Three dimensional unsteady flow for an oscillating turbine blade

Abstract

An experimental and computational study, motivated by the need to improve current understanding of blade flutter in turbomachinery and provide 3D test data for the validation of advanced computational methods for the prediction of this aeroelastic phenomenon, is presented. A new, low speed flutter test facility has been developed to facilitate a detailed investigation into the unsteady aerodynamic response of a turbine blade oscillating in a three dimensional bending mode. The facility employs an unusual configuration in which a single turbine blade is mounted in a profiled duct and harmonically driven. At some cost in terms of modelling a realistic turbomachinery configuration, this offers an important benefit of clearly defined boundary conditions, which has proved troublesome in previous work performed in oscillating cascade experiments. Detailed measurement of the unsteady blade surface pressure response is enabled through the use of externally mounted pressure transducers, and an examination of the techniques adopted and experimental error indicate a good level of accuracy and repeatability to be attained in the measurement of unsteady pressure. A detailed set of steady flow and unsteady pressure measurements, obtained from five spanwise sections of tappings between 10% and 90% span, are presented for a range of reduced frequency. The steady flow measurements demonstrate a predominant two-dimensional steady flow, whilst the blade surface unsteady pressure measurements reveal a consistent three dimensional behaviour of the unsteady aerodynamics. This is most especially evident in the measured amplitude of blade surface unsteady pressure which is largely insensitive to the local bending amplitude. An experimental assessment of linearity also indicates a linear behaviour of the unsteady aerodynamic response of the oscillating turbine blade. These measurements provide the first three dimensional test data of their kind, which may be exploited towards the validation of advanced flutter prediction methods. A three dimensional time-marching Euler method for the prediction of unsteady flows around oscillating turbomachinery blades is described along with the modifications required for simulation of the experimental test configuration. Computationalsolutions obtained from this method, which are the first to be supported by 3D test data, are observed to exhibit a consistently high level of agreement with the experimental test data. This clearly demonstrates the ability of the computational method to predict the relevant unsteady aerodynamic phenomenon and indicates the unsteady aerodynamic response to be largely governed by inviscid flow mechanisms. Additional solutions, obtained from a quasi-3D version of the computational method, highlight the strong three dimensional behaviour of the unsteady aerodynamics and demonstrate the apparent inadequacies of the conventional quasi-3D strip methodology. A further experimental investigation was performed in order to make a preliminary assessment of the previously unknown influence of tip leakage flow on the unsteady aerodynamic response of oscillating turbomachinery blades. This was achievedthrough the acquisition of a comprehensive set of steady flow and unsteady pressure measurements at three different settings of tip clearance. The steady flow measurements indicate a characteristic behaviour of the tip leakage flow throughout the range of tip clearance examined, thereby demonstrating that despite the unusual configuration, the test facility provides a suitable vehicle for the investigation undertaken. The unsteady pressure data show the blade surface unsteady pressure response between 10% and 90% span to be largely unaffected by the variation in tip clearance. Although close examination of the unsteady pressure measurements reveal subtle trends in the first harmonic pressure response at 90% span, which are observed to coincide with localised regions where the tip leakage flow has a discernible impact on the steady flow blade loading characteristic. Finally, some recommendations for further work are proposed