COHERENT STRUCTURES AND DYNAMICS IN A NEUTRALLY STRATIFIED PLANETARY BOUNDARY-LAYER FLOW

Coherent structures and the dynamics of a neutrally stratified planeta ry boundary layer flow are studied through a large eddy simulation, wh ich includes surface roughness, Coriolis force, and a capping inversio n. Quadrant analysis and flow visualization show that low-speed negati ve momentum flux (ejection) is the dominant feature throughout most of the boundary layer. The initiation of vortical structures is observed to be associated with vorticity sheets and pressure maxima, which are formed dynamically when low-speed negative momentum flux collides wit h either high-speed negative momentum flux (sweep) or the mean flow. F our dimensional conditional averages are used to study the statistical behavior of ejections and sweeps. The shape, strength, lifetime, and origin of the conditionally sampled structures at three different heig hts are discussed. Near the surface, sweeps are observed to induce eje ctions when colliding with the surface. The evolution of sweep-induced ejections near the wall is discussed. (C) 1996 American Institute of Physics.