Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity.
Tidal turbines are deployed in sites which have elevated levels of free stream turbulence (FST).Accounting for elevated FST on their operation become vital from a design standpoint. Detailed experimental measurements of the dynamic near-wake of a tidal turbine model in elevated FST environments is presented; an active grid turbulence generator developed by our group was used to seed in the elevated FST and evaluate the influence of turbulence intensity (Ti) and inflow integral length scale (L) on the near-wake of the turbine. Three inflow conditions are tested: a quasi-laminar flow with Ti ~ 2.2% and two elevated Ti (~12-14%) cases, one with L ~ 0.4D (D is the turbine diameter) and the other where L~ D. Elevated Ti cases was found to increase the standard deviation of rotor torque by 4.5 times the value in quasi-laminar flow. Energy recovery was also found to be accelerated; at X/D=4, the percentage of inflow energy recovered was 37% and was twice the corresponding value in quasi-laminar flow. Elevated FST was observed to disrupt the rotational character of the wake; the drop in swirl number ranged between 12% at X/D=0.5 to 71% at X/D=4. Elevated Ti also resulted in L that were considerably larger (> 2 times) than the quasi-laminar flow case. An increase in inflow integral length scale (from 0.4D to D) was observed to result in enhanced wake Ti, wake structures and anisotropy; however, no noticeable influence was found on the rate of wake recovery.
The objective of the current experimental work is to investigate the effectiveness of surface protrusion type modifications to a circular cylinder in augmenting fluid induced motions at 3 × 103 < Re < 3 × 104 and falls within the TrSL2 (Transition in Shear Layer) Reynolds number regime. The current experiments build on the previous successful efforts to intentionally enhance oscillations by Bernitsas and group at the University of Michigan, for use in their Fluid Induced Motion based energy harvester Vortex induced vibration for aquatic clean energy (VIVACE) at higher Reynolds number (2–4 × 104 < Re < 1–2 × 105) that falls in the TrSL3 regime. Surface protrusions tested in the current work include three different sandpaper strips of widely varying roughness; and smooth strips that only had a thickness with no embedded roughness. Amplified vortex induced vibrations and galloping oscillations were observed while using cylinder configurations with all strips. The grit size of the sandpapers used also seemed to have an effect on the vibration amplitudes. Different positions of the strip (with respect to the frontal stagnation point) were tested and proved to be of crucial importance as the response showed a drastic variation depending on the position. The variation in response of the cylinder to roughness and strip-position that were observed in the current experiments are different from the experiments performed at TrSL3 regime; thereby suggesting a strong Reynolds number effect. The power harnessing potential of VIV based devices with different surface-protrusion configurations was evaluated based on the experimental results. There appears to be a very conspicuous difference in potential of the energy available especially while executing galloping oscillations. Experiments were also repeated with springs of different stiffnesses which proved to have an effect on the incited galloping oscillations depending on the type of strip employed.
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