Vortex shedding occurs when fluid flows past a blunt surface. This often results in a vortex-induced vibration due to equal frequency between that of vortex shedding and natural frequency of the body in contact. The vortex-induced vibration serves as a source of a renewable energy when certain converters and transducer mechanisms are added for electricity generations.
In order to increase the efficiency of the converters, the aerodynamic efficiency of the blade converters and the angular velocity of the transducers is an important factor that needs to be considered.
Previous research has been implemented in view of increasing the energy conversion efficiency. However, their analyses were conducted based on a non-factual low Reynolds number specifying that energy conversion efficiency increases as the Reynolds number increases. It is therefore important to carry out investigations at higher Reynolds number which relates to real-life situations and confirm that the efficiency of energy conversion increase at higher Reynolds number.
Professor Yoshiki Nishi and Hamid Arionfard from Yokohama National University in Japan evaluated ways whereby uses of a vortex-induced vibration of a pivoted cylinder converter can be enhanced realistically by increasing its angular velocity and aerodynamic efficiency over a high range of Reynolds number. The work published now in Journal of Fluids and Structures included experimental measurement and numerical analysis on the effects of pivot point placement, arm length ratio and natural frequency on the pivoted cylinder converter.
The numerical, analytical model involved the study of drag effects on the cylinder converter by balancing of moments with respect to the pivot point either at the upstream or downstream position, simulating a vortex-induced vibration. The generated maximum power output was also derived, and the natural frequency of the system serves as a function of both flow velocity and its structural properties. The authors also observed occurrence of large vibrations which happens during the lock-in as a result of closeness between the vortex shedding frequency and natural frequency of the structure.
When the pivot was located at the downstream, experimental results indicated that the lock-in range increases with respect to increase in natural frequency while at the upstream, the range was smaller compared to the former. From the numerical results, they found that when the pivot is located at the upstream, the drag force induced a moment, which steadies the cylinder converter as flow velocity increases, while the drag force enhanced the motion when the pivot was located downstream. An intense increase in generating power was noticed when the pivot was located at the downstream.
Due to the fact that the energy conversion rate was limited by certain instability induced by a drag force, higher values of flow velocity beyond the limits resulted in a decrease in efficiency of the system and as a result, the wider Reynolds number doesn’t essentially increase the efficiency of the system.
The authors further investigated the effects of arm length on the vortex-induced vibration of the pivoted cylinder converter at a certain stiffness of spring while the reduced velocity served as a function of the flow velocity. The maximum generated power was found at a wider range of reduced velocities and narrow range of arm length when the pivot was located at the downstream. At the upstream pivots, a reduced maximum generated power was noticed at a narrow range of reduced velocities and wide range of arm length. An optimum value of arm length was discovered at the downstream and upstream, however, the former was lower.
This study was able to provide suggestions at which actual performance of a vortex-induced vibration converter could be enhanced coupled with its efficiency at higher Reynolds number.
Arionfard, H., Nishi, Y. Experimental Investigation of a Drag Assisted Vortex-Induced Vibration Energy Converter, Journal of Fluids and Structures 68 (2017) 48–57.
Department of Systems Design for Ocean-Space, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Japan.Go To Journal of Fluids and Structures