Force drafting of three-dimensional flags in tandem arrangement

The flapping of a flag in a uniform flow is a canonical example of flow-induced vibrations. There have been several experimental and numerical studies on a viscous flow over two two-dimensional flags in tandem arrangement, finding that the downstream flag experiences larger drag than the upstream one if its amplitude is larger. We assume that this is due to the lack of three-dimensional effects. To verify this assumption, an immersed boundary-lattice Boltzmann method is employed to study the force drafting of two three-dimensional flags in tandem arrangement. It is found that for large aspect ratio flags, the three-dimensional force drafting is the same as that of two-dimensional case, while for small aspect ratio flags, the three-dimensional force drafting is the same as two rigid bodies, i.e. the downstream flag experiences smaller drag than the upstream one even its amplitude is larger.