Limitation of expiratory airflow from mammalian airways is currently understood to be due to choking at wave speed (S. V. Dawson and E. A. Elliott. J. Appl. Physiol. 43: 498-515, 1977). A critical weakness of the theory is the lack of a mechanism for the dissipation of energy when effort exceeds that needed for maximal flow. We have observed substantial wall motion with flow limitation in a physical model of a trachea. Therefore we have examined a simple two-dimensional mathematical model, designed to approximate the behavior of the physical model of the trachea, to try to identify a relationship between flow limitation and wall oscillation. The model matches wave-speed predictions when only long waves are considered. The model predicts that aerodynamic flutter will occur in the zone of supercritical flow described in wave-speed theory. Aerodynamic flutter in the zone of supercritical flow provides a potential mechanism for the energy dissipation necessary for transition from supercritical to subcritical flow and explains the high-frequency pure tone heard with flow limitation.
Operational experience in Canada during the early seventies showed that hovercraft can make very effective icebreakers. In a low speed mode, the hovercraft causes an air cavity to form under the sheet as it rides onto it. This causes a section of the sheet to become unsupported and fail in bending under the action of its own weight. In a high speed mode, the motion of the craft over the sheet sets up large amplitude flexural gravity waves. Research into both modes of operation has been going on at Memorial University of Newfoundland (MUN) since the mid-eighties. This paper summarizes this work. For the low speed case, scaling laws for the resistance to forward motion of two new geometries were developed and confirmed with data obtained in the ice tank at the Institute for Marine Dynamics (IMD) in Newfoundland. For the high speed case, measurements of sheet deflections in the MUN wave tank and the IMD ice tank showed that a critical speed exists for motion over a sheet. At this speed, sheet deflections are limited only by dissipation and nonlinearities. We believe this critical speed is the source of high speed mode hovercraft icebreaking. Key words: hovercraft, icebreakers, air cavity mode, wave mode.
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