Craig Hill

Local Scour around a Model Hydrokinetic Turbine in an Erodible Channel

AbstractLaboratory experiments were performed to study the effect of an axial-flow hydrokinetic turbine model on an erodible channel under both clear water and live-bed conditions. Clear water experiments were performed at two scales with a local bed shear stress just below the critical state. Live-bed experiments, performed at small scale, examined the interactions between relatively large-scale bedforms and the flow induced by an axial flow turbine. Spatiotemporal topographic measurements were obtained by sonar and by a state-of-the-art high-resolution scanning system integrated into an automated data acquisition carriage designed and fabricated at the St. Anthony Falls Laboratory at the Univ. of Minnesota. Results indicate that the presence of the turbine rotor increases the local shear stress resulting in accelerated and expanded scour development when compared with typical bridge pier scour mechanisms. The inferred key difference is the alteration of the flow patterns in the rotor wake leading to an ...

Effects of energetic coherent motions on the power and wake of an axial-flow turbine

A laboratory experiment examined the effects of energetic coherent motions on the structure of the wake and power fluctuations generated by a model axial-flow hydrokinetic turbine. The model turbine was placed in an open-channel flow and operated under subcritical conditions. The incoming flow was locally perturbed with vertically oriented cylinders of various diameters. An array of three acoustic Doppler velocimeters aligned in the cross-stream direction and a torque transducer were used to collect high-resolution and synchronous measurements of the three-velocity components of the incoming and wake flow as well as the turbine power. A strong scale-to-scale interaction between the large-scale and broadband turbulence shed by the cylinders and the turbine power revealed how the turbulence structure modulates the turbine behavior. In particular, the response of the turbine to the distinctive von Karman-type vortices shed from the cylinders highlighted this phenomenon. The mean and fluctuating characteristics of the turbine wake are shown to be very sensitive to the energetic motions present in the flow. Tip vortices were substantially dampened and the near-field mean wake recovery accelerated in the presence of energetic motions in the flow. Strong coherent motions are shown to be more effective than turbulence levels for triggering the break-up of the spiral structure of the tip-vortices.

Experimental and field observations of breach dynamics accompanying erosion of Marmot cofferdam, Sandy River, Oregon

A key issue faced in dam removal is the rate and timing of remobilization and discharge of stored reservoir sediments following the removal. Different removal strategies can result in different trajectories of upstream sediment transport and knickpoint migration. We examine this issue of for the Marmot Dam removal in Sandy River, Oregon, USA using both physical experiments and field studies accompanying removal of the dam in October 2007. The physical experiment was designed to provide insights on how and if the position of a cofferdam notch will affect how reservoir sediments are remobilized, with the goal of minimizing the volume of sediment stranded in terraces. Data and observations indicate that at lower failure discharges, notch position impacts the location of cofferdam failure as well as the location of the first major knickpoint and its trajectory. In particular, notch positions that force the river to migrate laterally in order to adjust to natural valley orientation and morphology were most effective in removing larger volumes of sediment and reducing terrace heights. Actual cofferdam notching to maximize erosion produced extremely rapid and significant erosion of reservoir sediments. Comparison of model results with field observations suggests that the physical experiments provided solid predictions of rates of erosion and overall knickpoint trajectory.

On the turbulent flow structure around an instream structure with realistic geometry

We investigate the flow dynamics around a rock vane, a widely-used instream structure for stream restoration, by conducting laboratory flume experiments, and carrying out high-resolution Large Eddy Simulation (LES) taking advantage of parallel computing. The flume experiments are conducted under fixed- and mobile-bed conditions, where the velocities and bed elevations are measured, respectively. The LES is carried out for the fixed-bed experiment by directly resolving the details of the rocks that constitute the vane and the individual roughness elements on the channel bed. The LES-computed mean flow statistics show good agreement with the measurements, and the analysis of the computed flow field reveals the existence of two counter-rotating secondary flow cells downstream of the vane, which originate from the plunging of the three-dimensional streamlines onto a lower part of the sidewall downstream of the vane. To further examine the role of the secondary flow cells under a mobile-bed condition, the LES results are compared with the equilibrium bed elevation measured in the mobile bed experiment. The mobile-bed experiment reveals the existence of an oblique sand ridge downstream of the vane that is aligned with the line of flow convergence caused by the collision of the two secondary flow cells. The results indicate that the two counter-rotating cells downstream of the rock vane has a profound impact on the mean flow and bed shear stress as well as on the bed morphodynamics. This article is protected by copyright. All rights reserved.

Experimental Test Plan - DOE Tidal And River Reference Turbines

Our aim is to provide details of the experimental test plan for scaled model studies in St. Anthony Falls Laboratory (SAFL) Main Channel at the University of Minnesota, including a review of study objectives, descriptions of the turbine models, the experimental set-up, instrumentation details, instrument measurement uncertainty, anticipated experimental test cases, post-processing methods, and data archiving for model developers.

Simulation-based optimization of in-stream structures design: rock vanes

We employ a three-dimensional coupled hydro-morphodynamic model, the Virtual Flow Simulator (VFS-Geophysics) in its Unsteady Reynolds Averaged Navier–Stokes mode closed with

Flow structure interaction around an axial-flow hydrokinetic turbine: Experiments and CFD simulations

k-\omega

High-resolution numerical simulation of turbulence in natural waterways

k-ω model, to simulate the turbulent flow and sediment transport in large-scale sand and gravel bed waterways under prototype and live-bed conditions. The simulation results are used to carry out systematic numerical experiments to develop design guidelines for rock vane structures. The numerical model is based on the Curvilinear Immersed Boundary approach to simulate flow and sediment transport processes in arbitrarily complex rivers with embedded rock structures. Three validation test cases are conducted to examine the capability of the model in capturing turbulent flow and sediment transport in channels with mobile-bed. Transport of sediment materials is handled using the Exner equation coupled with a transport equation for suspended load. Two representative meandering rivers, with gravel and sand beds, respectively, are selected to serve as the virtual test-bed for developing design guidelines for rock vane structures. The characteristics of these rivers are selected based on available field data. Initially guided by existing design guidelines, we consider numerous arrangements of rock vane structures computationally to identify optimal structure design and placement characteristics for a given river system.