Numerical study on the energy extraction performance by flapping foils in a density stratified flow

Abstract

An energy extractor based on fully-activated flapping foils operating in a density stratified flow is studied numerically. The effect of stratification strength characterizing as the internal Froude number on the energy extraction performance is first investigated by fixing a typical set of parameters. The hydrodynamic characteristics of flapping foils with different kinematics in the stratified regime are then examined by varying the pitching amplitude. Our results suggest that the stratification effect decreases the energy harvesting efficiency owing to resulting poor synchronization between forces and flapping motion. Within the considered parametric space, the minimum energy extraction performance is identified at the Froude number around Fr=2, giving rise to an attached boundary layer without the favorable flow separation and leading edge vortex shedding. Nevertheless, a sudden increase of extracted power is observed when the stratification level is enhanced to Fr=1, which is attributed to the distinct flow structures generated by the induced internal waves. For the varying kinematics, the Froude number corresponding to the minimum performance is found to increase as the pitching amplitude becomes higher. It is indicated that the induced internal waves tend to dominate the flow patterns at a highly strong stratification, whereas the kinematics of flapping foils still play a crucial role in determining the vortex dynamics for the moderate and weak stratification strength.