<title>Vibration measurements in a rotating blisk test rig using an LDV</title>

Abstract

Three methods of using an LDV to measure the vibration of a bladed disc in a rotating rig are described. I.e. (i) the response of one blade is monitored continuously by diverting the laser beam via a small mirror on the rotating disc, and a fixed, annular, conical mirror. Any of the blades may be targeted, by indexing the rotating mirror. (ii) circular scans give circumferential response mode shapes directly, or (iii) via spectra, as nodal diameter coefficients. Some natural modes of blisks (integral bladed discs) occur in isolated pairs, with amplitude distributions which are sinusoidal around a circular scan line. The natural frequencies increase with the number of sinusoids around the circumference, asymptotically to the cantilever blade frequency. Forced vibration of these modes, in a rotating situation, produces various combinations of standing and rotating waves. With a mistuned blisk, the close modes couple to create natural mode shapes which are very irregular. Excitation frequencies are at multiples of rotation speed, exciting modes with equivalent numbers of nodal diameters, sometimes aliased with the number of blisk blades. The LDV techniques described can cover all these effects, and some example measurements are included to illustrate their potential.