Buoyancy effects on large scale motions and amplitude modulation

Recent studies of high-Re wall turbulence have revealed the existence of large scale motions (LSMs) that populate the logarithmic layer and amplitude modulate small-scale turbulent fluctuations near the wall. However, the extent to which LSMs occur in flows with unstable thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number) is not well understood. In this study, we consider the effects of buoyancy on LSMs through large eddy simulations (LES) of the convective atmospheric boundary layer spanning weakly to highly unstable thermal stratification. It is found that, as the CBL becomes increasingly unstable, the inclination angle of structures near the ground increases from 12-15° to nearly 90°. Furthermore, the scale separation between the inner and outer peaks in the premultiplied velocity spectra decreases until only a single peak remains (comparable in magnitude to the boundary layer depth). Under weakly convective conditions, significant amplitude modulation occurs due to both the large-scale streamwise and vertical velocity; however, under highly convective conditions amplitude modulation occurs due to the large-scale vertical velocity alone. Connections between the topology of turbulent structures and flow modulation will also be discussed.