Schooling behavior of heaving flexible airfoils

The schooling behavior of rigid and flexible NACA0017 airfoils in the heaving motion is experimentally explored in a merry-go-round equipment. The airfoil was attached to the end of a horizontal support bar whose other end was connected to the freely rotating vertical axis. The axis was forced to undergo a sinusoidal motion in the vertical direction to make a pure heaving motion of the airfoils in the frequency range of 0.5 to 5 Hz. The propulsion due to the heaving airfoils is expressed by a horizontally rotating speed of the support bar. This experimental setup is simulating infinite schooling situations of airfoils in an in-phase heaving motion with the streamwise distance $d$. The ratio of the distance to the chord length $d$/$c$ was determined by the number of airfoils $(1\le n\le 8)$. The rotational frequency $F$ according to the heaving frequency $f$ was measured with different experimental parameters. The schooling number $S=f$/(\textit{nF}), representing the number of heaving oscillations between each airfoil, was introduced to explain the schooling behavior of the airfoils. The effects of the flexibility, $d$/$c$ and $f$ on the propulsive performance were examined with the schooling behavior of the airfoils.