Coupling performance of two tandem and side-by-side inverted piezoelectric flags in an oscillating flow

The flow-induced vibration characteristics and energy extraction performance of two flexible inverted piezoelectric flags arranged in tandem and side-by-side configurations in an oscillating flow are studied. An immersed boundary-lattice Boltzmann method is employed for Reynolds number of 100, mass ratio of 2.9 and non-dimensional bending stiffness of 0.26 which correspond to the maximum flapping amplitude for a single inverted flag in a uniform flow. 2D simulations are conducted by varying the ratio R of the frequency of the oscillating flow to the fundamental natural frequency of the flag and the horizontal velocity amplitude (A_u) of the flow. Three coupling regimes at A_u=0.5 are identified for both tandem and side-by-side flags: chaotic oscillations regime I (0.1≤R≤1), large periodic and symmetric oscillation regime II_a1.1≤R≤1.5andII_b(2.1≤R≤3.0), and small periodic and asymmetric oscillation regime III(1.6≤R≤2). The maximum mean electrical power coefficient C¯_P occurs in regime II_a at R=1.5 with, α (piezo-mechanical coupling parameter) = 0.5, and β (piezo-electric tuning parameter) = 1.5. C¯_P is 0.1 for a tandem upstream flag, 0.068 for the tandem downstream flag and 0.1 for both side-by-side flags, and is respectively 120%, 300% and 213%, higher than that of the corresponding flag in the uniform flow. This improvement is attributed to the higher flapping angular amplitude (180°), the higher ratio of the flapping frequency of the flags to the oscillating frequency of the flow (virtually constant at 0.5), and constructive vortex interaction in regime II_a.