Analytic and Experimental Studies of a True Airspeed Sensor

The objective of this study was to analyze and design a true airspeed sensor which will replace the conventional pitot-static pressure transducer. This sensor should have the following characteristics: the flow phenomenon is vortex procession, or the vortex whistle , it should have no moving parts, and also is to be independent of temperature, density, altitude or humidity changes. This sensor has been designed mainly for small commercial aircraft with the airspeed up to 321.9 km/hr (200 mph). In an attempt to model the complicated fluid mechanics of the vortex precession, three-dimensional, inviscid, unsteady, incompressible fluid flow was studied by using the hydrodynamical linearized stability theory. The temporal stability approach was used to derive the relationship between the true airspeed and frequency response. The results show that the frequency response is linearly proportional to the airspeed. The designed sensor basically consists of a vortex tube, a swirler, and a pick up system. When air passed through the swirler, a precessional flow was generated at the region before and after the sudden enlargement area. Also an audiable vortex whistle is generated. The signal is picked by the microphone and the frequency response is shown in a frequency counter. The measurement for both closed conduit tests and wind tunnel tests were recorded. A computer program was developed for obtaining the numerical solution to the theoretical model. The parameters described the sensor geometry were introduced into the calculations. Computational results for various combinations of vortex tubes and swirlers have been obtained. For a specific flow rate or airspeed, the larger the exit swirler angle, the greater the frequency response. For a smaller cross-sectional area at the precessional flow region, the frequency response is higher. It was observed that as the airspeed was increased the Strouhal number remained constant for a fixed design. The Strouhal number was found to be only dependent on the exit angle on swirler. In some cases, the experimental results were found to be in a reasonable agreement with the inviscid theoretical predictions. Viscous effects on the performance of the sensor was believed to be appreciable when experiment and theory differed