Generalised prediction methods for flow-generated noise produced by multiple in-duct elements

In recent years, several studies have sought to develop a generalised prediction method for flow-generated noise that is produced by the interaction of multiple elements in the airflow duct of an air-conditioning system. The prediction methods for single or multiple in-duct discontinuities are based on the assumption that there is a constant of proportionality between the fluctuating drag force and the steady drag force that acts on the flow spoilers. To develop a prediction method for multiple in-duct discontinuities, the concept of partially coherent sound fields is adopted to formulate the sound powers that are produced by the interaction of multiple in-duct elements at frequencies both below and above the cut-on frequency of the lowest transverse duct mode. An interaction factor is defined as the result of a simple relationship between the sound power that is due to the interaction of multiple in-duct elements and the sound power that is due to a single in-duct element. The method has been found to agree well for two in-duct elements with experimental studies. This paper introduces the recent developments in the prediction methods, and discusses the problems that are encountered in the development of the technique.