The anomalous behavior of Pitot tubes in the turbluent pipe flow of a dilute polymer solution is investigated and discussed. Pitot tube error is measured using an alternative optical means of local velocity measurement. Unlike previous research in this field a strict control is kept on the state of degradation of the solutions and Pitot tube error is measured directly whilst the tube is under test. Errors which occurred in total head pressure measurement were found to be negative and in some cases were as high as 40%. Factors which influence the Pitot tube accuracy are identified and a dynamical similarity argument is used to express the error as a function of these. Pitot tube error is attributed to the polymer solutions storing energy viscoelastically at the tube tip. A critical review of explanations previously put forward for Pitot error with reference to the results is included in the discussion. The polymer used was polyox WSR-301.
A series of tests on guide vane and compressor cascades is reported. The Reynolds number was varied in the guide vane cascade tests, and the Reynolds number and the cascade aspect ratio were varied in the compressor cascade tests. The substantial laminar separation observed in the compressor cascades at high aspect ratio (and low Reynolds number) was suppressed in the cascade tests at low aspect ratio, 2:1. Effects of Reynolds number variation on the performance of a single stage axial flow compressor are also given, and compared with predictions of performance using the cascade tests. Calculations of laminar separation points agree quite well with the experimental observations. It appears that transition due to laminar boundary layer instability is unlikely to occur on compressor blades, in the normal operating range of Reynolds number.
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