Constructive foil-wake Interactions of tandem flapping hydrofoils for propulsion and energy harvesting

• Main Author: Wang, Guangjian
• Other Authors: Ng Bing Feng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/169877
• Institution: Nanyang Technological University

Bio-inspired flapping hydrofoils have emerged as a promising solution for propulsion and energy harvesting due to their high efficiency and minimal interferences to the environment. A tandem flapping configuration with a forefoil upstream of a hindfoil in a short spatial arrangement can achieve augmented force production and outperform that of two isolated single-foils, as the hindfoil can attain additional thrust and lift by interacting with the shed vortices from the forefoil. Presently, the developments of high-efficient flapping systems are centered on how to leverage the constructive effects of such interactions, which are termed constructive foil-wake interactions, to enhance force production. Existing studies demonstrate that the foil-wake interactions are sensitive to the relative positioning of the two foils, e. g. the phase angle, inter-foil spacing, flexibility, etc. By properly tuning the foil kinematics, the hindfoil can capture a portion of the vortices which rotate in the same direction as its leading edge vortices (LEV), thus achieving constructive foil-wake interactions for augmenting the force production. Albeit the prominence of tandem hydrofoils, the optimal tandem configurations for propulsion and energy harvesting are still less understood, due to the complicated unsteady flow characteristics. Thus, the objectives of the present study are to investigate the underlying dynamics of foil-wake interactions for tandem flapping hydrofoils, and to further maximize their performances in propulsion and energy harvesting. The underlying physics of foil-wake interactions on tandem hydrofoils are investigated to grasp a fundamental knowledge of the constructive foil-wake interactions. Specifically, a pair of elliptical hydrofoils with a Strouhal number of 0.32 and a Reynolds number of 5000 is introduced in a tandem configuration. The phase angle between the fore and hindfoils, which ranges between -180° and 180°, is considered the primary kinematic parameter of interest that affects the foil-wake interactions. Therefore, unsteady incompressible Navier-Stokes equations are solved directly to predict the force production on the hydrofoils and to investigate the associated physics. Meanwhile, experimental validations are performed using digital particle image velocimetry (DPIV) in water tunnel experiments. The numerical and experimental outputs demonstrate consistent results with existing studies, where the foil-wake interactions can remarkably affect the force production on tandem hydrofoils. Moreover, in-phase flapping with a phase angle of 0° enables the hydrofoils to achieve the highest force production, while out-of-phase with a phase angle of 180° the lowest, as compared to other scenarios. Moreover, enlarging the magnitude of the phase angle can result in a reduction in thrust production. To leverage the constructive foil-wake interactions for maximizing the force production on tandem hydrofoils, time-asymmetric flapping kinematics with unequal up and downstroke durations are thereafter proposed in the present study for high-efficient propulsion. Specifically, an asymmetry ratio, ranging from 0 to 0.4, is introduced to quantify the degree of the stroke time-asymmetry and to serve as the primary kinematic parameter that affects the hydrofoil performances. The effects of stroke time-asymmetry are investigated at three phase modes, namely the 0° in-phase flapping, the 180° counterstroke flapping, and the 90° hindfoil-leading flapping. Likewise, unsteady Navier-Stokes equations are solved and DPIV experiments are conducted to predict the force production and the flow structures of the tandem hydrofoils. The results suggest that the propulsive and lift performances of the tandem hydrofoils can be remarkably augmented by the introduction of the stroke time-asymmetry at proper phase angles. The counterstroke flapping mode enables the tandem hydrofoils to achieve a noticeable enhancement in thrust production by 2.5 times, while the in-phase flapping mode results in a 15% increment, as the asymmetry ratio increases from 0 to 0.4. Such enhancements are achieved through the changes in hydrofoil flapping velocities and foil-wake interactions between the unequal up and downstrokes. These findings not only provide insights into the characteristics of tandem flapping hydrofoils which are operated in non-sinusoidal stroke cycles but also offer a reference to the design of efficient foil kinematics for high-performance biomimetic propulsors. Although the force production of tandem hydrofoils undergoing time-asymmetric flapping is investigated numerically and experimentally, the computational burdens for directly solving the unsteady Navier-Stokes equations are explicitly high. Therefore, a lightweight neural network is trained as a reliable and efficient surrogate modeling to significantly reduce the computational burdens. Specifically, the instantaneous force coefficients of the tandem hydrofoils at multiple asymmetry ratios are simulated as the input data. Subsequently, a neural network is trained to approximate the thrust and lift of the hydrofoils at any given asymmetry ratio. The predicted results indicate that the deep learning model can attain nearly identical thrust and lift coefficients to the computational fluid dynamics modeling with much higher computational efficiency. Aside from propulsion, tandem hydrofoils have also been adopted to harvest hydrokinetic energy from fluid flows. In order to maximize their power production and economic viability, the hydrofoils need to be densely populated which however poses interference challenges that affect their energy harvesting performances. To overcome this, an efficient tandem-hydrofoil based closely interconnected tidal array is explored and proposed in the present study, where the fore and hindfoils are closely spaced and interconnected to enhance the energy extraction and reduce the levelized cost of electricity. To quantify the performances of the proposed tidal array, the power production, energy efficiency, capacity density, and levelized cost of electricity are obtained using a comprehensive approach that combines water tunnel experiments and computational fluid dynamics modeling. In particular, water tunnel experiments are conducted to measure the heaving and pitching displacements of the hydrofoils for power production. Unsteady Reynolds-Averaged Navier-Stokes equations are solved using the finite volume method to obtain the force production on hydrofoils. The results demonstrate that the capacity density of the proposed tandem configuration achieves at least 3 times higher than those of existing studies, while the hydrofoil deployment density is 4 times higher. Moreover, the energy efficiency and capacity density can be enhanced by enlarging the forefoil pitching amplitude or shortening the inter-foil spacing. Furthermore, the levelized cost of electricity is minimized to 50% that of existing studies, achieved by increasing the energy capture and reducing the capital expenditures, thereby maximizing the commercial potential of hydrofoil-based tidal arrays. In conclusion, the foil-wake interactions of tandem hydrofoils are investigated experimentally and numerically in the present study. Moreover, time-asymmetric flapping kinematics and closely interconnected tandem configurations are proposed to enhance the propulsive and energy harvesting performances of tandem hydrofoils, respectively.