A submerged, neutrally buoyant cylinder may be an attractive option for a wave energy converter design due to its potential for highly efficient energy capture, advantageous power scaling relative to its length, and potential to weather storms by submerging to safer depths. Additionally, it serves as an ideal testbed to better understand heave and surge forcing interactions on wave energy systems. Understanding the hydrodynamic interactions between the cylinder and incoming waves can lead to better modeling and control design and in turn more efficient energy absorption. This paper presents the design, implementation, and experimental results of a submerged cylinder wave energy experimental testbed made using a two degree of freedom gantry (T-bot) in surge and heave controlled by coupled motors that are programmed to apply desired forces on the cylinder. We demonstrate the efficacy of the approach by applying forces to emulate mechanical springs and dampers with user specified spring and damping coefficients. System identification of the gantry is used to produce a feedback controller that counteracts the gantry friction in addition to applying the virtual spring and damping forces. Experiments in a 116 m wave tank with waves of amplitude up to 31.75 mm and periods between 1 s and 2.5 s show agreement with linear model predictions, experiments with a passive spring oscillator system, and WEC-Sim simulations. The results provide the groundwork for future rapid prototyping of advanced control designs and further study of the near-cylinder hydrodynamic interactions that drive energy absorption of wave energy converters.

Flow field results are presented for the near-wake of an axial-flow hydrokinetic turbine in the presence of surface gravity waves. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine based on the U.S. Department of Energy's Reference Model 1 tidal current turbine. Measurements were obtained in the large towing tank facility at the U.S. Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio selected to provide maximum power. The turbine has been shown to be nearly scale independent for these conditions. The selected wave form was intended to represent oceanic swell encountered off the U.S. eastern seaboard. The resulting model wave is a deep water wave, in terms of relative depth, traveling with the "current", in the opposite direction of the towing carriage. Velocity measurements were obtained using a submersible, planar particle image velocimetry (PIV) system at streamwise distances of up to two diameters downstream of the rotor plane. PIV ensembles were obtained for phase locked conditions with the reference blade at the horizontal position. Phase averaged results for no-wave and wave conditions are presented for comparison showing further expansion of the wake and shear layer in the presence of waves as compared to the no-wave case. When ensembles are selectively sampled on the wave phase, a high degree of coherency is shown to remain and the wake width is shown to undulate with the passing of the wave, with the vertical displacement range on the same order as that of a particle under similar conditions. The impact of waves on turbine tip vortex helical structure is also examined. Waves are shown to change the location of adjacent helices. In the streamwise direction, this changes the pitch of the helix, shown in previous studies to affect the downstream wake recovery distance. In the vertical direction, depending on the wave-induced flow field at the time they were created, vortices are forced outward, into the mean flow or inward, into the wake core, potentially enhancing kinetic energy transport and accelerating the re-energization process. Published by Elsevier Ltd.

Three-dimensional particle image velocimetry (PIV) experiments were conducted in the immediate near wake and up to seven diameters downstream of a three-bladed marine propeller model operating in two different inflow conditions: one with imposed freestream turbulence with intensity of 7% and streamwise integral length scale comparable to propeller geometry, and the second experiment with a quiescent inflow conditions as a reference. The resulting Reynolds number based on propeller chord and relative velocity is Re0.7R = 4.7 x 10(5). All components of radial transport of mean flow kinetic energy are analyzed and the largest contributor to the fluxes is found to be correlated to Reynolds shear stresses, resulting in radially outward flux in the wake. Two regions of the near wake are distinguishable with downstream extent dependent on the level of external turbulence. In the first region, immediately behind the propeller, shed tip vortices are very coherent and undergo grouping and roll-up around each other and the second region where the vortex merger process is complete and characterized by breakdown of vortices into small-scale turbulence. The latter region was found to occur earlier in the experiment with external turbulence. Conditional statistics of velocity fluctuations were employed and they show that outward interactions and sweep events contribute the most to the transfer of mean flow kinetic energy from the inner wake to the freestream.

Flow field results are presented for the near-wake of an axial flow hydrokinetic turbine in quiescent flow conditions. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine modeled after the Sandia National Laboratory Reference Model 1 Tidal Current Turbine. All measurements were obtained in the large towing tank facility at the United States Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio corresponding to maximum power production. The turbine is scale independent with respect to lift and very slightly dependent with respect to drag for these conditions (Re-c@0.7R approximate to 4 x 10(5)). The wake velocity field data was obtained using a two-dimensional particle image velocimetry (PIV) system. PIV ensembles were obtained for phase locked conditions. This paper focuses on characterizing the velocity and the mean flow structure in the near wake. Specifically, the downstream evolution of coherent tip vortices shed by the rotor blades were examined. Vortex aperiodicity was shown to increase with downstream distance. The streamwise spacing between adjacent vortex cores was shown to be constant within a diameter downstream of the rotor. Further downstream, significant vortex filament interaction was observed, including leapfrogging. This interaction is thought to be the primary mechanism for wake breakdown and re-energization. Published by Elsevier Ltd.

