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Author: Blair Anne Johnson Publisher: ISBN: Category : Languages : en Pages : 358
Book Description
We perform an experimental study to investigate turbulent boundary layers in the absence of mean shear at both stationary solid and mobile sediment boundaries. High Reynolds number (Re[lamda] ~ 300) horizontally homogeneous isotropic turbulence is generated via randomly actuated synthetic jet arrays (RASJA Variano & Cowen 2008). Each of the arrays is controlled by a spatio-temporally varying algorithm, which in turn minimizes the formation of secondary flows or mean shear. One array consists of an 8 x 8 grid of jets, while the other is a 16 x 16 array. By varying the operational parameters of the RASJA, we also find that we are able to control the turbulence levels, including integral length scales and dissipation rates, by changing the mean on-times in the jet algorithm. Acoustic Doppler velocimetry (ADV) and particle image velocimetry (PIV) measurements are used to study the isotropic turbulent region and the boundary layer formed beneath it as the turbulence encounters the bottom boundary. Time-lapsed photography is used to monitor large-scale bed morphology of the sediment. The flow is characterized by statistical metrics including the mean flow and turbulent velocities, turbulent kinetic energy, temporal spectra, integral scales of the turbulence, and terms in the turbulent kinetic energy transport equation including energy dissipation rates, production, and turbulent transport. We evaluate the implications of assuming isotropy in computing dissipation by comparing several methods commonly used in measurements, includ- ing second-order structure functions, spatial spectra, scaling arguments, and direct computations. With our dissipation results, we calculate the empirical constant in the Tennekes (1975) model of Eulerian frequency spectra. This model allows for the determination of dissipation from temporally resolved single-point velocity measurements when there is no mean flow. We compare our boundary layer characterizations to prior literature that addresses mean shear free turbulent boundary layers via grid-stirred tank (GST) experiments, moving bed experiments, rapid distortion theory (RDT), and direct numerical simulations (DNS) in a forced turbulent box. We draw comparisons between an impermeable flat boundary, a flat permeable sediment boundary, and a rippled sediment boundary. In experiments examining turbulence above a sediment boundary, we observe sediment suspension primarily via vortical pick-up and splats. Additionally, we observe the development of ripple patterns in the sediment, which is unexpected in a facility absent mean shear or oscillations. We find a relationship between the integral length scale of the turbulent flow with the ripple spacing, suggesting a link between the turbulence levels and sediment transport. Because traditional viscous stresses due to mean velocity gradients suggest no bed friction or sediment transport, we develop a method for considering Reynolds stresses over short time periods as a surrogate for understanding the importance of bed stress in a zero mean shear environment.
Author: Blair Anne Johnson Publisher: ISBN: Category : Languages : en Pages : 358
Book Description
We perform an experimental study to investigate turbulent boundary layers in the absence of mean shear at both stationary solid and mobile sediment boundaries. High Reynolds number (Re[lamda] ~ 300) horizontally homogeneous isotropic turbulence is generated via randomly actuated synthetic jet arrays (RASJA Variano & Cowen 2008). Each of the arrays is controlled by a spatio-temporally varying algorithm, which in turn minimizes the formation of secondary flows or mean shear. One array consists of an 8 x 8 grid of jets, while the other is a 16 x 16 array. By varying the operational parameters of the RASJA, we also find that we are able to control the turbulence levels, including integral length scales and dissipation rates, by changing the mean on-times in the jet algorithm. Acoustic Doppler velocimetry (ADV) and particle image velocimetry (PIV) measurements are used to study the isotropic turbulent region and the boundary layer formed beneath it as the turbulence encounters the bottom boundary. Time-lapsed photography is used to monitor large-scale bed morphology of the sediment. The flow is characterized by statistical metrics including the mean flow and turbulent velocities, turbulent kinetic energy, temporal spectra, integral scales of the turbulence, and terms in the turbulent kinetic energy transport equation including energy dissipation rates, production, and turbulent transport. We evaluate the implications of assuming isotropy in computing dissipation by comparing several methods commonly used in measurements, includ- ing second-order structure functions, spatial spectra, scaling arguments, and direct computations. With our dissipation results, we calculate the empirical constant in the Tennekes (1975) model of Eulerian frequency spectra. This model allows for the determination of dissipation from temporally resolved single-point velocity measurements when there is no mean flow. We compare our boundary layer characterizations to prior literature that addresses mean shear free turbulent boundary layers via grid-stirred tank (GST) experiments, moving bed experiments, rapid distortion theory (RDT), and direct numerical simulations (DNS) in a forced turbulent box. We draw comparisons between an impermeable flat boundary, a flat permeable sediment boundary, and a rippled sediment boundary. In experiments examining turbulence above a sediment boundary, we observe sediment suspension primarily via vortical pick-up and splats. Additionally, we observe the development of ripple patterns in the sediment, which is unexpected in a facility absent mean shear or oscillations. We find a relationship between the integral length scale of the turbulent flow with the ripple spacing, suggesting a link between the turbulence levels and sediment transport. Because traditional viscous stresses due to mean velocity gradients suggest no bed friction or sediment transport, we develop a method for considering Reynolds stresses over short time periods as a surrogate for understanding the importance of bed stress in a zero mean shear environment.
