Optimal frequency for flow energy harvesting of a flapping foil
Optimal frequency for flow energy harvesting of a flapping foil
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DOI：
10.1017/s0022112011000334
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发表时间：
2011-03
影响因子：
3.7
通讯作者：
Q. Zhu
中科院分区：
文献类型：
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作者：
Q. Zhu
受翼桨推进效率与尾流稳定性之间的相关性（导致推进的最佳Strouhal数）的启发，数值模拟了升力/俯仰翼桨能量收集区，并研究了尾流稳定性与能量收集效率之间的关系。基流的计算采用Navier-Stokes算法，稳定性分析通过Orr-Sommerfeld方程进行。发现尾流是对流不稳定的，并确定了最不稳定模式fw的频率。当fw ~ f与系统的最大能量收集效率（f为翼型振荡频率）一致时，表明流动能量提取与尾流的有效演化密切相关。这发生在f ~ 0.15的频率（f由弦长和流速归一化），条件是在足够大的有效迎角下，前缘有明显的旋涡脱落。实际上，这种“翼-尾流共振”通常与前缘的多涡脱落有关。此外，从升沉和俯仰运动的能量提取的详细检查表明，在最佳性能点附近，从俯仰运动提取的平均能量接近于零。这表明通过我们之前提出的简单的完全被动系统实现高效能量收集的可行性，其中不需要激活。
Inspired by the correlation between the propulsion efficiency of a flapping foil propeller and stability of the wake behind it (which leads to the optimal Strouhal number for propulsion), we numerically simulated a heaving/pitching foil in energy harvesting regime, and investigated the relation between wake stability and the energy harvesting efficiency. The base flow is computed using a Navier–Stokes algorithm and the stability analysis is performed via the Orr–Sommerfeld equation. The wake is found to be convectively unstable and the frequency of the most unstable mode fw is determined. The case when fw ~ f coincides with maximum energy harvesting efficiency of the system (f is the frequency of foil oscillation), suggesting that flow energy extraction is closely related to efficient evolution of the wake. This occurs at a frequency of f ~ 0.15 (f is normalized by the chord length and the flow speed), under the constraint that there is significant vortex shedding from the leading edge at sufficiently large effective angles of attack. Indeed, this ‘foil–wake resonance’ is usually associated with multi-vortex shedding from the leading edge. Furthermore, detailed examination of energy extractions from the heaving and the pitching motions indicates that near the optimal performance point the average energy extraction from the pitching motion is close to zero. This suggests the feasibility of achieving high-efficient energy harvesting through a simple fully passive system we proposed earlier in which no activation is needed.