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Numerical Investigation of Oscillating Flapping Foils for Energy Harvesting in Freestream and Ground Effect

Numerical Investigation of Oscillating Flapping Foils for Energy Harvesting in Freestream and Ground Effect
Numerical Investigation of Oscillating Flapping Foils for Energy Harvesting in Freestream and Ground Effect
In recent years, as an alternative to rotary wind turbines, flapping foils or oscillating airfoils are under increasingly active investigation to extract energy from wind/water. The aim of this research is to investigate numerically the role of flapping foils on the performance of energy harvesting in freestream and in ground effect. Their potentials for the generation of electric power are studied here using two- and three-dimensional, unsteady Navier-Stokes solver (Ansys Fluent) with a dynamic mesh and sliding interface, in both laminar and turbulent flows. The performance efficiency and mean power coefficient are investigated here for different geometry modifications, namely thickness distribution (NACA0012, NACA0015 and NACA0018) and trailing edge shape modification (sharp, blunt and round) at a low and a high Reynolds numbers (푅푒 = 1100 and 5 × 105) and under different operational parameters, to explore the effect of geometry change on the energy harvesting performance efficiency. The purpose of the truncation process is to eliminate the sharp and steep curvature at the trailing edge portion which may in turn reduces the adverse pressure gradient caused in that area, and thus helps in delaying flow separation. Modifying the airfoil trailing edge and thickness are found to have influence on the lift coefficient, especially for a high Reynolds number, and that the blunt trailing edge has a better efficiency (7% improvement) than the sharp and rounded trailing edges. For all simulations which have been carried out for the 2D single oscillating airfoils, the highest efficiency performance is found to occur at reduced frequencies 푓 ∗ = 0.14 − 0.18. This increase in efficiency is mainly due to the LEVs (leading edge vortices) and a better synchronization between CY and VY (i.e., good timing in the sign switch of CY and VY ). This study is then extended to multiple oscillating bodies to investigate the interaction effect on the power extraction for the two objects in tandem, which are airfoil-airfoil, and cylinder-airfoil. From this study, it is observed that the multiple configurations of airfoil-airfoil interaction (for low inter airfoil distance) can generate up to 30% higher efficiency more than the optimal single oscillating airfoil, and the use of airfoil-airfoil interaction with blunt trailing edge airfoil is found to have a better energy harvesting/efficiency for the two oscillating bodies, with a total efficiency of more than 5% in comparison to the sharp airfoil-airfoil. Finally, for the cylinder-airfoil interaction, the efficiency performance is found to decrease when compared to the two airfoils interaction. Next, a parametric study on the ground effect has been conducted to optimise the power efficiency. The effects of Reynolds numbers at laminar and turbulent flows, the location of airfoil pitching axis, the distance between the airfoil pitching axis and the ground, the amplitude and frequency of oscillation on power extraction by the flapping wing were examined using URANS. From this study, it is found that in all the cases there is an increase in the peak lift coefficient, as height decreases. However, the improvement in power efficiency in ground effect is found to be depended mainly on the perfect synchronization of the heaving velocity and the instantaneous lift that happens at 0.12 < 푓 ∗ < 0.2. For lower and higher reduced frequencies, the increase in lift is not always reflected in a better power efficiency, as the motion of the airfoil and the forces are not well synchronized. Finally, a turbulent case with a higher Reynolds number was also evaluated in ground effect, and it found that an improvement of more than 8% in the energy efficiency was observed in comparison to its freestream case. Finally, the 2D simulation has been extended to the 3D simulation at a low and a high aspect ratios of 3.5 and 7. In addition, the effect of trailing edge geometry variation was investigated. From these results, it is found that the 2D case over-predict the power efficiency as compared to the 3D case, which is about 22% and 10% efficiency drop for aspect ratios of 3.5 and 7, respectively for the laminar case, and this is due to the well-known limits of the 2D models which do not take into account the 3D effects like tip vortex. Increasing the aspect ratio of the wing leads to a higher contribution to efficiency from heaving, as expected. Turbulent flow also shows a similar efficiency drop in both aspect ratios. In addition, the effect of the aspect ratio on efficiency from pitching is found to be negligible for laminar flow but quite significant for turbulent flow. The 3D geometrical shape variation has also been investigated, where the results of NACA0015 sharp trailing edge has been compared to NACA0018 blunt trailing edge in both laminar and turbulent flow, and the results of the study are found to agree with the 2D results where NACA0018 blunt trailing edge gives slightly better efficiency. Thus, it may be concluded that, the aspect ratio, 3D effects and geometrical shape modification have influence on the power efficiency in both laminar and turbulent flows.
University of Southampton
Abdullah Sani, Sarah A'Fifah, Binti
8a9dd5f7-6923-4efb-8af2-f5e267a136f6
Abdullah Sani, Sarah A'Fifah, Binti
8a9dd5f7-6923-4efb-8af2-f5e267a136f6
Djidjeli, Kamal
94ac4002-4170-495b-a443-74fde3b92998

Abdullah Sani, Sarah A'Fifah, Binti (2021) Numerical Investigation of Oscillating Flapping Foils for Energy Harvesting in Freestream and Ground Effect. University of Southampton, Doctoral Thesis, 168pp.

