Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A13: Focus Session: Marine Hydrokinetic Energy Conversion I |
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Chair: Laura Beninati, Bucknell University Room: 301 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A13.00001: Interaction between an axial-flow model hydrokinetic turbine and an erodible channel Craig Hill, Mirko Musa, Leonardo P. Chamorro, Michele Guala Laboratory experiments were carried out to examine the effect of relatively large-scale bedforms on the performance of a model axial-flow hydrokinetic turbine. The turbine rotor, $d_{T} =0.15$m, was attached to a miniature DC motor, and allowed for voltage data acquisition at 200 Hz along with 3D hub-height inflow velocity, $U_{hub} $, approximately 7$d_{T} $ upstream of the turbine. Spatio-temporal bed elevations were acquired along three longitudinal sections and at least one transverse transect within the flume providing the temporally-averaged scour and deposition patterns characterizing the turbine near-field region. Turbine-turbine interaction was investigated under aligned configurations in the streamwise direction with variable spacing both in clear water scour and live bed transport conditions. Effects from both migrating bedforms and the upstream turbine were observed in the long-term and short-term voltage fluctuations of the downstream turbine. Combined measurements of inflow velocity, bed topography and turbine voltage were used to obtain joint statistics and correlations, which provided an indication of the variability in environmental exposure and performance that hydrokinetic turbines will encounter in natural erodible rivers. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A13.00002: Tow tank measurements of turbulent flow in the near wake of a horizontal axis marine current turbine under steady and unsteady inflow conditions Luksa Luznik, Max Van Benthem, Karen Flack, Ethan Lust Near wake measurements are presented for a 0.8 m diameter (D) two bladed horizontal axis tidal turbine model for two inflow conditions. The first case had steady inflow conditions, i.e. turbine was towed at a constant carriage speed and the second case had a constant carriage speed and incoming regular waves with a period of 1.6 seconds and 0.09 m wave height. The test matrix in the wake covered four radial positions from r/D$=$0.3 to 0.5 and five axial positions from x/D$=$0.19 to 0.95. All measurements were performed at the nominal tip speed ratio (TSR) of 7.4. The distribution of mean velocities for the steady inflow case exhibit significant spatial variability in the wake region. Normalized mean streamwise velocity show a decrease in magnitude with the axial direction for all radial locations ranging from U/Utow$=$0.55 at r/D$=$0.49 to 0.35 at r/D$=$0.3. Vertical and lateral mean velocities are small but consistent with counterclockwise fluid angular momentum for a clockwise rotor rotation. The Reynolds shear stresses consistently show elevated levels for measurements near the rotor tip (r/D$=$0.49) and are significantly reduced by x/D$=$0.6 downstream. This suggests low turbulence levels in the wake which is consistent with very low free stream turbulence. For the case with waves, evidence of enhanced turbulence intensities and shear stresses within spatial coverage of the experiment suggest increased in localized turbulence production in the blade tip region over the entire near wake region. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A13.00003: An investigation into blockage corrections for cross-flow hydrokinetic turbine performance Robert Cavagnaro, Brian Polagye The performance of hydrokinetic turbines is augmented in confined channels, such that the coefficient of performance is elevated versus free-stream conditions. This often introduces uncertainty when characterizing prototype-scale turbines in flume or tow tank facilities. Performance of a one-quarter scale helical, cross-flow turbine is characterized over a range of operating conditions (inflow velocity and tip-speed ratio) at blockage ratios (ratio of rotor swept area to channel area) of $\sim$10 and $\sim$25{\%}. Particle image velocimitry is used to characterize rotor induction, as well as the turbulent wake produced by the turbine. Performance at the different blockage ratios is compared to corrections derived from actuator disk theory and to full-scale field performance in the absence of blockage. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A13.00004: Performance and cavitation characteristics of bi-directional hydrofoils Ivaylo Nedyalkov, Martin Wosnik Tidal turbines extract energy from flows which reverse direction. One way to address this bi-directionality in horizontal axis turbines that avoid the use of complex and maintenance-intensive yaw or blade pitch mechanisms, is to design bi-directional blades which perform (equally) well in either flow direction. A large number of proposed hydrofoil designs were investigated using numerical simulations. Selected candidate foils were also tested (at various speeds and angles of attack) in the High-Speed Cavitation Tunnel (H\textsc{i}C\textsc{a}T) at the University of New Hampshire. Lift and drag were measured using a force balance, and cavitation inception and desinence were recorded. Experimental and numerical results were compared, and the foils were compared to each other and to reference foils. Bi-directional hydrofoils may provide a feasible solution to the problem of reversing flow direction, when their performance and cavitation characteristics are comparable to those for unidirectional foils, and the penalty in decreased energy production is outweighed by the cost reduction due to lower complexity and respectively lower installation and maintenance costs. