Application of thermal anemometry and high-frequency measurement of mass flux to aeolian sediment transport research

Abstract

Recent research on sediment transport by temporally varying winds has stimulated the need for high-frequency measurements of wind velocity, turbulence and transport rate. Conventional technologies yielding only temporally-averaged estimates of wind velocity and mass flux are unable to resolve the transitional and self-regulating behaviour of the aeolian saltation system in unsteady winds. The characteristics, calibration and potential of thermal velocity probes suitable for research on aeolian transport are analysed. Recent developments in particle impact sensing and the use of sand trap/force transducer combinations are reviewed, and the design of a new optical mass flux sensor is presented. An upper frequency limit of about 1 Hz is shown to be essential to transport rate relations in unsteady forcing winds or during intermittent transport. Higher frequencies are required if Reynolds stress or potentially influential flow structures and sediment/fluid interactions in the saltation cloud are to be resolved. Armoured hot-wire sensors measure u-component air velocity to 1 Hz in grain clouds, and some less robust two-dimensional thermal sensors may be used at higher frequencies to determine bed shear from Reynolds stress u′w′ during sand transport by wind. Grain impact devices and automated sand traps provide high-frequency measurements of transport rate, but they are intrusive and their output signals may be ambiguous. This paper reports the design and testing of a new instrument system providing high-frequency measurements of aeolian sand transport. It comprises a non-intrusive, optical mass flux sensor synchronized to an array of armoured hot-wire anemometers. The sensor uses a linear image array to detect, at a sampling frequency of 4–40 Hz, the transit of wind-driven particles through a laser sheet transverse to the flow in a wind tunnel. The physics of the sensor have been rigorously explored in static grain tests and calibrations to mass flux. Its principal advantages over existing devices are its non-intrusive design and ability to measure near-bed mass flux with precision at high frequency. Example applications demonstrate that this instrumentation is able, for the first time, to recover high-frequency aspects of the temporal behaviour of aeolian saltation, previously revealed only by numerical models.