Development of a 17-Hole Omnidirectional Pressure Probe

Abstract

M ULTIHOLE pressure probes are the most common measurement technique to measure velocity vector, static and total pressure in three-dimensional flowfield. Their general principles, calibration methods, and data reduction algorithms have been studied in depth and documented well [1–10]. Conventional fiveor seven-hole probes are capable of measuring flow velocity vector up to 75 deg from the probe axis [6]. For highly three-dimensional flow, the 18or 12-hole probes recently developed by Rediniotis and Kinser [11] and Ramakrishnan and Rediniotis [12] are capable of measuring velocity vector up to nearly 160 deg from the probe axis. A general data reduction algorithm has been developed for these kinds of omnidirectional probes [13]. The algorithm is based on extensive calibration and coordinate transformation technique, which is suitable for the nonconventional probes since it does not require the ports to be distributed symmetrically. Nevertheless, conventional data reduction algorithms based on interpolation still find their merit with simplicity, robustness, and accuracy. By using partitioning technique, an interpolation-based algorithm is fully competent for the omnidirectional probes. In this work, a 17-hole omnidirectional pressure probe was designed and fabricated for three-dimensional flowfield measurement. The idea is initiated from the 18-hole probe and the port arrangement is essentially same as the 18-hole one but with one port substituted by the sting.With this configuration, all pressure ports are symmetric to the probe axis, whichmakes holding and positioning of the probe easier in fabrication as well as in measurement. Furthermore, in the 18-hole probe, the distance from the rim of the three holes beside the sting to the sting surface is around 35% of the sting diameter. The pressure reading from these holes is influenced by the sting remarkably. In the 17-hole probe, this distance extends to 70% of the sting diameter.Meanwhile, the 17-hole probe is still capable of measuring similar angular range as the 18-hole probe. Six zones are used in the 18-hole probe in [11] with each zone resembles afive-hole probe and is in charge of a cone angle of 60 deg, which makes some ports locate at cone angle of 105 deg maximally. This angle range is problematic since the boundary layer separates at around 85 deg for a laminar flow. In [13], this problem is solved with a newly developed data reduction algorithm that uses three ports for angle detection. The new algorithm is still not an optimal choice because the fewer ports are used (smaller angle difference between the ports equivalently), the less sensitive pressure coefficients are to the objective angle. Measurement accuracy for flow angle is then compromised, especially when pressure measuring error is relatively large. A more adaptable and straightforward data reduction algorithm was developed for the 17-hole probe. Seventeen zones are schemed that can be classified into five-hole-pattern zones and seven-holepattern zones. For a five-hole-pattern zone, cone angle of the farthest port is 67.5 deg; for a seven-hole-pattern zone, cone angle of the farthest port is 90 deg. Furthermore, the seven-hole-pattern zone is divided into three subzones, which makes cone angle of the farthest port reduce to 70 deg. The remainder of this paper discusses the design and fabrication of the probe, data reduction algorithm, and its calibration.