Investigation into the anomalous behavior of Pitot tubes in dilute polymer solutions

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.