Choke flutter instability sources tracking with linearized calculations

TL;DR

This paper introduces a linearized calculation method to identify and analyze the main excitation sources responsible for choke flutter instability in UHBR fan blades, aiding in safer fan design.

Contribution

It presents a novel linearized approach to track local vibration contributions to global damping in fan blades, specifically addressing choke flutter sources in UHBR fans.

Findings

Main excitation sources are localized at the blade trailing edge.

Shock-wave motion and shock-boundary layer interaction are key phenomena.

Linearized method effectively decomposes vibration contributions.

Abstract

Purpose :The choke flutter is a fluid-structure interaction that can lead to the failure of fan or compressor blade in turbojet engines. In Ultra High Bypass Ratio (UHBR) fan, the choke flutter appears at part speed regimes and at low or negative incidence when a strong shock-wave chokes the blade to blade channel. Localization of main excitation sources and the understanding of the different work exchange mechanisms will help to avoid deficient and dangerous fan design. Design/methodology/approach :In this paper, an UHBR fan is analyzed using a time-linearized Reynolds-Averaged Navier-Stokes equation solver to investigate the choke flutter. The steady state and the imposed vibration (inter blade phase angle, reduced frequency and mode shape) are selected to be in choke flutter situation. Superposition principle induced by the linearization allow to decompose the blade in numerous small subsections to track the contribution of each local vibration to the global damping. All simulations have been performed on a 2D blade to blade extraction. Findings :Result analysis points to a restricted number of excitation sources at the trailing edge which induce a large part of the work exchange in a limited region of the airfoil. Main phenomena suspected are the shock-wave motion and the shock-wave/boundary layer interaction. Originality/value :An original excitation source tracking methodology allowed by the linearized calculation is addressed and applied to a UHBR fan test-case.