Investigation on flapping dynamics and wake characteristics of a flexible plate in nonlinear hysteresis region

In this paper, the flapping dynamics and wake flow characteristics in the nonlinear hysteresis region are investigated experimentally by immersing a cantilevered flexible plate in uniform airflow. The experimental results show that the flapping mode of a cantilevered flexible plate in hysteresis region will be transited from periodical traveling wave mode to limited cantilever-like mode with the variation of Reynolds number. The flapping mode will greatly influence the kinetic parameters of a flexible plate and also wake flow characteristics. The comparison of Strouhal number values between flapping flexible plate and animals further indicates that the fluid dynamics between passive flapping and active swimming is similar to the obtained optimal propulsive efficiency. Flow visualization reveals that the Karman vortex street appears, vanishes and coherent structures of turbulent flow arise behind the stable flexible plate with increasing Reynolds number. Meanwhile, the measurements of pressure distribution in wake flow provide a good physical understanding of the energy-saving mechanism and the optimal arrangement in fish school. Moreover, the fast Fourier transform (FFT) spectra of fluctuating velocity indicate that the characteristics of wake flow are closely related to the flapping mode of flexible plate. The wake flow will come into strong harmonic excitation state when flapping mode transits into a limited cantilever-like mode.