The Train of Frozen Boxcars (TFB) model has been developed for a continuous piezoelectric cantilever fluidic harvester to simplify the effective one-way interaction between the fluid and the structure for certain flows. The TFB model treats the force due to vortex or turbulent flow as a series of boxcars of different amplitudes, widths and separations advected with a constant velocity over a piezoelectric beam. In this paper, the effect of five parameters, namely the number, amplitude, width, spatial separation and advection speed of the boxcars in the TFB forcing model, is studied for four different forcing scenarios. It has been observed that an increase in the amplitude or advection velocity of the boxcars leads to an increase in the power output, whereas a saturation limit in the power output is observed with an increase in the width or number of boxcars. More importantly, however, it is concluded that the separation between boxcars is the determining factor in maximizing or minimizing the power output from the harvester.
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.
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.