Duct effects on the dynamic fan characteristics of air cushion systems

During dynamic operation of an air cushion system, the fan operating point as seen at the cushion does not move along a static characteristic. Instead, it moves on a loop. It is shown that loop type behavior can be predicted theoretically. The theory models the fan-duct-plenum system as a one dimensional acoustic vibration system. It uses the method of characteristics and a finite difference technique known as the method of specified time intervals to solve for the pressure and flow variations which occur along the duct during unsteady operation. Several practical situations where the loop behaviour may be of importance are discussed.