Application of holography to acoustically induced vibrational fields

Experience in the gas, petrochemical, aerospace and power industries has demonstrated that high capacity piping or duct systems under conditions of high speed flows sustain acoustically induced vibrations. Quite often, higher order acoustic modes couple selectively with certain structural pipe or duct modes, producing significant increases in pipe/duct wall vibrations. These coincidences, when superimposed on other dynamically originated structural deformations, can produce high local stress levels that could damage the medium conveying system. Hence, the detection and immediate estimation of the stresses associated with complex vibration is highly desirable. Currently, one of the best suited tools to estimate stresses in complex vibrational fields is pulsed holography. The application of this tool to complex field vibration evaluation is documented in this work. This paper focuses on pulsations generated by a turbocompressor and their impact on piping components. The piping immediate to the compressor discharge nozzle is investigated. The acoustic field interaction with piping walls is briefly described. The unique, in-situ holographic measurements of large diameter piping shell vibrating under unsteady resonant and forced excitations are performed and documented. The results are presented in the form of printed images from holograms which illustrate three dimensional wave patterns in vibrational field. The correlation of pipe wall vibration and dynamic strain is shown on a sample hologram. In-depth analysis of the holograms leads to the relaxation of the in-house high frequency vibration criterion.