Experimental Modal Analysis of a Turbine Blade Using Scanning Laser Doppler Vibrometry in Combination with Air-Coupled Impulse Acoustic Excitation

Abstract

A comparative analysis of contact and air-coupled approaches to acoustic excitation of a turbine blade was carried out using scanning laser Doppler vibrometry. The objective was to identify the natural frequencies and mode shapes of the blade. When contact excitation was applied using an electrodynamic shaker, modal analysis revealed eight vibration modes of the blade in the frequency range from 100 Hz to 9.5 kHz. Air-coupled acoustic excitation was implemented using a pulsed gas-discharge electroacoustic transducer operating based on a spark discharge in air at atmospheric pressure. It is shown that the gas-discharge electroacoustic transducer enables excitation of more than 30 vibration modes in the frequency range from 100 Hz to 38.5 kHz. When the air-coupled transducer was positioned 40 mm away from the blade, the vibration velocity amplitude on its surface reached 30 μm/s. Thus, the experiment confirms the feasibility of performing experimental modal analysis of a metallic part using air-coupled excitation and noncontact vibration measurement techniques.