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An Experimental Investigation of a Passively Flapping Foil in Energy Harvesting Mode
Author(s): Muhammad Nafees Mumtaz Qadri, Aamer Shahzad, Fuwang Zhao, Hui Tang
Keywords: Fluid-Structure Interaction, Bio-inspired Energy Harvesting, Leading Edge Vortex, Flow-induced passive flapping motion
Energy extraction through flapping foils is a new concept in the domain of renewable energy, especially when the system is fully driven by incoming free-stream flow, a phenomenon known as flow-induced vibration. To investigate this concept, a water tunnel test-rig was designed and fabricated, where the foil performs two-degrees of freedom pitch and plunge motion under the influence of incoming water flow. For this study a power-takeoff system was not introduced, hence energy harvesting performance was evaluated through real-time force and motion measurements with the help of sensors. A flat plate foil made from plexiglass was used and during our preliminary testing it was found that it didn’t perform self-sustained flapping motion unless the free-stream velocity reached a certain value, known as the cut-off velocity, which for this test-rig is approximately 0.40 m/s. When equipped with sensors, the cut-off velocity increased to approximately 0.50 m/s. To support these self-sustained flapping motions inertial mass blocks were placed to provide the necessary inertia especially when the flat plate foil performed the pitching or stroke reversal action. Different inertial mass units (mib = 0.45, 0.90 & 1.35 kg/block) were tested to analyze their effect on the flat plate foil kinematics and its energy harvesting performance. Other parameters such as pitching amplitude (θo = 30°, 43° & 60°) and free-stream velocity (U∞ = 0.57 m/s, 0.65 m/s and 0.78 m/s) were varied at fixed pivot location (xp = 0.65 chords (c)) to augment the varying inertial mass unit study. In the first section at fixed inertial mass (mib) of 0.45 kg/block and xp = 0.65c from leading edge, energy harvesting performance (C̅p & η) was observed to increase linearly with increase in pitching amplitude, while it degraded as the free-stream velocity increased. Best energy extraction performance with mib = 0.45 kg/block was achieved at θo = 60° and U∞ = 0.57 m/s with 52.5% efficiency (η) at C̅p of 1.124. In the second part, comparative analysis between different inertial mass blocks as a mechanical load is discussed and how it affects the energy harvesting performance of the test rig. The test-rig was sensitive to changes in inertial loads and affected the force-motion synchronization which affected the energy harvesting performance. The analysis discussed in this second part was done at θo = 60° and U∞ = 0.57 m/s since best performance was achieved at these parameters in first part while keeping the inertial mass fixed.