Author: Blair Anne Johnson Publisher: ISBN: Category : Languages : en Pages : 128
Book Description
Sediment resuspension is initiated through a variety of mechanisms across the wave breaking, run-up, and run-down stages that occur in the wave breaking, surf, and swash zones. When a wave breaks offshore, the plunging jet rarely reaches the bed directly; however, this injection of momentum generates turbulence that propagates through the water column, creating local stresses as turbulent eddies interact with the seafloor. When these stresses are sufficient to resuspend sediment from the bed, the sediment can become entrained in the flow, and thus both turbulence and sediment are advected shoreward during onshore directed phases of the wave cycle. It is at this time that the fluid is highly turbid, filled with sediment and debris. As the flow enters the swash zone, it runs upshore and slows, and the sediment settles out of entrainment and is deposited onshore. Then, as this relatively clear fluid flow retreats and regains speed in flowing down the beach face, sediment is again resuspended into the flow. But upon careful observation, the turbid region is constrained to the base of the water column due to shear-dominated resuspenions as a boundary layer grows. The resuspension during the latter event can be attributed to an increased bed shear stress, due to the uniform mean flow along the sediment bed. This mechanism for resuspension is well-understood, as thorough experimental research has led to well-characterized parameters such as the Shields Curve that identify requisite shear stresses for sediment resuspension. However, the former mechanism, in which sediment is resuspended due to the interaction of large-scale turbulent motions, has seen far less research, because of the difficulty in isolating this particular flow in the laboratory and identifying meaningful parameters of the flow necessary for incipient particle motion from the bed. We have chosen to isolate the phenomenon of sediment resuspension by turbulence absent mean shear in the laboratory to better understand this fundamental process. By adapting a turbulence chamber in the DeFrees Hydraulics Lab developed by Variano & Cowen (2008), in which a Randomly Actuated Synthetic Jet Array (RASJA; Variano and Cowen, 2008) is suspended above a tank of water, with jets directed downward, we have a nearly ideal facility for examining decaying horizontally homogeneous isotropic turbulence above a bed. The jets fire according to a specified random algorithm to generate homogeneous isotropic turbulence without inducing mean flows. With a solid glass bottom boundary in place, data were collected with Acoustic Doppler Velocimetry (ADV) and Particle Image Velocimetry (PIV) measurements to characterize profiles of the mean flow, turbulence intensity, and kinetic energy. Statistics such as spatial and temporal spectra and parameters of mean flow strength were computed to understand the nature of the turbulence in the facility, revealing a well developed inertial subrange and very weak mean flows. The glass was then replaced with a sediment bed, and the tests were repeated to make a direct comparison between the solid and sediment beds. During the sediment tests, resuspension was observed intermittently. We were surprised to find that a rippled pattern quickly evolved in the sand bed, even with relatively few visible resuspension events. Preliminary tests have been performed to examine the relationships between the forced turbulence levels and the mechanisms of resuspension, and we have also performed qualitative studies to investigate the influence of the presence of solid boundaries and turbulence on bed morphology. In order to quantitatively analyze the nec- essary stresses and fluid structures present, a conditional quadrant analysis is performed on the Reynolds stresses. A small project was included to test the performance of a Nortek Aquadopp High Resolution Profiler in the facility. Though the profiler is designed to capture large-scale environmental flows that are uniform across its beams at a given elevation from its transducer face, this investigation was performed to test the instrument's capability to capture high levels of turbulence in a small facility where this assumption breaks down. Direct comparisons were made to simultaneous PIV measurements, and measurements from the Profiler were included in the overall tank characterization.