Record type: Thesis (Doctoral)

Abstract

In recent years, as an alternative to rotary wind turbines, flapping foils or oscillating airfoils are under increasingly active investigation to extract energy from wind/water. The aim of this research is to investigate numerically the role of flapping foils on the performance of energy harvesting in freestream and in ground effect. Their potentials for the generation of electric power are studied here using two- and three-dimensional, unsteady Navier-Stokes solver (Ansys Fluent) with a dynamic mesh and sliding interface, in both laminar and turbulent flows. The performance efficiency and mean power coefficient are investigated here for different geometry modifications, namely thickness distribution (NACA0012, NACA0015 and NACA0018) and trailing edge shape modification (sharp, blunt and round) at a low and a high Reynolds numbers (푅푒 = 1100 and 5 × 105) and under different operational parameters, to explore the effect of geometry change on the energy harvesting performance efficiency. The purpose of the truncation process is to eliminate the sharp and steep curvature at the trailing edge portion which may in turn reduces the adverse pressure gradient caused in that area, and thus helps in delaying flow separation. Modifying the airfoil trailing edge and thickness are found to have influence on the lift coefficient, especially for a high Reynolds number, and that the blunt trailing edge has a better efficiency (7% improvement) than the sharp and rounded trailing edges. For all simulations which have been carried out for the 2D single oscillating airfoils, the highest efficiency performance is found to occur at reduced frequencies 푓 ∗ = 0.14 − 0.18. This increase in efficiency is mainly due to the LEVs (leading edge vortices) and a better synchronization between CY and VY (i.e., good timing in the sign switch of CY and VY ). This study is then extended to multiple oscillating bodies to investigate the interaction effect on the power extraction for the two objects in tandem, which are airfoil-airfoil, and cylinder-airfoil. From this study, it is observed that the multiple configurations of airfoil-airfoil interaction (for low inter airfoil distance) can generate up to 30% higher efficiency more than the optimal single oscillating airfoil, and the use of airfoil-airfoil interaction with blunt trailing edge airfoil is found to have a better energy harvesting/efficiency for the two oscillating bodies, with a total efficiency of more than 5% in comparison to the sharp airfoil-airfoil. Finally, for the cylinder-airfoil interaction, the efficiency performance is found to decrease when compared to the two airfoils interaction. Next, a parametric study on the ground effect has been conducted to optimise the power efficiency. The effects of Reynolds numbers at laminar and turbulent flows, the location of airfoil pitching axis, the distance between the airfoil pitching axis and the ground, the amplitude and frequency of oscillation on power extraction by the flapping wing were examined using URANS. From this study, it is found that in all the cases there is an increase in the peak lift coefficient, as height decreases. However, the improvement in power efficiency in ground effect is found to be depended mainly on the perfect synchronization of the heaving velocity and the instantaneous lift that happens at 0.12 < 푓 ∗ < 0.2. For lower and higher reduced frequencies, the increase in lift is not always reflected in a better power efficiency, as the motion of the airfoil and the forces are not well synchronized. Finally, a turbulent case with a higher Reynolds number was also evaluated in ground effect, and it found that an improvement of more than 8% in the energy efficiency was observed in comparison to its freestream case. Finally, the 2D simulation has been extended to the 3D simulation at a low and a high aspect ratios of 3.5 and 7. In addition, the effect of trailing edge geometry variation was investigated. From these results, it is found that the 2D case over-predict the power efficiency as compared to the 3D case, which is about 22% and 10% efficiency drop for aspect ratios of 3.5 and 7, respectively for the laminar case, and this is due to the well-known limits of the 2D models which do not take into account the 3D effects like tip vortex. Increasing the aspect ratio of the wing leads to a higher contribution to efficiency from heaving, as expected. Turbulent flow also shows a similar efficiency drop in both aspect ratios. In addition, the effect of the aspect ratio on efficiency from pitching is found to be negligible for laminar flow but quite significant for turbulent flow. The 3D geometrical shape variation has also been investigated, where the results of NACA0015 sharp trailing edge has been compared to NACA0018 blunt trailing edge in both laminar and turbulent flow, and the results of the study are found to agree with the 2D results where NACA0018 blunt trailing edge gives slightly better efficiency. Thus, it may be concluded that, the aspect ratio, 3D effects and geometrical shape modification have influence on the power efficiency in both laminar and turbulent flows.

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Submitted date: December 2021

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Local EPrints ID: 456931
URI: http://eprints.soton.ac.uk/id/eprint/456931
PURE UUID: b4d794ab-60e6-41d4-b8df-8121a71861da

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Date deposited: 17 May 2022 16:52
Last modified: 16 Mar 2024 17:35

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Contributors

Author: Sarah A'Fifah, Binti Abdullah Sani
Thesis advisor: Kamal Djidjeli

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