Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research

Abstract

Recently, ultrasonic anemometers (UAs) have become available for precise, high-frequency measurement of three-dimensional velocity and turbulence properties. Except for a few wind tunnel and computational fluid dynamics (CFD) simulations, advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence (e.g., cup anemometers) or unable to withstand sediment-laden airflow (e.g., hotfilms). In contrast, extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time. This paper provides a pragmatic discussion on using UAs in aeolian research. Recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near-surface shear stress using UAs over complex terrain are discussed. Physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non-logarithmic velocity profiles resulting from roughness- and topographically induced effects and the inability of instrumentation to measure within the thin constant-stress region. These constraints hinder accurate shear velocity ( u *), shear stress and sand transport estimation. UAs allow measurement of turbulent Reynolds stress (RS) that, in theory, should equal profile-derived shear stress. Discrepancies often exist between these quantities however due to three-dimensional (spanwise) flow components and rapid distortion effects (i.e., unbalanced production and dissipation of turbulence) common in flow over complex terrain. While the RS approach yields information on turbulent contributions to near-surface stress generation, little evidence exists showing that RS is a better measure of forces responsible for sediment transport. Consequently, predictive equations for sediment transport using RS do not exist. There is also a need to identify the role of micro-turbulent events (e.g., burst–sweep cycles) and macro-turbulent structures (e.g., separation cells, shear layers) in aeolian dynamics in field settings to validate recent wind tunnel and CFD simulations. A conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects. In unsteady, non-uniform flow over complex terrain, streamline angles vary spatially and temporally with height and location. Thus, determination of, and correction to, true streamline coordinates is difficult. Caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour (e.g., lift at a dune crest) and may complicate interpretations of RS. Instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines. Logistical considerations include sensor design advantages and operational limitations, data communication formats and field deployment strategies—each can affect measurement accuracy and are easily overlooked. Sampling range, frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized. Ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings. With proper considerations of their limitations, UAs may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics.