Air Film Dampers and Their Optimisation

Vibration control is an important consideration in the design of weight-critical machinery and components operating at high speed in harsh environments. Increasing damping levels is often the easiest way to reduce the vibration of the system without sacrificing performance. The air film damper is interesting because its damping performance cannot be significantly influenced by ambient temperature. However, it is difficult to develop an air film damper with satisfactory performance, because the damping efficiency of the air gap is very sensitive to some factors, such as the gap thickness, the air viscosity and the ambient pressure. For all but the simplest of geometries, the effects of such factors are not easy to identify using analytical methods. This makes the task of designing an air film damper challenging, particularly in practical applications where other factors such as manufacturability must be accounted for. The rapid development of computer technology now presents an opportunity to solve much more complex tasks and consequently, the design of components is now frequently supported by appropriate finite element (FE) analysis. This thesis, therefore, sets out to use the structural-acoustic coupling routines, now available in commercial finite element software, to improve the practical understanding of the air film damper and to demonstrate a design methodology for their optimisation. For this purpose, an FE model is developed to deal with more complex geometries of the plate/air gap system. Numerical methods are used to predict the damping ratio of a vibrating plate separated from a solid wall by a narrow air gap. Validation of the numerical approach is achieved by comparison with physical experiments and an existing theoretical model for a range of air gap thicknesses and air pressures. The numerical approach is then used to investigate air gap designs that have previously not been considered because of their complex geometry. These studies show that by partially restricting the ability of the air to flow, through the use of channels, the requirement for a very narrow gap can be relaxed. For practical designs, this is important as manufacturing large, thin gaps are very expensive. The limiting factor in the length of such channels is shown to be the compressibility of the air as particularly high restriction to flow can result in local pressure changes.