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Fluxatron and sonic anemometer measurements of the momentum flux at a height of 4 metres in the atmospheric boundary layer McDonald, John William

Abstract

At the International Comparison of Turbulence Measuring Instruments, 1970, velocity components and momentum flux measurements were compared using propeller-type Fluxatrons (Hicks, 1970) and sonic anemometers from Kaijo-Denki, Japan and the Institute of Atmospheric Physics, U.S.S.R. There were distinct differences found in the measurements of the vertical velocity from the propeller sensors. The propeller's momentum flux measurements computed from its velocity components were also different. The U¹ propeller was found to be linear for lower frequencies with an associated distance constant of about 7 metre. Measurement of the variance of U¹ for f < 0.16 hz. showed the U¹ propeller in excess of both sonics by 20%. However, with the propeller's high frequency loss beyond f = 0.2 hz. the discrepancy was reduced to only an 8% excess for .00055 hz. < f < 10.8 hz. The W¹ propeller response was non-linear and had an upper cut-off frequency of 1hz. Because of its non-linear response and stalling characteristics at low wind speeds and also its high frequency cut-off the W¹ propeller was observed to measure only about 50% of the total fluctuating W¹ energy available. Analysis of the sonic cospectra of momentum showed that significant contributions to the momentum flux were to be found in the frequency domain 0.001 hz. < f < 5.0 hz. The combined response effects of the propeller were enough to reduce the Fluxatron’s estimate of this momentum flux by 32.5%.

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