Physics of environmental flows interacting with obstacles

2017 Fall.Includes bibliographical references.The effects of natural and man-made obstacles on their surrounding environmental flows such as rivers, lakes, estuaries, oceans and the atmosphere has been the subject of numerous studies for many decades. The flow-obstacle interaction can lead to the generation of turbulence which determines local flow dynamics and even large-scale circulations. The characteristic chaotic and enhanced mixing properties of turbulence in conjunction with other environmental conditions such as the clustering of multiple obstacles and density variations raise a number of interesting problems pertaining to both fundamental fluid dynamics and practical engineering applications. Insights into these processes is of fundamental importance for many applications, such as determining the fate of deep water-masses formed in the abyssal ocean, optimizing the productivity and environmental impact of marine farms, predicting the amount of power that a group of turbines can generate, estimating carbon dioxide exchange between the forests and the atmosphere or modeling flood routing in vegetated rivers. The main aim of this dissertation is to use high-resolution numerical simulations to study environmental flows of different forcing mechanisms interacting with obstacles of different geometries. The objectives are multi-fold: (i) To gain insights into the three-dimensional hydrodynamics of constant-density flows interacting with a finite canopy; (ii) To develop an unambiguous geometrical framework for characterizing canopy planar geometry; (iii) To explore the fundamental differences in the flow dynamics between porous canopies and their solid counterpart; and (iv) To investigate the effect of ambient density stratification on flow-obstacle interactions. The first part of this dissertation focuses on the mean three-dimensional hydrodynamics in the vicinity of a suspended cylindrical canopy patch with a bulk diameter of D. The patch was made of Nc constituent solid circular cylinders with h in height and d in diameter, and was suspended in deep water (H/h ≫ 1 where H is the total flow depth). After the validation against published experimental data, large eddy simulations (LES) were conducted to study the effects of patch density (0.16 ≤ φ = Nc(d/D)2 ≤ 1, by varying Nc) and patch aspect ratio (0.25 ≤ AR = h/D ≤ 1, by varying h) on the near-field flow properties. It was observed qualitatively and quantitatively that an increase in either φ or AR decreases bleeding velocity along the streamwise direction but increases bleeding velocities along the lateral and vertical directions, respectively. A close examination at the flow inside the patch reveals that despite the similar dependence of vertical bleeding on φ and AR, the underlying physics are different. However, in contrast to the bleeding velocity, a flow-rate budget shows that the proportion of the vertical bleeding flow leaving the patch with respect to the total flow entering the patch (i.e. relative vertical bleeding) decreases with increasing AR. Finally, the interlinks between patch geometry, flow bleeding and flow diversion are identified: the patch influences the flow diversion not only directly by its real geometrical dimensions, but also indirectly by modifying flow bleeding which enlarges the size of the near-wake. While loss of flow penetrating the patch increases monotonically with increasing φ, its partition into flow diversion around and beneath the patch shows a non-monotonic dependence, highlighting the fundamental differences in the flow dynamics between porous patches and their solid counterpart. Next, the propagation of full-depth lock-exchange bottom gravity currents over a submerged array of circular cylinders is investigated using laboratory experiments and LES. Firstly, to investigate the front velocity of gravity currents across the whole range of array density φ, the array is densified from a flat-bed (φ = 0) towards a solid-slab (φ = 1) under a particular submergence ratio H/h, where H is the flow depth and h is the array height. The time-averaged front velocity in the slumping phase of the gravity current is found to first decrease and then increase with increasing φ. Next, a new geometrical framework consisting of a streamwise array density μx = d/sx and a spanwise array density μy = d/sy is proposed to account for organized but nonequidistant arrays (μx 6 ≠ μy), where sx and sy are the streamwise and spanwise cylinder spacings, respectively, and d is the cylinder diameter. It is argued that this two-dimensional parameter space can provide a more quantitative and unambiguous description of the current-array interaction compared with the array density given by φ = (π/4) μxμy. Both in-line and staggered arrays are investigated. Four dynamically different flow regimes are identified: (i) through-flow propagating in the array interior subject to individual cylinder wakes (μx: small for in-line array and arbitrary for staggered array; μy: small); (ii) over-flow propagating on the top of the array subject to vertical convective instability (μx: large; μy: large); (iii) plunging-flow climbing sparse close-to-impermeable rows of cylinders with minor streamwise intrusion (μx: small; μy: large); and (iv) skimming-flow channelized by an in-line array into several sub-currents with strong wake sheltering (μx: large; μy: small).Finally, the flow dynamics of intrusive gravity currents past a bottom-mounted obstacle in a continuously stratified ambient was numerically investigated, highlighting the effect of ambient stratification which is not considered in the previous sections. The propagation dynamics of a classic intrusive gravity current was first simulated in order to validate the numerical model with previous laboratory experiments. A bottom-mounted obstacle with a varying non-dimensional height of ˜D = D/H, where D is the obstacle height and H is the total flow depth, was then added to the problem in order to study the downstream flow pattern of the intrusive gravity current. For short obstacles, the intrusion re-established itself downstream without much distortion. However, for tall obstacles, the downstream flow was found to be a joint effect of horizontal advection, overshoot-spring back phenomenon, and associated Kelvin-Helmholtz instabilities. Analysis of the numerical results show that the relationship between the downstream propagation speed and the obstacle height can be subdivided into three regimes: a retarding regime (˜D ≈ 0 ∼ 0.3), an impounding regime (˜D ≈ 0.3 ∼ 0.6), and a choking regime (˜D ≈ 0.6 ∼ 1.0).Overall, at a fundamental level, this dissertation aims to contribute to an improved understanding of the physics associated with environmental flows interacting with obstacles. Moreover, the results from this research are expected to facilitate better parameterizations of this important class of flows

