An experimental investigation of turbomachine blade row aeromechanics

This research was directed at understanding two major issues in turbomachine unsteady aerodynamics: (1) two-dimensional modeling of the unsteady aerodynamic excitation to a blade row within the constraints of linearized theory, and (2) the resulting unsteady aerodynamic loading of a blade row utilizing linearized theory analysis. These objectives were pursued by means of a series of experiments in the Purdue Annular Cascade Research Facility. This facility experimentally reproduces the fundamental unsteady flow phenomena inherent in axial flow turbomachines. The unsteady periodic flow field generated by rotating rows of perforated plates and airfoil cascades was measured with a two-component hot-wire anemometer and an unsteady total pressure probe and characterized in terms of the two-dimensional unsteady velocity and unsteady static pressure perturbations. The resulting unsteady periodic chordwise surface pressure distributions on a downstream stator row were measured with miniature high-frequency response pressure transducers mounted within the stator airfoils. Thus the unsteady aerodynamic excitation and resulting unsteady aerodynamic response were quantitatively ascertained. The periodic unsteady flow perturbations were analyzed as superpositions of harmonic vortical and potential flow perturbations, with each of these fundamental perturbations modeled as a spatial flow nonuniformity which is temporally steady in an appropriately rotating reference frame. The unsteady velocity associated with an harmonic vortical perturbation was shown to be parallel to the mean velocity vector in the rotating relative reference frame. The unsteady potential perturbations were shown to either propagate or decay axially depending upon flow conditions, with the propagation or decay determined by the mean relative Mach number in the rotating reference frame. Unsteady flow fields generated by rotating rows of perforated plates were found to be almost purely vortical perturbations and to conform closely with the linearized theory vortical gust model. Unsteady flow fields generated by rotating airfoil cascades were shown to be combined vortical/potential perturbations, with the vortical component propagating unattenuated and the potential component decaying at the rate predicted by the linearized theory. The stator response to both the perforated plate and airfoil excitations was accurately predicted by utilizing the linearized theory to calculate the response to harmonic vortical and potential excitations of unit magnitude, scaling these unit responses by the vortical and potential excitations extracted from the unsteady flow measurements, and summing the results. A classical flat plate cascade analysis was extended to account for camber, mean incidence, and the streamwise component of the incident unsteady flow. For cambered airfoils, this thin airfoil cascade analysis yielded better results than the flat plate analysis when the incident unsteady flow perturbation was dominated by the chordwise component