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

Research output: Contribution to journalReview article

62 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)57-76
Number of pages20
JournalGeomorphology
Volume68
Issue number1-2
DOIs
StatePublished - May 15 2005
Externally publishedYes

Fingerprint

anemometer
sediment transport
turbulence
complex terrain
airflow
shear stress
instrumentation
sensor
computational fluid dynamics
wind tunnel
eolian process
three-dimensional flow
morphodynamics
sampling
bedform
velocity profile
simulation
roughness
visualization
dune

Keywords

  • Aeolian
  • Dune
  • Reynolds stress
  • Rotation
  • Sediment transport
  • Shear stress
  • Ultrasonic anemometer

ASJC Scopus subject areas

  • Earth-Surface Processes

Cite this

Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research. / Walker, Ian.

In: Geomorphology, Vol. 68, No. 1-2, 15.05.2005, p. 57-76.

Research output: Contribution to journalReview article

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KW - Aeolian

KW - Dune

KW - Reynolds stress

KW - Rotation

KW - Sediment transport

KW - Shear stress

KW - Ultrasonic anemometer

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