

(1)

Suction

320

(2)

Coherent Structures

322

(2)

Scales

324

(5)

Sensor Requirements

327

(2)

Reactive Control

329

(9)

Introductory Remarks

329

(1)

Targeted Control

330

(4)

Reactive Feedback Control

334

(2)

Required Characteristics

336

(2)

Magnetohydrodynamic Control

338

(5)

Introductory Remarks

338

(1)

The von Karman Equation

339

(1)

Vorticity Flux at the Wall

340

(1)

EMHD Tiles for Reactive Control

341

(2)

Chaos Control

343

(6)

Nonlinear Dynamical Systems Theory

343

(2)

Chaos Control

345

(4)

Microfabrication

349

(3)

Soft Computing

352

(5)

Neural Networks for Flow Control

355

(2)

Parting Remarks

357

(2)

Epilogue

359

(4)

Bibliography

363

(54)

Index

417

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Flow Control: Passive, Active, and Reactive Flow Management

Summary

Author Biography

Table of Contents

Supplemental Materials