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A13.00005: Experimental characterization of marine hydrokinetic (MHK) turbine array performance Nickolas Stelzenmuller, Alberto Aliseda Three scale model horizontal axis MHK turbines (1:45) were tested in a flume at various array spacings. The scale rotors are based on the full-scale Department of Energy Reference Model 1, modified to reproduce the hydrodynamic performance of the full-scale turbine at the reduced experimental Reynolds number ($10^5$ vs $10^6$, based on chord length). Flow incident on the turbines and in the wakes was characterized via PIV and ADV measurements. Tip speed ratio (TSR) similarity of the turbines is achieved by controlling the applied load with magnetic particle brakes. Single turbines were characterized at various mean freestream velocities to explore the effect of Reynolds number on turbine performance. Measured turbine efficiencies of $\sim $40\% are similar to efficiencies predicted from full-scale simulations, indicating similar energy extraction at scale. Wake characteristics and turbine efficiencies have been investigated at a range of TSRs, with the goal of determining array spacing and operating conditions that maximize overall array efficiency. Free surface deformations were measured near the rotor plane for various vertical positions of the turbine relative to the free surface and compared to numerical simulation results. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A13.00006: Reynolds number effects on the performance and near-wake of a cross-flow turbine Peter Bachant, Martin Wosnik To design wind or marine hydrokinetic (MHK) turbine farms with high efficiency, interactions between turbine wakes must be accurately predicted. However, to date numerical models predicting detailed wake properties of cross-flow (or vertical-axis) turbines have been validated with experimental data taken at Reynolds numbers significantly lower than those of full scale devices, casting doubt on the models' accuracy. To address this uncertainty, we investigated the effects of Reynolds number on the performance and near-wake characteristics of a 3-bladed cross-flow turbine, both experimentally and numerically. Mechanical power output and overall streamwise drag were measured in a towing tank at turbine diameter Reynolds numbers $Re_D = 0.5 \times 10^5$--$2.0 \times 10^6$. A detailed map of the near-wake one turbine diameter downstream was acquired via acoustic Doppler velocimetry for each Reynolds number case, from which differences in the mean velocity, turbulence intensity, and Reynolds stresses are highlighted. Finally, Reynolds-averaged Navier-Stokes (RANS) numerical simulations were performed, the results from which are compared with the experimental data. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A13.00007: Comparison of spatio-temporal resolution of different flow measurement techniques for marine renewable energy applications Vincent Lyon, Martin Wosnik Marine hydrokinetic (MHK) energy conversion devices are subject to a wide range of turbulent scales, either due to upstream bathymetry, obstacles and waves, or from wakes of upstream devices in array configurations. The commonly used, robust Acoustic Doppler Current Profilers (ADCP) are well suited for long term flow measurements in the marine environment, but are limited to low sampling rates due to their operational principle. The resulting temporal and spatial resolution is insufficient to measure all turbulence scales of interest to the device, e.g., ``blade-scale turbulence.'' The present study systematically characterizes the spatial and temporal resolution of ADCP, Acoustic Doppler Velocimetry (ADV), and Particle Image Velocimetry (PIV). Measurements were conducted in a large cross section tow tank (3.7m x 2.4m) for several benchmark cases, including low and high turbulence intensity uniform flow as well as in the wake of a cylinder, to quantitatively investigate the flow scales which each of the instruments can resolve. The purpose of the study is to supply data for mathematical modeling to improve predictions from ADCP measurements, which can help lead to higher-fidelity energy resource assessment and more accurate device evaluation, including wake measurements. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A13.00008: Upstream blockage effect on the thrust force of a marine hydrokinetic device Giulio Soliani, Maria Laura Beninati, Michael Krane, Arnold Fontaine The study evaluates the interaction of two model marine devices axially arranged one in front of the other, in a tandem configuration. Particular focus is given to the change that occurs in the thrust of the downstream marine hydrokinetic (MHK) device when the spatial arrangement of the two elements is varied. At critical spacing there is no thrust generation. The study is motivated by the need to predict the thrust behavior of MHK devices and determine the minimum separation distance to avoid the no thrust condition. The downstream element is a two-bladed, horizontal axis turbine, while the upstream blockage is a perforated disk with similar geometric properties intended to approximate the wake of the MHK device. Testing is conducted in the flume facility at Bucknell University. Experiments are performed for a fixed range of spacing between the perforated disk and the turbine. For each separation distance, the span-wise velocity profile upstream and downstream of the turbine is measured, as well as the device's rotational speed. The turbine's thrust coefficient is calculated. Plots of the thrust coefficient as a function of spacing depict the minimum separation distance to avoid the no thrust condition. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A13.00009: Scour around a submerged cylinder and marine hydrokinetic (MHK) device in live-bed conditions Maria Laura Beninati, Michael Volpe, Michael Krane, Arnold Fontaine Experiments are presented to explore how sediment scour around a single Marine Hydrokinetic (MHK) turbine varies with flow speed. Three Reynolds numbers, based on support structure diameter were used to induce live-bed scour conditions. Based on results from previous studies on submerged cylinders, differences in scour patterns between a single cylinder and MHK device can be determined. In the case of MHK energy, many devices are submerged in the flow. Thus, it is important to analyze the impact of both the support structure and the addition of the rotating blades. The experiments were performed in the small-scale testing platform in the hydraulic flume facility at Bucknell University. For each test case, bed form topology was measured after a three-hour time interval using a traversing two-dimensional bed profiler. During the experiment, scour depth measurements at the front face of the cylindrical support structure were taken to estimate the scour rate. Measurements of the bed form were taken across the width of the test section. Results show that the scour hole dimensions increase in the presence of the MHK device. These dimensions also increase with increasing Reynolds number. [Preview Abstract] |
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