Energy Extraction from a Steady Flow Using Vortex Induced Vibration.

Abstract

Ocean renewable energy is abundant and worldwide available in the form of waves, currents, tides, thermal gradient, and salinity gradient. The VIVACE Converter (Vortex Induced Vibration Aquatic Clean Energy) is introduced to extract energy from a steady flow using Vortex Induced Vibrations (VIV). In our early experiments on energy harnessing using VIVACE, energy density of 51W/m3 was achieved. Scalability of VIV and the VIVACE Converter makes it possible to harness energy even from low current speeds. Our experiments were performed in the TrSL3 regime (2Xl04- 4Xl04 < Re < lXl05- 2Xl05). This regime had not been explored before. A strong dependence of VIV on Reynolds number was observed. In the TrSL3 regime, amplitude ratio (A/D) of 1.9 was achieved for a smooth cylinder in VIV even with high damping imposed. Our VIV tests fall in the tail-end of the Griffin-plot. A/D achieved and maintained are 2-3 times higher than previously measured data. Our experiments showed that proximity of the VIVACE Converter to free surface reduces the range of synchronization of VIV and the A/D. Proximity to free surface introduced hysteresis due to flipping between wake states. At low gap ratio hysterisis was observed at both ends of the synchronization range. At a critical value of the Froude number, VIV is suppressed and it is attributed to change in stability of vortex shedding. At high Froude numbers and low gap ratios, the vortex formation transforms from absolute to convective instability. Our experiments showed that proximity of the VIVACE Converter to bottom boundary reduced the A/D for intermediate gap ratio (0.6 < G/D <3.0), but the range of synchronization remained unaffected. For low gap ratios (G/D < 0.6), the positive A/D reached a value of 2. Surface roughness of predetermined distribution and magnitude was added to the surface of the VIVACE cylinder in VIV. The added roughness, when designed and implemented appropriately, affects in a predetermined way the boundary layer, the separation point, the wake, the drag and lift forces, and consequently the VIV. In our experiments properly designed surface roughness increased the range of synchronization and A/D (2.7) of VIV.