Author: N. J. Clifford Publisher: ISBN: Category : Science Languages : en Pages : 386
Book Description
Turbulence: Perspectives on Flow and Sediment Transport outlines the fundamental principles of the measurement and analysis of turbulent flow and sediment transport. It reviews laboratory work over the last three decades on near-boundary flow structure, as well as providing more recent research dealing with the high frequency monitoring and modelling of sediment transport. The research sections address aspects of turbulence characteristics and sediment behaviour in fluvial, tidal and aeolian boundary layers. Throughout, emphasis is upon the practicalities of measurement and modelling under field conditions. The effects of roughness transitions are extensively discussed and some of the most popular instrumentation is reviewed. This book brings together contributions from twenty researchers worldwide from many disciplines connected with earth sciences, engineering and fluid dynamics, providing both review material and some of the latest research into the small-scale interactions of turbulence and sediment transport in geophysical boundary layers. The book is ideally suited to a wide audience, from advanced undergraduate level onwards.
Author: Peter Nielsen Publisher: World Scientific Publishing Company ISBN: 9813103582 Category : Technology & Engineering Languages : en Pages : 340
Book Description
This book is intended as a useful handbook for professionals and researchers in the areas of Physical Oceanography, Marine Geology, Coastal Geomorphology and Coastal Engineering and as a text for graduate students in these fields. With its emphasis on boundary layer flow and basic sediment transport modelling, it is meant to help fill the gap between general hydrodynamic texts and descriptive texts on marine and coastal sedimentary processes. The book commences with a review of coastal bottom boundary layer flows including the boundary layer interaction between waves and steady currents. The concept of eddy viscosity for these flows is discussed in depth because of its relation to sediment diffusivity. The quasi-steady processes of sediment transport over flat beds are discussed. Small scale coastal bedforms and the corresponding hydraulic roughness are described. The motion of suspended sand particles is studied in detail with emphasis on the possible suspension maintaining mechanisms in coastal flows. Sediment pickup functions are provided for unsteady flows. A new combined convection-diffusion model is provided for suspended sediment distributions. Different methods of sediment transport model building are presented together with some classical models.
Author: B Mutlu Sumer Publisher: World Scientific ISBN: 9813234326 Category : Technology & Engineering Languages : en Pages : 758
Book Description
This book discusses the subject of turbulence encountered in coastal and civil engineering.The primary aim of the book is to describe turbulence processes including transition to turbulence; mean and fluctuating flows in channels/pipes, and in currents; wave boundary layers (including boundary layers under solitary waves); streaming processes in wave boundary layers; turbulence processes in breaking waves including breaking solitary waves; turbulence processes such as bursting process and their implications for sediment transport; flow resistance in steady and wave boundary layers; and turbulent diffusion and dispersion processes in the coastal and river environment, including sediment transport due to diffusion/dispersion.Both phenomenological and statistical theories are described in great detail. Turbulence modelling is also described, and several examples for modelling of turbulence in steady flow and wave boundary layers are presented.The book ends with a chapter containing hands-on exercises on a wide variety of turbulent flows including experimental study of turbulence in an open-channel flow, using Laser Doppler Anemometry; Statistical, correlation and spectral analysis of turbulent air jet flow; Turbulence modelling of wave boundary layer flows; and numerical modelling of dispersion in a turbulent boundary layer, a set of exercises used by the authors in their Masters classes over many years.Although the book is essentially intended for professionals and researchers in the area of Coastal and Civil Engineering, and as a text book for graduate/post graduate students, the contents of the book will, however, additionally provide sufficient background in the study of turbulent flows relevant to many other disciplines, such as Wind Engineering, Mechanical Engineering, and Environmental Engineering.
