Modelling Strategies to Obtain the Forcing Function for a Forced Response Analysis

Abstract

The forced response analysis of a turbomachinery component requires the transient pressure field over that component. For an accurate prediction, this pressure field or forcing function needs to contain the frequency signatures of adjacent components. This paper compares various efficient modelling strategies to include the effect of these adjacent components in obtaining this pressure field. The example used in this work is a hydro turbine though the strategies could be applied to other turbo machines. The Hydro turbine example comprises a spiral casing (or a volute), inlet guide vanes, stay vanes and a rotating runner. The pitch ratio between the guide vanes and runner is 14:15. The desire is to compute the unsteady pressure field in the runner domain. The flow solver employed has a variety of simulation techniques available to compute flows in cases with unequal numbers of blades/vanes in adjacent rows (“unequal pitch”). These techniques, range from mixing plane and frozen rotor methods for steady flows to various transformation methods for unsteady flows. The transformation methods remove the need for large full or part wheel calculations. Therefore, solution can be obtained at fraction cost of the full wheel simulation. The Fourier transform method is used in this paper to model the pitch change between stator and rotor. Three transient pitch-change modelling strategies are presented, and its accuracy and solution efficiency are compared to full wheel simulation. The three pitch-change variations are: a single-frequency frozen gust analysis, a blade coupling between runner and guide vanes using pitch-change interface, and multiple-frequency frozen gust analysis which will account for the asymmetry due to presence of the spiral casing.