Aero-elastic instability is often a constraint on the design of
modern high by-pass rat.io aero engines. Unstalled supersonic flutter
is an instability which can be encountered in shrouded fans, in which
mechanical vibrations give rise to unsteady aerodynamic forces which
couple further energy into the mechanical vibration. This phenomenon
is particularly sensitive to the deflection shape of the mechanical
A detailed measurement of the vibrational deflection shape of a
test fan undergoing supersonic unstalled flutter was sought by the author.
This measurement was required in order to assess the current theoretical
understanding and modelling of unstalled fan flutter, The suitability
of alternative techniques for this measurement was assessed, Pulsed
holographic interferometry was considered optimum for this study because
of its full field capability, large range of sensitivity, high spatial
resolution and good accuracy. A double pulsed holographic system,
employing a rnirror~Abbe image rotator, was built specifically for this
study, The mirror-Abbe unit was employed to rotate the illuminating
beam and derotate the light returned from the rotating fan. This
therefore maintained correlation between the two resultant holographic
images. The holographic system was used to obtain good quality interferograms
of the 0.86m diameter test fan when rotating at speeds just
under 10 000rpm and undergoing unstalled flutter. The resultant interferograms
were analysed to give the flutter deflection shape of the fan.
The study of the fan in flutter was complemented by measurement of the
test fan's vibrational characteristics under non-rotating conditions.
The resultant experimental data were in agreement with the current
theoretical understanding of supersonic unstalled fan flutter. Many of
the assumptions employed in flutter prediction by calculation of unsteady
work were experimentally verified, The deflection shapes of the test
fan under non-rotating and flutter conditions were compared with those
predicted by a finite element model of the structure and reasonably good
agreement was obtained.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.