Stability of slender inverted flags and rods in uniform steady flow
Cantilevered elastic sheets and rods immersed in a steady uniform flow are known to undergo instabilities that give rise to complex dynamics, including limit cycle behaviour and chaotic motion. Recent work has examined their stability in an inverted configuration where the flow impinges on the free end of the cantilever with its clamped edge downstream: this is commonly referred to as an 'inverted flag'. Theory has thus far accurately captured the stability of wide inverted flags only, i.e. where the dimension of the clamped edge exceeds the cantilever length; the latter is aligned in the flow direction. Here, we theoretically examine the stability of slender inverted flags and rods under steady uniform flow. In contrast to wide inverted flags, we show that slender inverted flags are never globally unstable. Instead, they exhibit bifurcation from a state that is globally stable to multiple equilibria of varying stability, as flow speed increases. This theory is compared with new and existing measurements on slender inverted flags and rods, where excellent agreement is observed. The findings of this study have significant implications to investigations of biological phenomena such as the motion of leaves and hairs, which can naturally exhibit a slender geometry with an inverted configuration.
© 2016 Cambridge University Press. (Received 2 June 2016; revised 10 October 2016; accepted 12 October 2016; first published online 21 November 2016) This research was supported by the Gordon and Betty Moore Foundation, the Charyk Family Foundation, the Charyk Laboratory for Bio-Inspired Design, Caltech's Kavli Nanoscience Institute and the Australian Research Council grants scheme. C.H.-C. acknowledges support through the 'la Caixa' Fellowship Grant for Post-Graduate Studies, 'la Caixa' Banking Foundation, Barcelona, Spain.