Abstract—The paper depicts air velocity values, reproduced by laser Doppler anemometer (LDA) and ultrasonic anemometer (UA), relations with calculated ones from flow rate measurements using the gas meter which calibration uncertainty is ± (0.15 – 0.30) %. Investigation had been performed in channel installed in aerodynamical facility used as a part of national standard of air velocity. Relations defined in a research let us confirm the LDA and UA for air velocity reproduction to be the most advantageous measures. The results affirm ultrasonic anemometer to be reliable and favourable instrument for measurement of mean velocity or control of velocity stability in the velocity range of 0.05 m/s – 10 (15) m/s when the LDA used. The main aim of this research is to investigate low velocity regularities, starting from 0.05 m/s, including region of turbulent, laminar and transitional air flows. Theoretical and experimental results and brief analysis of it are given in the paper. Maximum and mean velocity relations for transitional air flow having unique distribution are represented. Transitional flow having distinctive and different from laminar and turbulent flow characteristics experimentally have not yet been analysed. Keywords—Laser Doppler anemometer, ultrasonic anemometer, air flow velocities, transitional flow regime, measurement, uncertainty. I. INTRODUCTION ITHUANIAN air velocity national standard is designed for air velocity units reproduction and its transfer to working standards and various type anemometers according to traceability chain. Declared and approved expanded uncertainties are ± (7 – 1) % for the velocity range (0.2 – 60) m/s, respectively. The validation of these values is found by air velocity measurement in an aerodynamic facility channel using converegent nozzles (CN) and static Pitot tubes (SPT). Reference SPT was calibrated at the laboratory of PTB (Germany) in a range of (1 – 40) m/s with the expanded uncertainty ± (3 – 0.3) %, CNs were calibrated by reference volumetric gas meters with the expanded uncertainty ± (0.25 – 0.3) %. Despite the calibration of CNs is quite precise, only mean velocity values’ reproduction at the measuring channel cross-section is warranted. Local velocity value, eg., in channel axis or flow core, could be calculated only after cognition of real velocity distribution in the cross-section. That means all fluid flow characteristics in the channel had to be investigated. It should be noticed that, in general, SPT usage for low velocity ((3 – 1) m/s) measurement is complicated because of precise measurement difficulties of low differential pressure values. According to the national laboratories of European countries, velocity unit values measurement capabilities and used methods are of very wide variety (Table I). The most reliable values are produced by Switzerland and Germany standards. Switzerland standard is the primary standard because it reproduces velocity values according to velocity unit definition, i.e. measured distance is divided by meter travelling time at a constant velocity. Germany standard is based on flow velocity measurement by laser Doppler anemometer (LDA). This method is very precise especially when a system is calibrated using rotating disc. Calibration uncertainty totals ± (5 – 0.08) % in a velocity range (0.2 – 60) m/s. Since 2006, LDA was applied for air velocity measurement research in the laboratory and the first investigations revealed air velocity reproduction and transfer in a range of (0.5 – 30) m/s with the uncertainty of ± (2.8 – 0.3) % [1]. The accuracy of measurement increased up to 3 times. Further investigation aims are: Ü research of LDA application capabilities, especially for low velocities; Ü the minimum velocity value identification capable to be reproduced in workable aerodynamic facility, up to nowadays practical needs 0.05 m/s or less; to evaluate influence quantities for this velocity range that determine measurement accuracy and to confirm the investigations by other method based on usage of ultrasonic anemometer (UA) that is of high resolution and need no particles in a flow; Ü measurements comparison with the ones of European countries national standards; Ü research on reproduced values transferring peculiarities to mostly used practical velocity measurement devices including relation of their geometrical dimensions to standard channel ones; Ü theoretical analysis of velocity distribution consistent pattern in an initial section of measurement channel. II. EXPERIMENTAL METHODOLOGY A. Experimental Equipment Low air velocity measurement range from 0.05 m/s to 0.5 m/s was analysed in order to evaluate measurement reliability. For the purpose, LDA was used to measure local velocity Low Air Velocity Measurement Characteristics’ Variation Due to Flow Regime A. Pedišius, V. Janušas, and A. Bertašienė L International Journal of Mechanical, Industrial and Aerospace Engineering 3:1 2009 25