Abstract
High demand for lower noise generation in the CPAP device industries necessitates the need to have a computational modelling for obtaining in-depth understanding of internal aerodynamic flows and aero-acoustic characteristics that can be used during the product development process. In this thesis, a comprehensive numerical study is conducted to investigate flow-induced noise and vibration of the CPAP devices with supports of experimental investigations for validations.
A 3-D flow system of a CPAP device is originally developed and then re-constructed and simplified for numerical simulation purposes. The CPAP flow system consists of inlet and outlet ducts, a centrifugal fan and a humidifier. A numerical simulation, which is a combination of well-established computational fluid dynamics (CFD) techniques with acoustic analogies (AA) and finite element analysis (FEA), is adopted and applied to each primary component of the CPAP devices to investigate noise and vibration generated by internal turbulent flows. The numerical results are compared with the experimental data for validations.
Significant insights into the noise and vibration characteristics of the CPAP components are obtained and compared with each other to identify the noise source power and its locations. The contribution of each element to the overall noise generation level is estimated.
Prior to the numerical studies, a thorough experimental investigation was conducted to investigate the flow performances, noise radiations and surfaces vibration of the CPAP devices and its main components. The results from the experimental investigation can be used for validation purposes as well as the boundary conditions for numerical studies.
Numerical results has found that the uneven flow structure or asymmetric internal flow is identified as the main cause of the noise generation. The specific geometry designed for the CPAP flow system has created flow patterns which contain many aerodynamic characteristics that are related to the aerodynamic noise generation such as flow attachment and separation, flow recirculation or rotation, etc.
Furthermore, individual noise power level at each components are estimated and compared with each other. It is found that the noise power generated from the centrifugal fan is the most contributed source to the overall noise level radiated from the CPAP device.
