Coherent Flow Structures at Earth's Surface

**An expert review of recent progress in the study of turbulent flows with a focus on recently identified organized structures.**This book reviews the recent progress in the study of the turbulent flows that sculpt the Earth’s surface, focusing in particular on the organized structures that have been identified in recent years within turbulent flows. These coherent flow structures can include eddies or vortices at the scale of individual grains, through structures that scale with the flow depth in rivers or estuaries, to the large-scale structure of flows at the morphological or landform scale. These flow structures are of wide interest to the scientific community because they play an important role in fluid dynamics and influence the transport, erosion and deposition of sediment and pollutants in a wide variety of fluid flow environments. Scientific knowledge of these structures has improved greatly over the past 20 years as computational fluid dynamics has come to play an increasing important part in building our understanding of coherent flow structures across a broad range of scales. Chapters comprise a series of major, invited papers and a selection of the most novel, innovative papers presented at the second Coherent Flow Structures Conference held August 3-5, 2011 at Simon Fraser University in Burnaby, British Columbia. Chapters focus on six major themes: * Dynamics of coherent flow structures (CFS) in geophysical flows * Interaction of turbulent flows, vegetation and ecological habitats * Coherent structure of atmospheric flows * Numerical modeling of coherent flow structures * Turbulence in open channel flows * Coherent flow structures, sediment transport and morphological feedbacks. Coherent Flow Structures at Earth’s Surface......Page 3 Contents......Page 7 List of Contributors......Page 9 Preface......Page 13 About the Companion Website......Page 15 1.1 Introduction......Page 17 1.2 From random turbulence to coherent flow structures......Page 18 1.3 Coherent flow structures in low Reynolds-number flows over smooth boundaries......Page 20 1.4 Large-scale, high Reynolds-number coherent flow structures......Page 23 1.5 Does scale matter?......Page 26 1.6 What is the difference between the mean flow and CFS?......Page 27 1.7 Coherent flow structures within geophysical flows: future research needs......Page 28 References......Page 30 2.1 Introduction......Page 33 2.2.2 Hairpins and hairpin vortex packets......Page 34 2.3 Interactions of eddies on different scales......Page 37 2.5 Conclusions......Page 38 References......Page 39 3.1 Introduction......Page 41 3.2 Experiments......Page 43 3.3.1 Instantaneous velocity fields......Page 46 3.3.2 Single-point statistics......Page 48 3.3.3 Two-point velocity correlations......Page 50 3.5 Conclusions......Page 55 References......Page 56 4 Coherent Flow Structures in the Pore Spaces of Permeable Beds underlying a Unidirectional Turbulent Boundary Layer: A Review and some New Experimental Results......Page 59 4.2 Flow across a permeable boundary layer: background......Page 60 4.3.2 Turbulence structure in the freeflow......Page 62 4.4.1 Conceptual models......Page 65 4.4.2 Experimental observations of coherent flow structures in the transition layer and their evolution......Page 66 4.4.3 Turbulence penetration into the bed: preliminary quantification using a novel refractive index matching (RIM) experimental approach......Page 71 4.6 Summary and challenges for future work......Page 74 Notation......Page 75 References......Page 76 5.1 Introduction to Kelvin–Helmholtz and Holmboe instabilities......Page 79 5.2 One-sidedness......Page 81 5.3 Application of the Taylor–Goldstein equation to asymmetric profiles......Page 82 5.4 Mixing......Page 83 5.5 Field observations......Page 85 References......Page 86 6.1 Introduction......Page 89 6.3 Scope......Page 91 6.4.1 The Turbulent flux budget......Page 92 6.4.2 Gradient-diffusion closure for the triple moment......Page 95 6.4.3 Linking the triple moment to the ejection-sweep cycle......Page 96 6.4.4 Scalar variance......Page 97 6.4.5 Scalar spectra......Page 100 6.4.6 Scalar co-spectra......Page 102 6.4.7 Scalar fluxes, fine-scale turbulence and ramp patterns......Page 104 6.6 Acknowledgements......Page 107 References......Page 108 7.1 Introduction......Page 113 7.2.1 SLTEST experimental setup......Page 115 7.2.2 Assessing the thermal stability regime......Page 116 7.3.1 Experimental setup......Page 117 7.4 Results......Page 118 7.4.1 A qualitative picture of