Coupled motion of two side-by-side inverted flags

The interaction and coupling between two inverted flags that are placed side-by-side in a uniform flow is investigated in an effort to determine the behavior of systems that are formed by arrays of cantilevered plates. Inverted flags are elastic plates that are free to move at their leading edge and clamped at their trailing edge. We show that placing two inverted flags of equal dimensions side-by-side will cause their motion to couple. In-phase, anti-phase, staggered, alternating and decoupled flapping modes are present, with the anti-phase mode being predominant at small flag distances and low wind speeds. Increases both in amplitude and frequency of flapping are observed in the two flag system with respect to a single flag. Two side-by-side inverted flags of different lengths are found to interact for small length differences, with the longer flag being able to induce a motion on the shorter flag even when the latter is outside of its flapping wind speed range.