The application of moderate pressure gradients can exercise a large influence on the decay of wakes produced by obstructions in a flow field. In fact, a positive pressure gradient may arrest completely the decay process and cause the relative wake size to grow rapidly. This phenomenon could exist in any diffusing passage whose entrance flow has a wake-type distortion. Such a nonuniformity, due to a blade or a strut or other cause of stagnation-pressure variation in the stream, could lead to diffuser stall, as has been demonstrated experimentally. An analytical study of the problem has been made on the basis of measurements made in the M.I.T. Gas Turbine Laboratory. Theoretical considerations have led to a very convenient calculation formula which shows quite acceptable agreement with experimental data. The methods employ momentum integral equations and an “eddy viscosity” which is uniform across the wake and proportional to the product of the local free-stream velocity and the momentum thickness.
The present work was conducted to extend information derived in a previous study where it was found that the performance of a transonic turbine nozzle of high turning angle was significantly affected by the design and operation of the downstream rotor and the detailed characteristics of the nozzle flow field. Measurements obtained in an operating stage environment were compared with information from simplified test arrangements. In particular, it was possible to make direct comparisons of static pressure distributions at the vane root as affected by downstream conditions. Detailed exit flow surveys indicated areas susceptible to the influence of rotor presence. Probe interaction effects also were assessed in a special series of tests conducted over a representative range of Mach numbers. Finally, experiments were performed with a perforated plate simulating rotor presence, in an attempt to determine the limitations of such simplified test arrangements in the generation of pertinent data. The results shed further light on a complex subject, and have been aimed at practical validation of test arrangements and techniques.
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