The Effect of Rotation on Very Large Scale Motions in the Atmospheric Boundary Layer

Turbulent flows in general are composed of constituent coherent structures and background flows. The dynamics and role of coherent structures are thus fundamental to the understanding of complex motions and mixing processes in Atmospheric Boundary Layer (ABL) flows. One of the distinguishing features of ABL flows is the presence of rotational effects manifested through the Coriolis force. Under the effect of the Coriolis force, Very Large Scale Motions (VLSMs) were studied and characterized. Previous studies confirmed the presence of VLSMs in channel and ABL flows through experimentation and simulations. However, due to limitations in experimentation and the lack of the account of Coriolis force in numerical simulations, the effects of rotation on VLSM have not been thoroughly examined. Here, the Large Eddy Simulation technique was used to simulate VLSMs under different degrees of rotation. Both traditional statistical and image processing techniques were used to characterize them. The traditional method of determination of length scales of VLSMs from per-multiplied spectra of the stream-wise velocity component proved to be inadequate when rotation was introduced. By identifying the footprints of VLSMs as connected regions of low momentum fluid and then identifying those regions through standard image processing tools, the difficulties in determining the VLSM length scales could be circumvented. It was found that rotation inhibited the coherence and suppressed the development of VLSMs in the ABL flows. Also, it was found that the usual practice of visually identifying VLSM on numerically filtered velocity fields as long regions of low momentum might not be sufficiently accurate.