A Novel Arrangement of Multiple Cylinders of Different Structural Conditions Dictating Wind Energy Harvesting at Very Low Reynolds Number

Vortex-induced vibration (VIV) of a single cylinder is different from the multiple cylinders, and energy harvesting from the VIV of multiple cylinders by utilizing the piezoelectric technique is one of the promising techniques. The number of cylinders, spacing between them, and structural condition of the interference cylinder influence the fluid forces on the cylinders, and consequently, the efficiency of the harvested output power is affected.

A two-dimensional unsteady laminar flow condition is coupled with a spring-mass-damper system to analyze the fluid-structure interaction. The influence of multiple cylinders arranged in tandem on the flow characteristics and energy harvesting from VIV are investigated systematically in the present study. The number of cylinders is varied as N = 1, 2, 3, and 4 and the spacing between the tandem cylinders is varied as, S^* (S/D, D is the cylinder diameter) = 3, 6, 9. Also, a wide combination of fixed and/or freely vibrating structural conditions are considered. The incoming flow velocity of wind is kept very low as Reynolds number, Re = 100.

It is observed that, irrespective of the structural condition, critical value of S^* is noticed at which there is no wake undulation. The strength of the vortex increases with increasing S^*, which is significantly dependent on the vibrational condition of the cylinders. The output power harvested from VIV is found to be considerably reliant on spacing as well as on the structural condition. A maximum harvested power of 10.55 mW is achieved at S^* = 9 and N = 4 in the present considered cases when all the cylinders are allowed to vibrate freely. The present results provide knowledge on the application of a set of tandem cylinders combining with vibrational mode for harvesting the power from VIV in a practically engineered nano/micro energy harvesting system, as demanded by the specific application on hand.