Transitional behaviour of saltation: wind tunnel observations of unsteady winds

Aspects of the transitional behaviour of aeolian saltation are investigated in a wind tunnel using non-intrusive, high frequency (approximate to 25 Hz) optical detection of sand transport. The experiments were conducted in concert with the development of a numerical model of sand transport in unsteady winds by Spies and McEwan. Comparisons are made with model outcomes and with data collected using a sand trap equipped with a sensitive load cell. A programmable velocity control was used to generate reproducible velocity excursions and sinusoidal gust sequences. For velocities less than approximate to 8.0 m s(-1) the performance of the wind tunnel meets the recommended value of the independence Froude number. Transport spikes observed at inception of mass flux are shown to be an artefact of sand trap and load cell operation. In unsteady winds such devices must be used with caution. Data from the optical sensor recover most temporal features of the numerical model, including the fast response of mass flux to velocity and the slower response of velocity to mass flux. Lack of transport overshoot and an unnaturally rapid feedback (< 1 s) between wind and grain cloud in velocity increments are functions of limited wind tunnel height; both features are in accord with model predictions for an equivalent simulation height. Surface creep detected by the sensor also enhances the observed primary response times. In sequences of sinusoidal velocity variations, sand transport rate is found to increase as gust frequency increases. This relation becomes more pronounced as gust amplitude increases. In the range of energy-containing atmospheric gusts with periodicities between 6 and 20s, sand transport occurs at rates in excess of that recorded for steady winds of the same mean velocity. This result has significant implications for the prediction of sand transport in unsteady winds. (C)1998 Academic Press Limited.