The inherent randomness and non-tunability associated with turbulent flow usually make it a poor candidate as an energy source for piezoelectric harvesters. We have shown in prior work that when the turbulence is generated by a grid, however, there are certain predictable relationships between the power output and the turbulent flow that could potentially make the deployment of piezoelectric harvesters in such settings viable. In this paper, we look at the voltages recorded from harvesters placed in turbulence generated by passive rectangular, fractal I, fractal cross, semi-passive and active grids and statistically characterize the coherent “beats” present in these “random” signals. The effects of beam distance from the grid, average flow velocity, grid type and separation between multiple beams have been considered in this analysis. An important conclusion from this work is that the beat duration distribution pattern of a beam that is close to or has entered into flutter vibration is clearly identifiable, potentially providing an opportunity to use piezoelectric materials as sensors to detect flutter.
Turbulence-induced vibration, particularly one generated by a fractal grid, has not been studied in much detail in the literature. This study focuses on the interaction of two side-by-side piezoelectric energy harvesters in fractal grid-generated turbulence. Three fractal grid patterns have been considered: “I”, square and cross. For each grid pattern, a full-factorial study of four important parameters has been considered: (i) Beam lengths and configurations, (ii) Mean flow velocity, (iii) Distance of the beams from the grids and (iv) the separation between the two beams. Experimental results show that all three fractal grids allowed for a significantly larger power output from the piezoelectric beams compared to a passive rectangular grid with a similar blockage ratio that was previously studied.
While the majority of the literature in energy harvesting is dedicated to resonant harvesters, non-resonant harvesters, especially those that use turbulence-induced vibration to generate energy, have not been studied in as much detail primarily due to their comparatively small power output, general non-tunability and difficulty in associating flow conditions to harvester behavior. In this extensive study, we look at the behavior of piezoelectric cantilever beams in different types of grid turbulence with the intention of identifying trends in the harvester output. Our results show that the power-law decay of the harvester output that had previously been observed for rectangular grids holds for a wide variety of fractal grids as well. Additionally, experimental data shows that the average harvester output in grid turbulence follows a power-law growth with respect to the mean flow velocity for relative short and stiff piezoelectric beams.
While the vast majority of the literature in energy harvesting is dedicated to resonant harvesters, non-resonant harvesters, especially those that use turbulence-induced vibration to generate energy, have not been studied in as much detail. This is especially true for grid-generated turbulence. In this paper, the interaction of two side-by-side fluidic harvesters from a passive fractal grid-generated turbulent flow is considered. The fractal grid has been shown to significantly increase the turbulence generated in the flow which is the source of the vibration of the piezoelectric beams. In this experimental study, the influence of four parameters has been investigated: Beam lengths and configurations, mean flow velocity, distance from the grid and gap between the two beams. Experimental results show that the piezoelectric harvesters in fractal grid turbulence are capable of producing at least the same amount of power as those placed in passive rectangular grids with a larger pressure loss, allowing for a potentially significant increase in the efficiency of the energy conversion process, even though more experiments are required to study the behavior of the beams in homogeneous, fractal grid-generated turbulence.
Resonant fluidic harvesters can typically be tuned to the frequency of the flow, so they yield a larger power output compared to their non-resonant counterparts. In order to explore increasing this output for non-resonance harvesters, a feasibility study has been performed to analyze the behavior of two side-by-side piezoelectric harvesters in low-intensity (less than 0.5%) grid-generated turbulence with respect to beam configurations, mean flow velocity, distance from the grid and separation between the two beams. Experimental results show that the potential for energy harvesting is perhaps not as great in the low mean-velocity flow as it is for the higher speed cases which are accompanied by flutter, but the side-by-side piezoelectric beams display potential for use as turbulence sensors at low speeds.
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