Design, realization, development and validation of an acoustic excitation system for a monobloc bladed wheel with dynamic measurement using a laser vibrometer placed on a robotic arm

Abstract

In turbomachinery, the expected new generation of rotors consists of a monobloc bladed disk, called blisk, with better performances and allowing to achieve higher pressure ratios. These structures have a cyclic symmetry and well-defined modes, characterized by a sinusoidal deformation along the circumference of the blisk, which allocate the deformation amplitude uniformly over the blades. In reality, blades have small randomly distributed variations, known as mistuning. In operation, these deviations can cause a localized forced response, leading to unexpected failures due to high cycle fatigue. Moreover, under nominal conditions, the air flow encounters some obstacles, periodically distributed in the turbomachinery, which leads to a periodic pressure variation along the blisk. Due to the rotating structure, the rotor is submitted to a traveling wave excitation of a certain order, whose shape coincides with the eigenmodes of the blisk, then likely to be excited. In addition to this, industrial blisks often have a high spectral density, which makes the identification of individual modes extremely complex with a classical base excitation.

To simulate engine order excitation, to perform modal appropriation, and to determine experimentally the mistuning, this work aims to design and implement a test bench that generates standing and traveling wave excitation of the desired order, on a compressor blisk. The solution proposed consists of an acoustic excitation system, exciting the structure in a non-intrusive way. This test bench is made up of multiple speakers driven by a voltage module, controlled by a software developed at V2i. One speaker is placed under each blade, which allows exciting the dedicated blade with a desired amplitude and phase. Then, the response of the blisk is measured with a laser Doppler vibrometer, placed on a robot arm.

In a first instance, a numerical study of the blisk is performed to identify its modal properties. In parallel with this, an experimental mistuning identification method, named the Component Mode Mistuning method, is presented and implemented. This method allows both to compute the mistuned modal properties of the investigated blisk for a given mistuning pattern and inversely, to identify the mistuning from experimental measures. Thirdly, the excitation system is developed, from the choice of the tools to the assembly. Thereafter, to excite each blade with the same amplitude, an accurate process of calibration is conducted. Finally, some tests are performed with the developed test bench: a classical modal analysis by acoustic excitation is made first, and then traveling and standing wave excitations are applied.