Flow-Structure-Interactions

Tapered Cylinder VIV & Energy Harvesting Eel (scroll down)

Piezoelectric Eels for Energy Harvesting in the Ocean H. Techet; J. J. Allen & A. J. Smits  Summary To perform long endurance military missions and power instruments at offshore ocean observatories, small, unattended sensor packages must generate and harvest power from their surroundings. The experiments performed at Princeton University with Prof. Lex Smits investigated the use of piezoelectric polymers as power generation devices in the wake of bluff bodies. Thin flexible piezoelectric membranes, or “eels,” were mounted aft of a rectangular bluff body and are excited by vortex shedding in the wake of the body. This flapping motion generated strain energy in the material that could be converted to electric power and stored in a battery to power small sensors and an acoustic modem. Multiple eels were stacked vertically behind a single bluff body and tested at various flow speeds. Experiments show a range of flow regimes, ranging from poorly coupled motions, where three-dimensionality in the vortex shedding is important, to an optimally coupled state, where the membranes oscillate at the natural frequency of the undisturbed wake. The effects of membrane length were also studied. Experiments revealed that a stack of eels will also lock-in with the vortex shedding frequency. This synchronized motion was easily disrupted if one single eel falls out of lock, twists, or sags near the tail; once disrupted the entire stack would stop flapping momentarily. The appearance of oblique shedding was noted to disrupt the stack as well. In addition to the behavior of the eel stack, the effect of the length of a single eel was investigated. The longest eels tended to have a smaller radius of curvature near the head, believed to be caused by the added weight and overall resistance on the membrane. This reduction in curvature resulted in a significantly lower strain energy density and maximum available strain energy. Decreasing length also tended to reduce flapping intermittency.    Images Clockwise from Top Left: Side view photo of an array of four (4) eels behind a bluff body (note the lack of synchronization between eels); Two time-step snapshots of the eel array viewed from the top; Square of the stain in the membranes over the length of the eel (strain was calculated from the radius of curvature and thickness of the material. Three different length eels are shown: 0.6 m, 0.91 m, and 1.2 m. The shortest eel contains the highest strain energy density in the head region) References Techet, A. H., J. J. Allen, and A. J. Smits, “Piezoelectric Eels for Energy Harvesting in the Ocean,” Proceedings of the 12th (2002) International Offshore and Polar Engineering Conference, Kitakyushu, Japan, v. II, pp. 713-718, May 25-31, 2002. Allen, J. J., A. H. Techet, R. M. Kelso, and A. J. Smits, “Energy Harvesting Eel,” 14th Australasian Fluid Mechanics Conference, Adelaide University, Adelaide, Australia, 10-14 December 2001.http://www.princeton.eduhttp://www.princeton.edu/mae/people/faculty/smits/Publications.htmlshapeimage_2_link_0shapeimage_2_link_1shapeimage_2_link_2
[scroll down for more information on Energy Harvesting Eels] Vortical patterns behind a tapered cylinder oscillating transversely to a uniform flow A. H. Techet & M. S. Triantafyllou Summary Visualization studies of the flow behind an oscillating tapered cylinder are performed at Reynolds numbers from 400 to 1500. The cylinder has taper ratio 40:1 and is moving at constant forward speed U while being forced to oscillate harmonically in the transverse direction. It is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span. Within certain frequency ranges a single mode dominates in the wake, consisting of shedding along the entire span of either two vortices per cycle (`2S' mode), or four vortices per cycle (`2P' mode); but within specific parametric ranges a hybrid mode is observed, consisting of a `2S' pattern along the part of the span with the larger diameter and a `2P' pattern along the part of the span with the smaller diameter. A distinct vortex split connects the two patterns which are phase-locked and have the same frequency. The hybrid mode is periodic, unlike vortex dislocations, and the location of the vortex split remains stable and repeatable, within one to two diameters, depending on the amplitude and frequency of oscillation and the Reynolds number.   [Left] Electrolytic precipitation visualization (Techet, et. al, 1998) of an oscillating tapered cylinder (40:1 taper ratio) compared to [right] numerical simulations performed at Brown University for a uniform cylinder in linear shear flow.   [Left] Particle Imaging Velocimetry (PIV) visualization (Techet, et. al, 1998) of an oscillating tapered cylinder (40:1 taper ratio) at two cross-sectional planes (z/d = 7.9 and 22.9) show distinct differences in shedding patterns at either end of the cylinder as a function of changing a/d with diameter. Red vorticity is counter-clockwise, blue is clockwise. [Right] A 3D model of the Techet ‘hybrid’ vortex shedding mode. References Hover, F. S., A. H. Techet, and M. S. Triantafyllou, “Forces on Oscillating Uniform and Tapered Cylinders in Cross Flow,” Journal of Fluid Mechanics, vol. 363, pp. 97-114, May 1998. Techet, A. H., F. S. Hover, and M. S. Triantafyllou, “Vortical Patterns Behind a Tapered Cylinder Oscillating Transversely to a Uniform Flow,” Journal of Fluid Mechanics, v. 363, pp. 79-96, May 1998. Publications.htmlshapeimage_3_link_0