Abstract
In recent years, an increasing number of breakthroughs have been made in the field of small-scale wind energy harvesting, where specialized materials are utilized to convert flow energy into electric power. Several studies on this power extraction rely on a common energy harvester setup in which a stiff cantilever beam is attached to the trailing edge of a miniature bluff body. At these small scales where boundary layer effects are appreciable in the laminar flow regime, periodic vortex shedding can be used to drive transverse vibrations in the beam. Interestingly, the fluid dynamics involved in this unsteady process have been studied for decades not to exploit their characteristics, but instead to eliminate potentially destructive effects. As a result, there is still much room for improvement and expansion on recent design studies. A study of how subtle changes in bluff body trailing edge geometry effect power output of a model will be presented in this paper. The model under consideration consists of a miniature bluff body on the order of tens of millimeters in diameter, to which a piezoelectric cantilever is attached at the trailing edge. This model is specifically designed for laminar to transitional Reynolds Number flows (500−2800) where the periodicity of vortex shedding approaches the natural frequency of the beam. As the flow speed is further increased, the effect of lock-in occurs where the resonant beam motion resists a change in vortex shedding frequency. Vibration amplitudes of the beam reach a maximum under this condition, thus maximizing power generation efficiency of the system and providing an optimal condition to operate the harvester. In an effort to meaningfully compare the results, a number of dimensionless parameters are employed. The influence of parameters such as beam length and natural frequency, fluid flow speed, and trailing edge geometry are studied utilizing COMSOL Multiphysics laminar, fluid-structure interaction simulations in order to create design guidelines for an improved energy harvester.
