Development of a 12-Hole Omnidirectional Flow-Velocity Measurement Probe

Abstract

M ULTIHOLE pressure probes are widely used in aerodynamic flow studies. Their general design and working principles have been widely studied and are well documented [1–5]. The most popular ones, the fiveand seven-hole probes, measure flow velocity up to 55 and 70 deg, respectively, from the probe axis. A spherical probe with 18 holes [6] was developed recently that considerably reduced the range limitation bymeasuring flows up to nearly 160 deg from the probe axis. The 18-hole probe was essentially an extension of the five-hole probe design. Its design was driven by the necessity to use the reduction method of a conventional five-hole configuration, the only method available at the time. Consequently, over a large region of its measurable range, there are more than four ports available with valid (attached-flow) pressure data. Because there are typically only four unknown variables during flow measurement, there is a redundancy in the number of ports in the 18-hole probe. When new calibration and reduction algorithms were developed by the authors [7] for tackling any generic arrangement of ports, it became possible to consider designing an optimized version of the omnidirectional probe, which would be free of redundancy. These algorithms do not depend on the axisymmetry of the port distribution pattern to define the nondimensional pressure coefficients. In this work, an optimal spherical-probe design for omnidirectional flow measurement is presented. The calculation of the four unknown flow quantities (two flow angles, flow speed, and static pressure) is achieved with the minimum necessary number of ports on the probe tip. This has significant implications in the instrument’s spatial resolution, frequency response as well as cost of interfacing, and usage. A prototype probe was fabricated with a spherical tip diameter of 0.375 in. (9.53 mm) and a sting diameter of 0.122 in. (3.1 mm). The probe was calibrated and its measurement accuracy assessed in a calibration facility. II. Design and Fabrication