Experimental Study of Drag from a Fluttering Flag

Abstract

As a result of flutter motion, centrifugal forces induce in-plane tension in the fabric. This tension acts to "flatten" the flag thereby opposing the fluid pressure forces which generate the out-of-plane curvature (motion). This dynamically induced tension is felt at the attachment point as increased drag force, above and beyond the magnitude of skin friction drag. It has been shown experimentally that the time-averaged drag depends only on the square of the velocity amplitude and the mass of the fluttering specimen. Experiments suggest that large centrifugal forces, because of large fabric curvature and low material stiffness can generate elevated drag over that of heavier, stiffer materials with identical dimensions. Furthermore, high speed photo data suggests flutter amplitudes are reduced at high wind velocities due to the induced in-plane tension. Additionally the makeup of flutter oscillations in a flexible flag were proven to consist of both traveling and standing waves. Standing wave ratio (SWR) approximations from superimposed photo data suggests an underlying dominance of traveling waves comprising the flutter motion.