Field measurements of turbulent flow structures over a nebkha

Abstract

This paper examines turbulent flow structures over a nebkha. We use high-frequency 3D ultrasonic anemometers to sample the wind velocity and turbulent flow across the longitudinal axis (center-line) of the nebkha. Results determined using quadrant analysis of wind velocity show that although the ejection and sweep events are dominant in the toe region of the nebkha, making a positive contribution to the Reynolds shear stress, the reversed wind flow can occur near the toe region when the stoss slope is sufficiently steep. The reversing flow causes an increase of the frequency of occurrence of inward and outward events in the lower stoss region, which suppresses the generation of Reynolds shear stress. Computational fluid dynamic (CFD) modelling demonstrates that when the stoss slope is lower than 20∘, flow reversing can not occur at the toe region, suggesting that 20∘ can be considered as the threshold stoss slope for the generation of reversing flow near the toe region of the nebkha, which is confirmed by the field observations. When the instantaneous Reynolds shear stress −u′w′¯ is one order of magnitude larger than the mean Reynolds shear stress, it contributes approximately 40% to the mean Reynolds shear stress with < 5% of the time share rate, suggesting that the instantaneous Reynolds shear stress might be the controlling factor for the entrainment of sand particles. As a consequence, when monthly, daily or hourly wind data are used to estimate the sand flux, some important instantaneous information (i.e., storm events) may be smoothed out by the averaging process.