Author: Johan C. Winterwerp Publisher: Elsevier ISBN: 0080473733 Category : Science Languages : en Pages : 577
Book Description
This book is an introduction to the physical processes of cohesive sediment in the marine environment. It focuses on highly dynamic systems, such as estuaries and coastal seas. Processes on the continental shelf are also discussed and attention is given to the effects of chemistry, biology and gas.The process descriptions are based on hydrodynamic and soil mechanic principles, which integrate at the soil-water interface. This approach is substantiated through a classification scheme of sediment occurrences in which distinction is made between cohesive and granular material. Emphasis is also placed on the important interactions between turbulent flow and cohesive sediment suspensions, and on the impact of flow-induced forces on the stability of the seabed. An overview of literature on cohesive sediment dynamics is presented and a number of new developments are highlighted, in particular in relation to floc formation, settling and sedimentation, consolidation, bed failure and liquefaction and erosion of the bed. Moreover, it presents a summary on methods and techniques to measure the various sediment properties necessary to quantify the various parameters in the physical-mathematical model descriptions. A number of examples and case studies have been included.
Author: Bernard P. Boudreau Publisher: Oxford University Press ISBN: 9780199770915 Category : Science Languages : en Pages : 430
Book Description
The benthic boundary layer is the zone of water and sediment immediately adjacent to the bottom of a sea, lake, or river. This zone is of considerable interest to biologists, geochemists, sedimentologists, and engineers because of very strong gradients of energy, dissolved and solid chemical components, suspended matter, and the number of organisms that live there. It is, for example, the sink for anthropogenic substances and the home of microscopic plant life that provides the nutrients that determine fish populations--and ultimately the size of the fisheries. This book of original chapters edited by Professors Boudreau and Jorgensen, both leading researchers in the field, will meet the need for an up-to-date, definitive text/reference on measurements, techniques, and models for transport and biochemical processes in the benthic boundary layer. Each chapter provides a comprehensive review of a selected field, with illustrated examples from the authors' own work. The book will appeal to professionals and researchers in marine biology, marine chemistry, marine engineering, and sedimentology.
Author: Lynne E. Frostick Publisher: CRC Press ISBN: 0415609127 Category : Technology & Engineering Languages : en Pages : 274
Book Description
A Users Guide to Hydraulic Modelling and Experimentation provides a systematic, comprehensive summary of the progress made through HYDRALAB III . The book combines the expertise of many of the leading hydraulic experimentalists in Europe and identifies current best practice for carrying out state-of-the-art, modern laboratory investigations. In addition it gives an inventory and reviews recent advances in instrumentation and equipment that drive present and new developments in the subject. The Guide concentrates on four core areas – waves, breakwaters, sediments and the relatively-new (but rapidly-developing) cross-disciplinary area of hydrodynamics/ecology. Progress made through the ‘CoMIBBS’ component of HYDRALAB III provides the material for a chapter focussed on guidance, principles and practice for composite modelling. There is detailed consideration of scaling and the degree of relevance of laboratory/physical modelling approaches for specific contexts included in each of the individual chapters. The Guide includes outputs from the workshops and several of the innovative transnational access projects that have been supported within HYDRALAB III, as well as the focussed joint research activities SANDS and CoMIBBS. Its primary purpose is to serve as a shared resource to disseminate the outstanding advances achieved within HYDRALAB III but, even more than this, it is a tribute to the human and institutional collaborations that led to and sustained the research advances, the human relationships that were strengthened and initiated through joint participation in the Programme, and the training opportunities that participation provided to the many young researchers engaged in the projects.
Author: Harold Barnes Publisher: CRC Press ISBN: 0203400682 Category : Science Languages : en Pages : 574
Book Description
A good quality annual review series that provides an important service to the sciences for both the general and the specialist reader. Oceanography and Marine Biology has succeeded in producing one admirably for more than 35 years. The quality of the paper, the printing and the presentation is excellent.--Times Higher Education Supplement
Author: Blair Anne Johnson
Author: Blair Anne Johnson
Author: Blair Anne Johnson
Author: N. J. Clifford
Author: Peter Nielsen
Author: 
Author: B Mutlu Sumer
Author: Johan C. Winterwerp
Author: Bernard P. Boudreau
Author: Lynne E. Frostick
Author: Harold Barnes