Sediment deposition within and around a finite patch of model vegetation over a range of channel velocity

The interaction between flow and vegetation creates feedbacks to deposition that vary with channel velocity. This experimental study describes how channel velocity and stem-generated turbulence influence the deposition within and around an emergent patch of model vegetation, with a particular focus on deposition within the patch. The Reynolds number threshold for stem-scale turbulence generation was determined using velocity spectra and flow visualization. At high channel velocity resuspension occurred in the bare regions of the channel and a nonuniform spatial distribution of net deposition was observed around and within the patch. In contrast, at low channel velocity there was no (or limited) resuspension and a uniform distribution of net deposition was observed around and within the patch. The deposition inside the patch was enhanced, relative to a bare-channel control, only when the following two criteria were met: (1) the absence of stem turbulence, and (2) the presence of sediment resuspension in the bare channel. Comparison to previous lab and field studies further support these criteria.National Science Foundation (U.S.) (Grant EAR-1414499

Environmental Hydraulics, Turbulence and Sediment Transport

In the research on environmental hydraulics, its turbulence, and its sediment transport, constant challenges have been faced. The complexity of hydraulic impacts on sediment transport and turbulent flow properties makes research in this area a difficult task. However, due to pressure from climate change and the mounting issue of pollution, environmental flow studies are more crucial than ever. Bedforming within rivers is a complex process that can be influenced by the hydraulics, vegetated field, and various suspended and bedload transports. Changes in flow conditions due to rain and flood can further complicate a hydraulic system. To date, the turbulence, morphologic, and bedforming characteristics of natural environmental flows are still not well understood. This book aims to bring together a collection of state-of-the-art research and technologies to form a useful guide for the related research and engineering communities. It may be useful for authorities, researchers, and environmental, civil, and water engineers to understand the current state-of-the-art practices in environmental flow modelling, measurement, and management. It may also be a good resource for research, post-, or undergraduate students who wish to know about the most up-to-date knowledge in this field

Turbulence and Flow–Sediment Interactions in Open-Channel Flows

The main focus of this Special Issue of Water is the state-of-the-art and recent research on turbulence and flow–sediment interactions in open-channel flows. Our knowledge of river hydraulics is deepening, thanks to both laboratory/field experiments related to the characteristics of turbulence and their link to erosion, transport, deposition, and local scouring phenomena. Collaboration among engineers, physicists, and other experts is increasing and furnishing new inter-/multidisciplinary perspectives to the research of river hydraulics and fluid mechanics. At the same time, the development of both sophisticated laboratory instrumentation and computing skills is giving rise to excellent experimental–numerical comparative studies. Thus, this Special Issue, with ten papers by researchers from many institutions around the world, aims at offering a modern panoramic view on all the above aspects to the vast audience of river researchers

Studying the Effect of Froude Number and Densimetric Froude Number on Local Scours around Circular Bridge Piers

A very large percentage of bridge failures are attributed to scouring around bridge piers and this directly influences public safety. Experiments are carried out in a 12-m long rectangular open channel flume made of transparent tempered glass. A 300 mm thick bed made up of sand particles is leveled horizontally to create the test bed and a 50 mm hollow plastic cylinder is used as a model bridge pier. Tests are carried out with varying flow depths and velocities. Data points of various scour parameters such as scour depth, width, and length are collected based on different flow conditions and visual observations of changes in the stream bed downstream the bridge pier are also made as the scour progresses. Result shows that all three major flow characteristics (flow depth, Froude number and densimetric Froude number) have one way or other affect the scour profile

Hydrodynamic and mass transfer study of micro-packed beds in sigle-and two-phase flow

