Characteristics of turbulent coherent structures in atmospheric flow under different shear-buoyancy conditions

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.