Large-scale PIV surface flow measurements in shallow basins with different geometries

Shallow depth flow fields and low velocity magnitudes are often challenges for traditional velocity measuring instruments. As such, new techniques have been developed that provide more reliable velocity measurements under these circumstances. In the present study, the two-dimensional (2D) surface velocity field of shallow basins is assessed by means of Large-Scale Particle Image Velocimetry (LSPIV). The measurements are carried out at the water surface, which means that a laser light sheet is not needed. Depending on the time scales of the flow and the camera characteristics, it is even possible to work with a constant light source. An experimental application of this method is presented to analyze the effects of shallow basin geometry on flow characteristics in reservoirs where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics. The flow and boundary conditions that give rise to asymmetric flow are presented. Asymmetric flow structures were observed starting from basin shape ratios that are less than or equal to 0.96. By decreasing the basin length and increasing the shape ratio to greater than 0.96, the flow structure generally tends towards a symmetric patter

LSPIV implementation for environmental flow in various laboratory and field cases

Large-Scale Particle Image Velocimetry (LSPIV) is an extension of a quantitative imaging technique to measure water surface velocities using simple and inexpensive equipment. This paper describes the implementation of imaged-based LSPIV in eight different environmental flow and hydraulic engineering applications for the investigation of complex configurations with and without sediment transport (bed and suspended loads). These applications include the investigation of sedimentation in shallow reservoirs, run-of-river hydropower plants, side weirs used to control bank overflow, flow fields in different spillway configurations with and without Piano KeyWeir (PKW), oil spills with flexible and rigid barriers, groin fields, river confluence, and sediment flushing in reservoirs. The paper summarises some special problems encountered in such study cases. The selection and adjustments of the parameters to solve them properly were examined. The potential of LSPIV to measure surface flow velocities in the context of river and dam engineering projects is shown. Despite significant variations of natural and artificial illuminations and seeding tracers in the laboratory, field, wind, and water surface elevation, LSPIV was applied successfully to obtain velocity measurements. LSPIV has proven to be a reliable, flexible, and inexpensive flow diagnostic tool that can be employed successfully in many engineering applications

How initial basin geometry influences gravity-driven salt tectonics: Insights from laboratory experiments

As a rifted margin starts to tilt due to thermal subsidence, evaporitic bodies can become unstable, initiating gravity-driven salt tectonics. Our understanding of such processes has greatly benefitted from tectonic modelling efforts, yet a topic that has however gotten limited attention so far is the influence of large-scale salt basin geometry on subsequent salt tectonics. The aim of this work is therefore to systematically test how salt basin geometry (initial salt basin depocenter location, i.e. where salt is thickest, as well as mean salt thickness) influence salt tectonic systems by means of analogue experiments. These experiments were analyzed qualitatively using top view photography, and quantitatively through Particle Image Velocimetry (PIV), and 3D photogrammetry (Structure-from-Motion, SfM) to obtain their surface displacement and topographic evolution. The model results show that the degree of (instantaneous) margin basin tilt, followed by the mean salt thickness are dominant factors controlling deformation, as enhancing basin tilt and/or mean salt thickness promotes deformation. Focusing on experiments with constant basin tilt and mean salt thickness to filter out these dominant factors, we find that the initial salt depocenter location has various effects on the distribution and expression of tectonic domains. Most importantly, a more upslope depocenter leads to increased downslope displacement of material, and more subsidence (localized accommodation space generation) in the upslope domain when compared to a setting involving a depocenter situated farther downslope. A significant factor in these differences is the basal drag associated with locally thinner salt layers. When comparing our results with natural examples, we find a fair correlation expressed in the links between salt depocenter location and post-salt depositional patterns: the subsidence distribution due to the specific salt depocenter location creates accommodation space for subsequent sedimentation. These correlations are applicable when interpreting the early stages of salt tectonics, when sedimentary loading has not become dominant yet

Chaotic advection and mixing in a western boundary current-recirculation system : laboratory experiments