large scale structures in the wall region......Page 119 7.4.3 A quantitative picture of ramplike structures......Page 120 References......Page 123 8 Critical Reflections on the Coherent Flow Structures Paradigm in Aeolian Geomorphology......Page 127 8.1 Introduction......Page 128 8.2.1 Topographically unconstrained......Page 129 8.2.2 Topographically forced......Page 130 8.3.1 Wind unsteadiness and quadrant signatures in aeolian transport......Page 131 8.3.2 Aeolian streamers as manifestations of coherent flow structures......Page 132 8.3.3 Flow structures around small obstructions......Page 135 8.4.1 Macro-scale structures......Page 138 8.4.2 Signatures of turbulence in flow over dunes......Page 141 8.5 Discussion......Page 142 8.6 Summary and conclusions......Page 145 References......Page 146 9.1.1 Canopy geometry......Page 151 9.2.1 Stem-scale coherent structures......Page 152 9.2.2 Coherent structures at a single canopy edge......Page 153 9.2.3 Canopies with two flow-parallel edges......Page 155 9.2.4 Circular patch......Page 157 References......Page 161 10.1 Introduction......Page 165 10.2.1 Field observations......Page 166 10.2.2 Large-eddy simulation......Page 167 10.3 Buoyancy forcing......Page 169 10.3.1 Similarity of momentum and scalar transport......Page 171 10.3.2 Influence of boundary-layer scale circulations......Page 172 10.4 Summary and conclusions......Page 174 References......Page 175 11.1 Introduction......Page 177 11.2.1 Wind tunnel......Page 178 11.2.3 Stereoscopic PIV setup......Page 179 11.3.1 Analysis of instantaneous velocity fields......Page 180 11.3.2 One-point statistics......Page 181 11.3.3 Two-point statistics......Page 182 11.3.4 Length scales of turbulent structures......Page 183 11.3.5 Qualitative evidence of very-large scale structures......Page 185 11.3.6 Conditional averaged velocity fields......Page 186 11.4 Discussion and conclusion......Page 187 References......Page 189 12.1 Introduction......Page 191 12.2 Method of LES......Page 194 12.3.1 Flow over smooth, rough and permeable beds......Page 195 12.3.2 Flow through vegetation......Page 200 12.3.3 Flow over dunes......Page 205 12.4 Conclusions......Page 210 References......Page 211 13.1 Introduction......Page 215 13.2.2 Flow measurement......Page 217 13.3 Methodology......Page 219 13.3.1 Velocity measurements......Page 220 13.4 Results......Page 221 13.4.2 The flow contained within each structure......Page 224 13.4.3 Linking trajectories of flow to flow structures......Page 225 13.5 Discussion......Page 227 References......Page 228 14.1.1 The COHSTREX project......Page 231 14.1.2 The cool skin effect and infrared remote sensing......Page 232 14.2 Stratified flow experiment......Page 233 14.2.1 Infrared signatures of thermal variability......Page 234 14.2.2 Acoustic imaging of boils......Page 235 14.2.3 Model for vertical boil propagation......Page 237 14.3 Unstratified flow experiment: thermal imaging......Page 239 14.3.1 Small-scale surface velocity......Page 241 14.3.2 Large-scale river flow......Page 244 14.4 Summary......Page 245 References......Page 246 15.1 Introduction......Page 247 15.2.2 Field campaign......Page 248 15.3.3 Turbulence parameters......Page 249 15.4.2 Hydrodynamic and sedimentological characteristics......Page 250 15.4.5 Flow structure during cloud observation......Page 251 15.4.6 Coherent flow and SSC structure variability on a tidal cycle......Page 252 15.5.1 The origin of the suspension clouds......Page 254 15.5.2 Hydrodynamic threshold for the occurrence of suspension clouds......Page 255 15.6 Conclusions......Page 256 References......Page 257 16.1 Introduction......Page 259 16.2.1 Field studies of pool hydrodynamics......Page 260 16.2.2 Flume and numerical experiments......Page 263 16.3.2 Instrumentation......Page 264 16.3.4 Experimental runs......Page 265 16.4.1 Distribution of flow and turbulence......Page 266 16.4.2 Large-scale flow structures......Page 268 16.4.3 Sensitivity to channel geometry......Page 270 16.5 Discussion and conclusion......Page 272 References......Page 274 17.1 Introduction and research context......Page 277 17.2.1 Data sampling......Page 279 17.2.2 Data analysis......Page 281 17.3.1 Time series characteristics......Page 284 17.3.2 Flow pulsation detection......Page 285 17.3.3 Flow pulsation scalings......Page 286 17.4 Discussion......Page 287 17.5 Implications and conclusions......Page 288 References......Page 289 18.1 Introduction......Page 291 18.2.2 Water-worked gravel bed......Page 294 18.2.3 Particle image velocimetry (PIV) flow measurement......Page 