REVIEW OF NOISE PREDICTION METHODS FOR AXIAL FLOW FANS

This paper will review some methods to predict aerodynamic noise produced by rotating blades in low Mach number, low to medium speed axial flow fans with an emphasis on broad band noise. The term 'method' used here indicates that the emphasis is put on schemes which include more or less the relevant source mechanisms. The literature surveyed is far from being complete and somewhat arbitrary. Some guidance was given by the idea, that the methods should not be too complex and relatively easy to handle by a fan designer. To the knowledge of the authors none of the more advanced noise prediction methods is used routinely in fan design. A reason might be that the required inputs parameters as inflow and boundary layer parameters are not known in a traditional aerodynamic design procedure. 1 - INTRODUCTION This paper will review some methods to predict aerodynamic noise produced by rotating blades in low Mach number, low to medium speed axial flow fans with an emphasis on broad band noise. The term 'method' used here indicates that the emphasis is put on schemes which include more or less the relevant source mechanisms. Following LOWSON [1] one can classify noise prediction methods into three groups: Class I: Predictions giving an estimate of overall level as a simple algebraic function of basic machine parameters Class II: Predictions based on separate consideration of the various mechanisms causing fan noise, using selected fan parameters Class III: Predictions utilising full information about the noise mechanisms related to a detailed description of geometry and aerodynamics, e.g. they require computation of local blade element velocities and angles of attack. The entire aerodynamic noise from fans considered here is usually caused by the fluctuating forces on the fan blades. The most important mechanisms are periodically unsteady blade force due to inflow distortions (spatially nonunifom inflow, unsteady inflow) stochastically unsteady blade forces due to incident turbulence (IT), turbulent boundary layer / trailing edge interaction (TBTE), turbulent boundary layer / blade surface interaction (TBS) and flow separation (FS) In this paper mainly the latter sources are considered. The literature is far from being complete. Their selection even seems somewhat arbitrary. Some guidance was given by the idea, that the methods should not be too complex and relatively easy to handle by a fan designer.