Development of a Nearly Omnidirectional Velocity Measurement Pressure Probe
The development of a nearly omnidirectional pressure probe for three-velocity-component and pressure measurements is described, focusing on the techniques employed in probe fabrication, calibration, and frequencyresponse study. The probe tip is a sphere with 18 pressure ports properly distributed over the spherical surface. The device eliminates the velocity directionality limitations of current multihole probes and constitutes a rugged tool for use in complex three-dimensional e ow mapping. An automated calibration system was used to generate a probe calibration database of approximately 10,000 individual probe orientations. Least-squares-based and neural-network-based calibration algorithms were developed. The reliability of the calibration procedures and algorithms is demonstrated e rst. Then probe utility is demonstrated in a e owe eld with e ow reversal, downstream of a backward-facing step. Last, a study of the probe frequency response is presented. I. Introduction T HE design, evaluation, and optimization of complex aerodynamicgeometriesinvolveextensivewind-tunneltestingand/or computationally intensive numerical simulations. Even in the latter case, high-quality experimental wind-tunnel results with minimal, quantie able errors are still necessary for code-validation purposes. Moreover, in aerodynamic testing facilities where large volumes of data need to be acquired in tight schedules, downtime due to poor instrumentation performance is highly undesirable. Such facilities include industrial testing wind tunnels, as well as high-productivity computational e uid dynamics code validation facilities. 1 In such environments, e ow measurement techniques such as laser Doppler velocimetry and particle image velocimetry, although powerful, usually require painstaking efforts toward their successful usage. Costly components; complex setups; troublesome e ow seeding requirements; lack of e exibility, ruggedness, and mobility; and ease of misalignment often render such techniques impractical. Moreover, in testing of complex three-dimensional geometries, accessibility of the entire e owe eld around the model is an essential issue. When employing optical techniques, large sections of the e owe eld are obstructed optically by the presence of the model. To access such regions, repositioning of the instrumentation setup is necessary, a time-consuming process with the associated potential pitfalls. Multihole pressure probes 2‐7 in many cases have provided the easiest-to-use and most cost-effective method for three-component e ow velocity measurements in research and industry environments. However, even with expanded measurement capabilities of such instruments, the current pressure probe cone gurations and techniques have a limited range of velocity inclinations that they can measure. The velocity inclination is indicated as the cone angle µ in Fig. 1. Let µmax be the maximum cone angle that can be measured reliably by the probe. In other words, a probe with a µmax of 40 deg can accurately measure any velocity vector that is contained within a cone with its apex at the probe tip, its axis along the probe axis, and with an apex included angle of 80 deg (Fig. 1). Measurable cone angles as high as 75 deg are not uncommon for seven-hole probes. 7 Following this reasoning, an omnidirectional probe is a probe that has a cone angle of 180 deg (360 deg included angle ); i.e., it can measure any velocity vector regardless of its orientation. The present work describes the development of a nearly omnidirectional probe that can measure up to cone angles of 170 deg