More advanced simulation tools are required to predict the loads experienced by a turbine in situ due to the surrounding environment including the effects of inflow turbulence, the impact of turbines operating upstream, the effect of the free surface, and the impact of surface waves, to name a few. These advanced models require detailed data sets for validation. At this time, few studies have focused on the near wake in an effort to provide such detailed data. It is critical to understand this region because it is where much of the high-energy phenomena occur that is most likely to impact turbine performance and reliability. To this end, a towing-tank particle image velocimetry (PIV) system was designed, built, and used to provide measurements of the flow in the near wake of a scale-independent marine current turbine. Included are preliminary results including a representative case and turbulent statistics for an ensemble of realizations.

Blade Element Momentum (BEM) theory is a well understood and proven method for modeling blade loads and determining steady state performance characteristics of wind turbines. Recently this theory has successfully been applied to horizontal axis marine current turbines when incorporating modifications that lead to better predictions of turbine performance for a range of operating conditions. Relatively little work exists in the implementation of BEM theory in a marine environment with surface gravity waves. A better understanding of the effects of waves on tidal turbines is necessary to predict fatigue loading that can eventually lead to blade failure. This paper presents a BEM numerical model that incorporates the unsteady velocities due to the presence of waves and assesses the effects of waves on tidal turbine performance. Numerical results of the coefficient of power (C-P) and coefficient of thrust (C-T) match closely with experimental results for the mean C-p and C-T for a range of tip speed ratios. The model is also able to predict the instantaneous amplitudes of the performance characteristics as compared with the measured values except for the BEM calculated thrust which shows a 20% reduction in amplitude.

Summer immersive experiences provide students the opportunity to explore the limits of their engineering education and develop a depth in a field of study. For institutions that centrally manage these experiences, ranging from experiments conducted at other academic locations to research and development with industry partners, to procurement and development with government laboratories and program offices, it can be difficult to ensure that all participants are receiving quality experiences. A survey had previously been administered to capture the value of student's summer immersive experience based on ABET Student Outcomes. Much of the data proved inconclusive due to the structure of the questions. However the data was used as a baseline for follow on research and guided the development of future surveys.Following the summer of 2013, a new survey was administered to students majoring in aeronautical, electrical, and mechanical engineering at three different colleges who had participated in institution-sponsored immersive experiences. The goal of the survey was to determine why students chose their summer experiences, what made these experiences successful, and how to improve experiences in the future to maximize return on investment. Success was measured not just in whether they experienced the ABET Student Outcomes (a)-(k) but to what level they were challenged in those domains. The results of the survey will be used next summer to influence which experiences are offered and refine how students are paired with a summer experience.

The impact of blade roughness and biofouling on the performance of a two-bladed horizontal axis marine current turbine was investigated experimentally and numerically. A 0.8 m diameter rotor (1/25th scale) with a NACA 63-618 cross section was tested in a towing tank. The torque, thrust and rotational speed were measured in the range 5 < lambda < 11 (lambda = tip speed ratio). Three different cases were tested: clean blades, artificially fouled blades and roughened blades. The performance of the turbine was predicted using blade element momentum theory and validated using the experimental results. The lift and drag curves necessary for the numerical model were obtained by testing a 2D NACA 63-618 aerofoil in a wind tunnel under clean and roughened conditions. The numerical model predicts the trends that were observed in the experimental data for roughened blades. The artificially fouled blades did not adversely affect turbine performance, as the vast majority of the fouling sheared off. The remaining material improved the performance by delaying stall to higher angles of attack and allowing measurements at lower lambda than were attainable using the clean blades. The turbine performance was adversely affected in the case of roughened blades, with the power coefficient (C-P) versus lambda curve significantly offset below that for the clean case. The maximum C-P for this condition was 0.34, compared to 0.42 for the clean condition. (C) 2013 Elsevier Ltd. All rights reserved.

Near wake measurements on a 0.8 m diameter two-bladed horizontal axis tidal turbine model were obtained with two Nortek Vectrino+ Acoustic Doppler Velocimeters (ADV) sampling simultaneously at 200 Hz. The test matrix covered four radial positions from r/D=0.3 to 0.5 and five axial positions from d/D=0.9 to 0.95. All measurements were performed at the nominal tip speed ratio (TSR) of 7.4. Two different cases were investigated. The first case had steady inflow conditions, i.e. 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 distribution of mean velocities for the steady inflow case exhibit significant spatial variability in the wake region. Mean streamwise velocity show a decrease in the axial direction for all radial locations with magnitudes ranging from 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 inflow turbulence verified by the upstream ADV. For the case with waves, evidence of enhanced turbulence intensities and shear stresses within spatial coverage of the experiment suggest increased localized turbulence production in the blade tip region over the entire near wake region.