295 18.3.1 Streamwise flow velocities......Page 296 18.3.2 Secondary flow velocities......Page 297 18.3.3 Turbulence......Page 299 18.4.1 Secondary flow circulation cells......Page 300 18.5 Conclusions......Page 303 References......Page 304 19.1 Introduction......Page 305 19.2 Grain-flow interaction: recent developments......Page 307 19.3 Fluctuating fluid forces......Page 309 19.4 Particle dislodgement paradox......Page 311 19.5 Resolution of the particle dislodgement paradox......Page 312 19.6 Analytical formulation......Page 314 19.7.1 Fluctuating force measurements on an instrumented immobile grain......Page 315 19.7.2 Measurements of mobile grain entrainment......Page 317 19.8 Thoughts on coherent structures and grain entrainment......Page 318 19.9 Some additional thoughts on the impulse concept and particle entrainment......Page 319 Notation......Page 320 References......Page 321 20.1 Introduction......Page 325 20.2 Methods......Page 327 20.3.1 Variations in suspended sediment concentration......Page 328 20.3.2 Time-mean and turbulent flow using clear-water conditions......Page 329 20.3.3 Sediment-laden versus clear-water turbulent flow conditions......Page 330 20.4 Discussion......Page 333 20.5 Conclusions......Page 336 References......Page 337 21.1 Introduction......Page 339 21.1.1 Experimental setup and data collected......Page 341 21.2.1 Physical characteristics of bed topography......Page 342 21.2.2 Multiscale statistics of bed topography......Page 343 21.3.1 Streamwise and vertical velocity spectra: implications for grain sorting......Page 346 21.4 Turbulence patterns modulated by bed forms......Page 349 21.5 Sediment transport modelling......Page 351 21.7 Acknowledgements......Page 352 References......Page 353 22.1 Introduction......Page 357 22.2.1 General methodology......Page 359 22.2.2 Estimating sand elevation......Page 361 22.2.3 Sediment transport......Page 362 22.2.4 Turbulence measurements......Page 363 22.3.1 Turbulence data......Page 365 22.3.2 Sediment transport relation for sand......Page 369 22.4 Conclusions......Page 370 References......Page 371 23.1 Introduction......Page 375 23.3.1 Location and conditions......Page 376 23.3.2 Measurements and sampling mode......Page 377 23.4.2 Frontal structure and turbulence......Page 378 23.4.3 Frontal mixing......Page 379 23.5 Comparison to prior field and laboratory results......Page 383 23.6 Summary......Page 384 References......Page 385 24.1 Introduction......Page 387 24.2.2 Data reduction and analysis......Page 390 24.3 Results and discussion......Page 391 References......Page 397 Index......Page 401 An expert review of recent progress in the study of turbulent flows with a focus on recently identified organized structures. This book reviews the recent progress in the study of the turbulent flows that sculpt the Earth’s surface, focusing in particular on the organized structures that have been identified in recent years within turbulent flows. These coherent flow structures can include eddies or vortices at the scale of individual grains, through structures that scale with the flow depth in rivers or estuaries, to the large-scale structure of flows at the morphological or landform scale. These flow structures are of wide interest to the scientific community because they play an important role in fluid dynamics and influence the transport, erosion and deposition of sediment and pollutants in a wide variety of fluid flow environments. Scientific knowledge of these structures has improved greatly over the past 20 years as computational fluid dynamics has come to play an increasing important part in building our understanding of coherent flow structures across a broad range of scales. Chapters comprise a series of major, invited papers and a selection of the most novel, innovative papers presented at the second Coherent Flow Structures Conference held August 3-5, 2011 at Simon Fraser University in Burnaby, British Columbia. Chapters focus on six major themes: Dynamics of coherent flow structures (CFS) in geophysical flows Interaction of turbulent flows, vegetation and ecological habitats Coherent structure of atmospheric flows Numerical modeling of coherent flow structures Turbulence in open channel flows Coherent flow structures, sediment transport and morphological feedbacks. "The book will review recent progress in the study of the turbulent flows that sculpt the Earth's surface, focusing in particular on the organized structures that have been identified in recent years within turbulent flows"-- Provided by publisher