Тhe error of a standard air flow velocity measurement procedure in wind tunnels of low subsonic speeds

Abstract

The standard air flow velocity measurement procedure has been developed in order to unify and standardize the measuring instruments used and data processing algorithms, reduce the development time and certify such standard methods. Sources of error in indirect measurements of air flow velocity in the range 3–105 m/s are investigated. The sources of instrumental error of indirect measurements are analyzed – pressure sensors with digital and analog output signals, pitot tube, temperature and relative air humidity sensors, atmospheric pressure sensors. It is shown that the greatest contribution to the instrumental error is made by measuring the pressure difference and atmospheric pressure. The errors of method, which depends on the measurement model, is considered. A new mathematical model of measurements is proposed, which includes optimal expressions for determining the density of moist air and corrections for compressibility and allows to reduce the methodological error, and requirements for the metrological characteristics of the measuring instruments used are developed, ensuring that the total error of indirect measurements of the flow velocity complies with the allowable limits of ±0,2 m/s. Recommendations are given on the use of measuring instruments and algorithms for processing measurement results, which make it possible to reduce the measurement error by 39 % and develop a standard air flow velocity measurement procedure. The standard procedure is relevant for aviation industry enterprises, where the air flow rate is measured by the pneumometric method.