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Investigation of Unsteady Flow in an Annular Cascade

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Final rept. Jul 1987-Oct 1991,

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The proper orthogonal decomposition POD was used to test the hypothesis that there exist a set of functions which characterize a turbine stator exit flowfield and whose dynamics may explain some of the complex behavior in downstream blade rows. The decomposition was applied to three component triple-wire probe measurements of a large-scale annular stator model exit flowfield. This study represents the first application of the orthogonal decomposition to directly measured three component data, and one of the first applications to an applied engineering flow. The full three-component cross- spectral tensor was produced from simultaneous multipoint triple-wire probe measurements. The measurements were taken across one stator pitch, at the passage midspan, 10 axial chord downstream of the stator trailing edge. The resulting 1089 cross-spectral estimates were then decomposed to obtain the eigenspectra and eigenmodes of the midspan flowfield. In addition, the POD was applied to two subdomains of the passage flowfield in order to increase the convergence rate of the energy representation. The two subdomains were taken to be the wake and outer flow region because of the large difference in scales between those regions. The wake spanned approximately 20 of the stator pitch at the measurement plane. Large distinct low frequency peaks below 200Hz were found in many of the eigenmodes although not consistently at the same frequencies. These peaks were considered the result of either inlet flow disturbances or migration of passage generated flows structures. More detailed measurements are needed however, to determine the nature of the source of these low frequency apparently large spatial scale structures.

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  • Fluid Mechanics

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