Experimental reconsideration of spatio-temporal dynamics observed in fluid-elastic oscillator arrays from complex system viewpoint: From vibrating pipes in heat exchangers to waving plants in agricultural fields

Transition from local complexity to global spatio-temporal dynamics in a two-dimensional array of fluid-elastic oscillators is examined experimentally with an apparatus comprising 90-1000 cantilevered rods in a wind tunnel as the Reynolds number (based on rod diameter) is increased from 200 to 900. A cluster-pattern entropy measure is introduced as a quantitative measure of local complexity. As the intensity of interaction among neighboring elements (in this case, frequency of collisions among rods) increases, a set of the elements (in this case, a rod-array) achieves globally better-organized behavior. On the basis of accelerometer data, the rod impact rate versus flow velocity shows a power-law scaling relation. Video images reveal that, initially, each rod moves individually; then clusters consisting of several rods emerge. Finally, global wave-like motion occurs at higher flow velocities. Each wave-like motion has its specific frequency and spatial wavelength, which vary according to wind velocity. (c) 2007 Wiley Periodicals, Inc.