Oscillating foil turbines can be utilized to extract hydrokinetic energy from tidal or river flows. When foils are placed in arrays, the reduced velocity between foils and the unsteady disturbances associated with the leading foil motion both affect the performance of downstream foils. To compare the performance between foils, a wide range of kinematics is numerically explored in a two-foil tandem configuration with matching strokes, but varying the inter-foil phase angle and spacing. The effects of the wake on the trailing foil performance are quantified by evaluating the difference between the normalized power extracted by each foil. The difference in normalized power extraction is a function of the wake phase parameter, Φ, and ranges from -65% to +6%, depending on the kinematic regime. It is also determined that the difference in normalized power is dominated by the pressure contribution from the heave stroke, whereas the viscous components are negligible. In general, these differences illustrate the unsteady effects within the wake of the first foil, and the various interaction modes of the downstream foil. These trends can be used to estimate power in other array configurations and provide a more robust model for wake-foil interactions for energy harvesting.
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Bistable Physical Geometries for Terahertz Plasmonic Structures Using Shape Memory Alloys
Shape memory alloy foils that are appropriately patterned are cycled between two different metal foil geometries resulting in two different terahertz (THz) plasmonic responses. This is accomplished by using patterned foils of a nickel–titanium alloy (Nitinol) that switches between the martensite phase below 31 °C, yielding one physical geometry, and the austenite phase, when the foil is heated above 51 °C, yielding a second physical geometry. In order to enable this reproducible switching, the sample is initially put through a two‐way training procedure, through which the two different desired physical geometries are imprinted. Specifically, the metal foils are trained to switch between a sinusoidal corrugation, either 1D or 2D, at close to room temperature and a flat metal sheet above the austenite phase transition temperature. The foils are found to switch reproducibly between geometries over at least 100 thermal cycles. Using THz time‐domain spectroscopy, the transmission properties of the foils are measured as a function of incident polarization and foil geometry. The changes in spectrum are explained qualitatively and through numerical simulation.
more » « less- NSF-PAR ID:
- 10034326
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Optical Materials
- Volume:
- 5
- Issue:
- 7
- ISSN:
- 2195-1071
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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