Flow-induced vibration of a web floating over a pressure-pad air bar

Abstract

Scope and Method of Study: The aeroelastic (air/web) model is developed by theories of elasticity and fluid dynamics. The flexible web is modeled as a traveling Euler-Bernoulli beam under tension which is exposed to high-speed air flows underneath it, and the aerodynamic pressure is developed from continuity and Navier-Stokes equations. The web is assumed to be a threadline traveling between two rollers over one air bar; there is no variation across the width. To keep a valid threadline model, two-dimensional flow is obtained by two air dams installed along both free edges of the web, to block air escaping in the cross-machine direction. Experiments are limited to a non-traveling web exposed to air-jet flows. The present study is focused on effects of high-speed air flows on the flexible tensioned web; the velocity of the web is neglected because the velocity of the air jet is much higher than the translational velocity in practical applications.

Findings and Conclusions: It is observed that divergence-type instability (static deflection) and flutter-type instability (vibration) both occur in the web due to the air-jet flow. The theory used to develop the computations is well supported by the experimental results. Stability criteria are provided and compared with analytical calculations and experiments. Flutter depends strongly on flow speed, and can be controlled by increasing tension, shortening the web span, and (for a single span) centering the air-bar position. The developed theoretical and computational approach could be used for multi-bar analysis.