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2001I study the exchange between a boundary current and flanking horizontal recirculations in a 'sliced-cylinder' rotating tank laboratory experiment. Two flow configurations are investigated: a single recirculation and a double, figure-8, recirculation. The latter case involves a hyperbolic point, while the former does not. I investigate the stirring and mixing under both steady and unsteady forcing. I quantify the mixing in each case using effective diffusivity, Keff, and a corollary effective length, Leff, as derived by Nakamura (1995, 1996). This approach involves diagnosing the geometric complexity of a tracer field. Geometric complexity is indicative of advective stirring. Because stirring creates high gradients, flows with high advective stirring also have high diffusion, and stronger overall mixing. I calculate effective length from images of dye in the tank and find much higher values of Leff in the unsteady hyperbolic cases than in the other cases. Slight unsteadiness in flows involving hyperbolic points gives rise to a chaotic advection mechanism known as 'lobe dynamics'. These lobes carry fluid in and out of the recirculations, acting as extremely effective stirring mechanisms. I demonstrate the existence of these exchange lobes in the unsteady hyperbolic (figure-8) flow. The velocity field in the tank is calculated utilizing particle image velocimetry (PIV) techniques and a time series U(t) demonstrates the (forced) unsteadiness in the flow. Images of dye in the tank show exchange lobes forming at this same forcing period, and caring fluid in and out of the recirculation. Based on the results of these experiments, I am able to confirm that, at least in this controlled environment, basic geometry has a profound effect on the mixing effectiveness of a recirculation. I demonstrate radically increased stirring and mixing in the unsteady hyperbolic flow as compared to steady flows and flows without hyperbolic points. Recirculations are ubiquitous in the world ocean; they occur on a variety of scales, in many different configurations, and at all depths. Some of these configurations involve hyperbolic points, while others do not. Chaotic advection via lobe exchange may be an important component of the mixing at multiple locations in the ocean where hyperbolic recirculation geometries exist.I am grateful for funding provided by a National Defense Science and Engineering Graduate Fellowship and for funding from ONR #N00014-99-1-0258 and NSF #OCE-961694

Composite modelling of subaerial landslide-tsunamis in different water body geometries and novel insight into slide and wave kinematics

This article addresses subaerial landslide-tsunamis with a composite (experimental-numerical) modelling approach. A shortcoming of generic empirical equations used for hazard assessment is that they are commonly based on the two idealised water body geometries of a wave channel (2D) or a wave basin (3D). A recent systematic comparison of 2D and 3D physical block model tests revealed wave amplitude differences of up to a factor of 17. The present article investigates two of these recently presented 2D-3D test pairs in detail, involving a solitary-like wave (scenario 1) and Stokes-like waves (scenario 2). Results discussed include slide and water particle kinematics and novel pressure measurements on the slide front. Instantaneous slide-water interaction power graphs are derived and potential and kinetic wave energies are analysed. Solitary wave theory is found most appropriate to describe the wave kinematics associated with scenario 1, whereas Stokes theory accurately describes the tsunami in scenario 2. The data of both scenarios are further used to calibrate the smoothed particle hydrodynamics (SPH) code DualSPHysics v3.1, which includes a discrete element method (DEM)-based model to simulate the slide-ramp interaction. Five intermediate geometries, lying between the ideal 2D and 3D cases, are then investigated purely numerically. For a “channel” geometry with a diverging side wall angle of 7.5°, the wave amplitudes along the slide axes were found to lie approximately halfway between the values observed in 2D and 3D. At 45°, the amplitudes are practically identical to those in 3D. The study finally discusses the implications of the findings for engineering applications and illustrates the potential and current limitations of DualSPHysics for landslide-tsunami hazard assessment

A Review of the State-of-the Art in Marine Hydrodynamics

The purpose of the present paper is to summarise the present situation in the field of marine hydrodynamics. Since the William Froude time there has been considerable development in all fields of marine hydrodynamics, both experimental and particularly in theoretical methods and their numerical implementation. The role of computational methods is becoming more important because modern technology requires analysis of increasingly complex phenomena. The hydrodynamics institutes make efforts to expand their activities through integration of experimental and computational approach in order to be able to successfully answer the increased demands of the related industries

Experimental study on the influence of the geometry of shallow reservoirs on flow patterns and sedimentation by suspended sediments

