Characteristics of Turbulent Coherent Structures in Atmospheric Flow Under Different Shear–Buoyancy Conditions

Eslam R. Lotfy, Ashraf A. Abbas, Sheikh Ahmad Zaki, Zambri Harun

Research output: Contribution to journalArticle

Abstract

Turbulent coherent structures in the atmospheric boundary layer exert unsteady loads on mechanical and civil structures and greatly contribute to pollutant dispersion and heat dissipation. Much has been deduced about the characteristics of these structures at the laboratory scale in pure shear-driven flows. We examine the influence of atmospheric stability (shear–buoyancy variation) on the newly discovered properties of these turbulence features using observations obtained from a test facility at an onshore site on the east coast of Malaysia. Three ultrasonic anemometers placed at 1.7, 3 and 12 m above ground collected 124 30-min samples of the undisturbed flow. Contrary to expectations, the decline in shear stress in stable stratification reduced the time delay of the peak cross-correlation, implying an increase in the inclination angle of these structures. A wavelet analysis shows that, although the time scale of the vortex packets decreases as the atmosphere becomes increasingly stable, the super-streak time scale increases. The monotonic increase in the energy content in the convective direction results in an enhanced modulating effect for the large super-streaks on the small vortex packets. Analyzing the structure coherence defined as the temporal extension of the streamwise velocity depression reveals two stages of the life cycle of convective rolls. In the first stage, a super-streak couple consisting of a warm updraft and a cold downdraft appears simultaneously at a Monin–Obukhov stability parameter ζ=-3.5. In the second stage, the warm updraft strengthens and the cold downdraft weakens.

Original languageEnglish
JournalBoundary-Layer Meteorology
DOIs
Publication statusPublished - 1 Jan 2019

Fingerprint

updraft
vortex
timescale
anemometer
wavelet analysis
shear stress
dissipation
stratification
life cycle
boundary layer
turbulence
atmosphere
coast
energy
cold
parameter
laboratory
effect
test
pollutant dispersion

Keywords

  • Amplitude modulation
  • Super-structures
  • Thermal stability
  • Vortex packets
  • Wavelet analysis

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

Characteristics of Turbulent Coherent Structures in Atmospheric Flow Under Different Shear–Buoyancy Conditions. / Lotfy, Eslam R.; Abbas, Ashraf A.; Zaki, Sheikh Ahmad; Harun, Zambri.

In: Boundary-Layer Meteorology, 01.01.2019.

Research output: Contribution to journalArticle

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