Book Description

The present paper describes work underway to develop a computational approach that can adequately simulate both the aerodynamic and acoustic behavior of a typical exhaust diffuser/volute combination, such as are commonly used in industrial gas turbines for power generation use. An experimental rig was constructed to obtain a detailed understanding of the flow and acoustic properties of the system, and to provide guidance for computational modelling. Two different approaches are described for analysis of this system. The first uses CFD predictions carried out with a time-averaged RANS-based approach and a statistical turbulence model. Examples of the flow-field from this approach are presented. The second approach uses Large Eddy Simulation CFD, on a simplified geometry chosen on the basis of the experimental evidence, to provide information on the unsteady flow behavior. This information is analyzed and used to specify parameters for an acoustic analogy model. The acoustic model is also a simplified representation of the dominant noise source constructed from an experimentally derived viewpoint. The model is based on a ring of dipoles simulating the fluctuating pressure field associated with the unsteady vortex shedding/growth/merging process in the shear layer emerging from the diffuser exit. Spectral analysis of the unsteady velocity field provided by the LES calculation is used to determine amplitude, frequency dependence and phase relationships in the acoustic model. The basis of the model is described and sample outputs from both LES and acoustic model components are used to illustrate its performance.