Hybrid Velocity Measurements: A Comparison of the roving Preston tube and Prandtl-Pitot tube

Abstract: The measurement of velocity in confined open channel geometries presents challenges that require the application of hybrid physical modeling techniques. Herein, velocity measurements were performed using the Prandtl-Pitot tube and the roving Preston tube (RPT). Each instrument has unique limitations in terms of measurement accuracy and ability to reach particular locations within a flow cross-section. When these instruments were collectively applied as a measurement suite, they overcame these limitations while providing useful measurement verification. The focus of this paper is the RPT, whose operations relied on reference pressure readings that varied with distance from flow boundaries. To address these variations, the RPT previously relied on bulk calibration methods to adjust measured velocities, but these methods resulted in inaccurate representations of time-averaged velocity, especially near boundaries. To investigate this limitation, the roving Preston tube was tested in a wind tunnel to examine wall effects on reference pressures. To complement this physical investigation, large eddy simulation (LES) numerical modeling was conducted to evaluate kinematic structures located at the orifice of the reference pressure tube and provide insight into why pressures vary with distance from fixed boundaries. Keywords: Velocity measurements, Roving Preston tube, Prandtl Pitot tube, Large Eddy Simulation