Boundary-Layer Transfer Coefficient for a Fluttering Leaf

Abstract

Laboratory experiments were performed to evaluate boundary-layer transfer coefficients for fluttering plant leaves in winds at various mean attack angles to the fluctuating flow. In these experiments, water vapor transfer was investigated by using rectangular flat leaf models of various dimensions fluttering resonantly with the fluctuation of artificial air streams with similar intensities of turbulence to natural flows within plant canopies. Most of the water-vapor transfer coefficients evaluated for upstream-and downstream-facing surfaces were larger than those estimated from the laminar boundary-layer theory. In a range of Reynolds numbers below a critical value between 3×103 and 3×104, the transfer coefficients were apparently proportional to the square root of the mean flow speed. Under this Reynolds number range, the ratio of the experimental transfer coefficient to that estimated from the theory was expressed as a function of the model dimensions at each fluttering number and at each attack angle.The enhancement of the transfer coefficients for fluttering leaf models at various attack angles is discussed in conjunction with fluid turbulence, large eddies, the pressure distribution over the model surface and the model-fluttering.