The effect of shear flexibility and rotary inertia on the vibration characteristics of some turbine blade configurations

• Main Author: Allen, V. C.
• Published: University of Surrey 1972
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.751774

Two basic configurations of non-rotating turbine blade have been considered and the effects of shear flexibility and rotary inertia investigated. In the first, simpler, configuration which is that of the single blade, with pretwisted form, the effects of a high thickness/length ratio is already well established. For this reason the investigation concentrates on establishing a technique of calculation which is both reliable and economical of computer time, and which at the same time can reasonably be extended to examine the second and more complex blade system. This second arrangement is that in which a number of identical, evenly pitched, blades are interconnected at their tips by some form of band or shroud. This configuration is conveniently referred to as a "blade package". Certain simplifications are made so as to limit the number of independent variables and thus not detract from the fundamental inquiry as to the effect of thickness. Consequently flexural-torsional coupling has been ignored which means that the individual blades have been treated as of zero incidence and without pretwist. In compensation flexural/longitudinal coupling has been included, as has a more realistic mathematical model of the shroud band than that adopted by earlier investigators such as Smith (ref. 7). The inherent tendency of calculated frequencies to be lowered when the effects of thickness are included tends to become offset, and, in some cases, swamped by the fact that the effective length of a shroud band segment is significantly less than the pitch of the blades in a "thin-line" model. Experimental Examination of two models, simulating packages each of five blades, confirms results of the theoretical analysis within the limits imposed by the absence of an accurate prediction of the effect of a junction between a sensibly thick blade and the adjacent bands.