The worst enemy of sustainable use of reservoirs is sedimentation. Often the main silting process is the result of settling down of suspended sediments. In shallow reservoirs the flow pattern and the sediment deposition processes are strongly influenced by the reservoir geometry. The trap efficiency of a shallow reservoir depends on the characteristics of the inflowing sediments and the retention time of the water in the reservoir, which in turn are controlled by the reservoir geometry. With the purpose of controlling the sedimentation in shallow reservoirs, the effects of the geometry on flow pattern and deposition processes were investigated with systematic physical experiments and numerical simulations. This allowed identifying ideal off-stream reservoir geometries, which can minimize or maximize the settlement of suspended sediments. The objective was also to gain deeper insight into the physical processes of sedimentation in shallow reservoirs governed by suspended sediments. The systematic experimental investigations were carried out in a 6m long, and 4m wide and 0.3m deep shallow basin. The influence of the shallow reservoir geometry was studied for the first time by varying the width, the length, and the expansion angle of the basin in the experiments for clear water and with suspended sediment. In total 11 different reservoirs geometries and 4 water depths were analyzed. During the tests several parameters were measured, as 2D surface velocities, 3D velocity profiles, thickness of deposited sediments, and sediment concentration at the inflow and outflow. Crushed walnut shells with a median grain size, d50, of 50 µm, and a density of 1500 kg/m3 were used to simulate the suspended sediments. After having reached a stable flow pattern with the clear water, velocity measurements were performed. In a second phase, the evolution of the flow and deposition patterns under suspended sediment inflow were investigated. Tests were carried out with durations from 1.5 hours up to 18 hours, in order to follow the morphological evolution. In order to investigate the efficiency of flushing, flushing operations at normal water level as well as with drawdown were examined after the sedimentation tests. Numerical simulations of the laboratory basin were performed and compared with experimental results. The purpose was to assess the sensitivity of the results on different flow and sediment parameters and different turbulence closure schemes. The experimental investigation of the flow and sediment behavior in axi-symmetric geometries with different shapes provides further information on the evolution of the flow pattern and the sediment deposition. Beside the expansion ratio and form ratio of the basin the flow regime was classified by the geometry shape factor SK and inlet Froude number Frin. The geometry shape factor, defined as a function of wetted perimeter, total reservoir surface area, aspect ratio and expansion density ratio, was used to compare and analyze the experimental results obtained from the different investigated basin geometries. The clear water experiments investigations revealed under what geometrical conditions the flow changes from symmetrical to asymmetrical behavior. The length of the basin has a strong influence on the change of the flow field. The water depth also significantly affects on the type of jet and vortex structure forming in the basin. On the other, hand the sediment deposition pattern was strongly influenced by the jet type and the flow behavior changed with increasing deposits. The prediction of sediment deposition is linked to the prediction of flow behavior. Furthermore it is very sensitive to the basin geometry and the boundary conditions of inflow and outflow. Some tests were performed until the sediment released at the outlet was equal to the sediments entering at inlet into the basin that means a quasi equilibrium was reached. Flushing at normal water level allows only a relatively small part of the deposited sediment to be evacuated. As expected, drawdown flushing is much more effective and a significant amount of sediment deposits could be flushed out of the basin. Regarding the flow pattern in shallow basins empirical relationships for the estimation of the reattachment length of gyres and the normalized residence time as a function of the geometry shape factor, SK, were established. Also empirical equations for the prediction of the jet flow type and velocity magnitude depending on the basin geometry could be formed. Finally empirical equations for the prediction of sedimentation index, silting ratio, trap efficiency and relative deposition thickness, as well as normalized residence time and flushing efficiency could be found. The numerical simulations revealed that the observed asymmetry in flow and deposition patterns can be explained by the sensitivity of the flow regarding geometry and boundary conditions. The influence of the length and width of the basin on the flow pattern can be predicted in good agreement with experiments by these simulations. Some recommendations are given for the design procedure of a new shallow reservoir in view of minimizing the sedimentation due to suspended sediment. The deposited sediment volume can be efficiently minimized by an optimal designed reservoir geometry