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The Aerodynamics of the Pitot-Static Tube and its Current Role in Non-Ideal Engineering Applications

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The Pitot-static tube is a traditional device for local point-wise measurement of airspeed. It is a typical component used in conjunction with most, if not all, wind tunnels in the undergraduate and graduate engineering laboratory. Depending on the application, the physical size of the probe can range from relatively small on the order of a millimeter in diameter, up to 5-10 mm in diameter for large “field” application devices. The principles of the Pitot-static tube (or Pitot- static probe) are typically introduced as an example application of Bernoulli’s equation in most undergraduate courses in fluid mechanics. Yet, in spite of the fact that it has been in use for decades, its use (and sometimes misuse) in non-traditional non-ideal flow situations is still the subject of investigation and uncertainty for important engineering applications. Consequently, it is necessary to acquaint engineering students not only with the normal operation of such instruments, but with their behavior in non-ideal flow situations as well. This is crucial from the standpoint of establishing the limits of applicability and accuracy, and in identifying flow situations where this device should and should not be implemented.

This paper attempts to investigate the non-traditional characteristics of the Pitot-static tube, in conjunction with an engineering laboratory for a senior-level elective introductory course in Aerodynamics. This is a natural place to introduce engineering students to the more detailed physical characteristics of this device, including the concept of pressure coefficient, dimensional analysis, streamlining and boundary layer separation phenomena. Equally important, and not generally given much consideration in a typical introductory fluid mechanics course, are the directional characteristics of the Pitot-static tube. The Pitot-static tube is not a directional device. It is generally well known that the Pitot-static tube is designed for use in relatively ideal parallel or nearly parallel flows. Such flows are typically encountered in a wind tunnel test section free stream, or nearby a smooth aerodynamic surface not encountering boundary layer separation. What may not be generally known, however, is that this device is commonly used by field engineers presently conducting “test and balance” of Heating, Ventilating and Air-Conditioning (HVAC) systems in large commercial buildings, to assess volumetric flow rate. Furthermore, the flows encountered in such applications can be quite far from the ideal parallel flows that the Pitot-static tube is designed to operate in. The focus of this paper is to demonstrate the full-range of Pitot-static tube directional characteristics to students using a simple physical setup in our Aerolab Educational wind tunnel facility. Quantitative measurements of Pitot-static tube pressure coefficient are complemented with flow visualization of the associated boundary layer separation phenomena to provide the students with insight into the measured Pitot-static tube behavior. The practical implications of these observations on the accuracy and reliability of modern field “test and balance” measurements are also investigated. Recommendations are proposed and investigated by the students for dealing with such non-ideal, but very important, field measurement situations.

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Beck, B. T., & Payne, G., & Heitman, T. (2010, June), The Aerodynamics Of The Pitot Static Tube And Its Current Role In Non Ideal Engineering Applications Paper presented at 2010 Annual Conference & Exposition, Louisville, Kentucky. 10.18260/1-2--16733