Blockage effect and ground effect on oscillating hydrofoil

Abstract

Oscillating hydrofoils located in a channel or limited zone by two side walls will be affected by both the blockage effect and ground effect. To study the contribution of these two effects on the energy-extraction performance of hydrofoils, a two-dimensional numerical model was established based on the overset grid technology. The differences in the energy extraction of hydrofoil with different wall settings were compared, and the blockage effect and ground effect in the double-wall case were mainly studied. The results show that compared with the open flow field, the single-wall can only improve the hydrofoil energy extraction by 1.75%, while the double-wall with the same foil-wall spacing can greatly improve the oscillating hydrofoil efficiency by 19.34%. By linearly fitting the results at low blockage ratios and performing the subtraction process, the influence of blockage effect and ground effect can be separated preliminarily. These two effects have the same order of magnitude in improving the energy extraction of hydrofoils when the blockage ratio is high. In addition, the present study proposes to add the correction of the drag coefficient to remedy the shortcoming of the correction method proposed by Barnsley and Wellicome that cannot deal with the high blockage ratio cases, which means the influence of the ground effect is also corrected. Then the blockage effect and the ground effect can also be separated by the subtraction process. Based on the above analysis, it is found that the ground effect has little influence when the hydrofoil oscillates at high frequencies, while the influence of the blockage effect is mainly related to the blockage ratio rather than the oscillating frequency.