## 1 Introduction
This force behaves as a damped harmonic oscillator that broadens the lock-in range [13]. Another study proposed a tuning system for VIV BWTs, featuring a secondary mass added to the main system to broaden the lock-in range. This dual-mass system functions as a tuned mass damper, thus widening the lock-in range [12]. Additionally, in [14], a tuning system that gradually adjusts the spring stiffness in response to changing wind speeds was developed, enabling the BWT system to maintain resonance over a wider range of conditions. This approach significantly enhanced the power generation efficiency of the system by adapting to varying wind speeds and ensuring extended resonance periods. This study takes a simpler but effective approach to designing a resonance-tuning system for VIV BWTs. The proposed resonance-tuning module uses a variable-stiffness smart material to adaptively control the system's natural frequency in response to varying wind speeds. There are several candidates for smart materials to adjust the stiffness. Shape-memory alloys (SMAs) are a widely applied smart material for stiffness control. However, these alloys are extremely rigid, and their response time is relatively long [15]. Magnetorheological elastomers (MREs) are flexible and elastic smart materials with the property of stiffness variation. MREs are elastomers that consist of a polymeric matrix with embedded micro-sized ferromagnetic particles. Under the influence of an external magnetic field, the iron particles align in a chain formation, causing the overall elastic modulus to increase [16; 17]. Furthermore, MREs have a relatively short response time for a stiffness change of a few milliseconds [18]. MREs are often applied to tune the resonant frequency of vibration systems, as reported in the literature [19; 20; 21; 22]. An MRE-based TVA was designed for adaptive vibration suppression by controlling MRE stiffness with permanent magnets [22]. As this material is flexible and can be designed in any shape, it can be easily integrated into existing BWT structures. Therefore, this study aims to design an MRE-based on-off resonance-tuning module for small-scale BWTs that can improve the overall energy generation efficiency by widening the operating wind speeds. The proposed resonance-tuning module has two shift modes: the low-stiffness (Low-K) mode and high-stiffness (High-K) mode. For a light breeze of wind, the BWT is tuned with the Low-K mode. When the wind begins to increase the drift from the structural resonance, the BWT is shifted to the High-K mode to increase the structural resonance of the BWT to match the new vortex frequency and maintain the lock-in vibration. The main contributions of this study are as follows:  
1. This study proposes a novel blackeless wind turbine that can shift the structural resonance to operate at a wider range of wind speeds to improve the effective power generation region. 2. This study designed an on-off resonance-tuning module based on a variable-stiffness smart material that can effectively shift the BWT resonance by 60%. 3. A series of experiments validated the widening of the operation range with the Low-K and High-K modes, which improved power generation efficiency by as much as 1.9 times for various wind speeds. This paper is structured as follows. In Section 2, a brief introduction to a cylinder-type BWT explains the basic principle of vortex-induced vibrations. The hardware design of the proposed system and the system modeling are presented in Sections 3 and 4. A series of experiments are conducted to validate the change in the natural frequency of the system to increase the lock-in vibration in Section 5. Finally, the conclusion is given in Section 6.