Effect of Frozen Turbulence Assumption on the Local Blades Vibration on the Choke Flutter Instability in Transonic UHBR Fan

Abstract

The choke flutter can lead to the failure of fan or compressor blade in turbojet engines. Choke flutter appears when a strong shock-wave chokes the blade to blade channel. In UHBR fan, choke flutter appears at part speed regimes and at low or negative incidence. In this paper, the choke flutter instability is analyzed based on the resolution of time-Linearised Reynolds-Averaged Navier-Stokes on a 2D blade to blade extraction of an Ultra High Bypass Ratio (UHBR) fan named ECL5v1. An analytical motion of rotation of the airfoil around its leading edge, without the deformation of the blade surface is imposed to a typical choke flutter fan operating condition. For a selected Inter Blade Phase Angle (90\(^\circ \)) and selected reduced frequency (0.15), choke flutter instability happens. The blade vibration is decomposed in a large set of subsection to determine the main local flutter sources. This paper aims to analyze the effect of the turbulence modelling on the position of the main local flutter sources and on the work exchange along the blade. Two cases are considered, with the frozen turbulence assumption or with the linearization of the turbulence model. The results locate the main local flutter sources at the same position, with the same work exchange direction but with a modulation of the work exchange magnitude. Amplification or restriction of the magnitude seems to be linked to the steady flow phenomena around both the excitation source and the receptor.