High Frequency Modal Testing of the Multiblade Packets Using a Noncontact Measurement and Excitation System

Abstract

High cycle failure of blades and vanes caused by the vibration is one of the major causes reducing the lifetime of turbomachines. For multiblade packets, the failure may occur at vibrations with high frequencies that can reach up to tens of kHz. The experimental modal testing of blades is crucial for the validation of numerical models and for the optimization of turbomachine design. In this paper, the test rig and procedure for measurements of dynamic characteristics of lightweight multiblade packets in wide and high frequency ranges are developed. The measurements are based on a noncontact excitation and noncontact measurement method, which allows the determination of the modal characteristics of the packets with high accuracy in wide frequency ranges. The responses of the multiblade packets are measured using a Scanning Laser Doppler Vibrometry (SLDV), while vibrations are excited by the acoustic excitation technique. Modal tests of the blade packet comprising 18 vane blades connected by shrouds are performed. The measurements are performed within the high frequency range of 0–30 kHz, and the natural frequencies and mode shapes are obtained for first 97 modes. To capture the complex high frequency blade mode shapes, each blade in the packet is scanned over 25 reference points uniformly distributed over the blade concave surface. In order to obtain the high frequency resolution, the frequency range used for the measurements is split into several frequency intervals accordingly to the number of spectral lines available in the used data acquisition system, and for each such interval, the test is performed separately. The finite model of the packet is created, and the numerical modal analysis is performed to compare the calculated natural frequencies and mode shapes with the experimental measurements. The comparison shows the satisfactory with those from finite element analysis. It illustrates the measurement method described in this work is effective and reliable.

Comparison and investigation of inverse identification techniques for the material properties of a brake disc through experimental and numerical modal analysis

“…Since there is no denominator term in the objective function Φ 3 , the error values are not normalized hence higher sum of squared error values are obtained. 4,31 Correspondingly, using the first 10 natural frequencies instead of 16 (i.e., using a smaller frequency range) in optimization resulted in improved Poisson’s ratio and Young’s modulus errors for both weighted and equally weighted optimizations by Φ 3 . In this study, it is proper to use the first 10 or fewer resonance frequencies in the optimization process.…”

“…In modal tests, some high-valued natural frequencies, most of which correspond to in-plane modes, could not be detected clearly due to the lack of excitation in radial direction, decreasing the accuracy of the accelerometer and energy density of the input by increasing frequency, 31 and low coherence of the relevant frequency region. Undefined natural frequencies, which are located above the 10th natural frequency value, are detected by comparing the mode shape results of modal tests and FEM analyses.…”

Operational Deflection Shape Measurements on Bladed Disks with Continuous Scanning Laser Doppler Vibrometry

“…The concept of ODS FRF on the basis of transfer function is first proposed by Richardson M H [ 23 ] in 1997. Similar to the transfer function, ODS FRFs are similarly calculated by the auto spectrum and cross-spectrum between the response of fixed reference point and the responses of the measuring points: …”