Les micros-lit fixes sont des milieux poreux miniaturisés ralliant les avantages à la fois des microréacteurs et des lits fixes, comme par exemple en terme de rapport surface/volume très élevé conduisant à des taux de transfert de chaleur et de matière intensifiés. Par conséquent, la caractérisation hydrodynamique des micro-lits fixes est nécessaire afin d’appréhender de manière objective les phénomènes de transfert et les modes de contact entre phases. Ensuite l'importance des micro-lits fixes est mise en évidence tandis que les approches pour construire des bases de recherche sur les micro-lits fixes y sont explicitées. Notre recherche commence par l'étude des régimes d'écoulement, des transitions de régime d'écoulement de la multiplicité de l’hydrodynamique et du transfert de matière liquide-solide dans les micro-lits fixes. Cette étude est réalisée au moyen d’une méthode de visualisation par microscopie optique à la paroi et le traitement d’image qui s’en suit pour la partie hydrodynamique et d’une méthode électrochimique basée sur l’oxydoréduction du couple complexes ferri/ferreux hexacyanure pour la partir sur le transfert de matière. Les résultats de perte de charge et de rétention de liquide ont été discutés par rapport aux régimes d’écoulement mis en place et des observations pariétales rendues possibles par microscopie optique. L'effet de la taille des particules et de la géométrie du canal sur les transitions de régimes d’écoulement, le comportement transitoire et le phénomène d'hystérèse ont également été abordés. Finalement, les résultats des expériences hydrodynamiques ont été obtenus en faisant face à de nombreux défis pour lesquels nous avons formulé de nombreuses recommandations en vue d’investigations futures. La détermination expérimentale du coefficient de transfert de masse liquide-solide (kLS) par la technique électrochimique a été effectuée dans un micro-lit fixe rempli de couches de particules de graphite non-sphériques servant de cathode et d'anode. Les expériences ont été réalisées pour un écoulement monophasique en régime de diffusion limitée. Finalement, la correspondance de valeurs de kLS avec les corrélations construites sur la base d’études sur les lits fixes à l’échelle macroscopique a été discutée.Micro-packed beds are miniaturized packed beds having the advantages of both microreactors (high surface-to-volume ratios leading to intensified heat and mass transfer rates, increased safety, etc.) and packed beds (effective contact between the phases) that have the potential to be successfully employed for purposes such as catalyst screening and production of hazardous materials. To assess this potential, hydrodynamic characterization of micro-packed beds is necessary as they address the actual flow phenomena and provide suggestions to improve the contacting patterns between phases for enhanced performances. This work starts with a brief review on process intensification via microreactors. Then the importance of micro-packed beds is highlighted while the approaches to build research foundations on micro-packed beds are discussed. Our research begins by studying the flow regimes, transitions in flow regime and hydrodynamic multiplicity in micro-packed beds mostly by means of microscopic wall visualization and image processing. Results on pressure drop and liquid holdup have been obtained and discussed in terms of flow regimes and wall-flow image analyses. In addition, residence time distributions of the liquid in micro-packed beds have been obtained according to two techniques, by an impulse tracing method (electrolyte tracer injection) and wall visualization with optical microscopy. The effect of particle size and channel geometry (circular vs. square) has also been investigated in terms of flow regime transitions, transient behavior and hysteresis. Finally, challenges and recommendations thereof to surpass the many difficulties encountered are methodically explained to facilitate future investigations. Experimental determination of liquid-solid mass transfer coefficient (kLS) via a linear polarization method was also carried out in a micro-packed bed filled with layers of non-spherical graphite particles serving as cathode and anode for the Redox ferri/ferrocyanide electrochemical reaction. Experiments concerned single-phase liquid flow within the diffusion-limited regime. Particle size analysis and image processing were used to evaluate deviations from spherical geometry of the graphite particles to determine liquid-solid mass transfer coefficient. Finally, the correspondence of kLS values with macro-scale packed bed correlations was discussed

Direct Simulation of Initial Value Problems for the Motion of Solid Bodies in a Newtonian Fluid Part 1. Sedimentation

This paper reports the result of direct simulations of fluid-particle motions in two dimensions. We solve the initial value problem for the sedimentation of circular and elliptical particles in a vertical channel. The fluid motion is computed from the Navier-Stokes equations for moderate Reynolds numbers in the hundreds. The particles are moved according to the equations of motion of a rigid body under the action of gravity and hydrodynamic forces arising from the motion of the fluid. The solutions are as exact as our finite-element calculations will allow. As the Reynolds number is increased to 600, a circular particle can be said to experience five different regimes of motion: steady motion with and without overshoot and weak, strong and irregular oscillations. An elliptic particle always turn its long axis perpendicular to the fall, and drifts to the centreline of the channel during sedimentation. Steady drift, damped oscillation and periodic oscillation of the particle are observed for different ranges of the Reynolds number. For two particles which interact while settling, a steady staggered structure, a periodic wake-action regime and an active drafting- kissing-tumbling scenario are realized at increasing Reynolds numbers. The nonlinear effects of particle-fluid, particle-wall and interparticle interactions are analysed, and the mechanisms controlling the simulated flows are shown to be lubrication, turning couples on long bodies, steady and unsteady wakes and wake interactions. The results are compared to experimental and theoretical results previously published