Flag flutter in inviscid channel flow

Using nonlinear vortex-sheet simulations, we determine the region in parameter space in which a straight flag in a channel-bounded inviscid flow is unstable to flapping mo-tions. We find that for heavier flags, greater confinement increases the size of the region of instability. For lighter flags, confinement has little influence. We then com-pute the stability boundaries analytically for an infinite flag, and find similar results. For the finite flag we also consider the effect of channel walls on the large-amplitude periodic flapping dynamics. We find that multiple flapping states are possible but rare at a given set of parameters, when periodic flapping occurs. As the channel walls approach the flag, its flapping amplitude decreases roughly in proportion to the near-wall distance, for both symmetric and asymmetric channels. Meanwhile, its dominant flapping frequency and mean number of deflection extrema (or “wavenum-ber”) increase in a nearly stepwise fashion. That is, they remain nearly unchanged over a wide range of channel spacing, but when the channel spacing is decreased be-low a certain value, they undergo sharp increases corresponding to a higher flapping mode