Numerical study of the wake structure behind a three-dimensional hill

A numerical study of the wake structure behind a three-dimensional hill is presented. Large eddy simulation is used to investigate the source of the wake vorticity and the variations of the wake structure with [U.sub.[infinity]]/Nh. Here, [U.sub.[infinity]] is the freestream velocity, N is the Brunt-Vaisala frequency, and h is the hill height. The simulation shows that when [U.sub.[infinity]]/Nh ~ 1, the wake vorticity originates from baroclinic processes at the initial stage of the wake development. Even though contributions from friction effects increase during the later stage of the wake development, the production of the wake vorticity from baroclinicity remains significant. The wake consists of a pair of vertically oriented vortices, which form vertical lateral boundaries for the wake. When [U.sub.[infinity]]/Nh [less than or equal to] 0.2, the vortical structure of the wake has a horseshoe shape, and the change of the wake width with height reflects the change of the radius of the hill with height. All evidence indicates that the wake at [U.sub.[infinity]] [less than or equal to] 0.2 follows the two-dimensional wake assumption, and thus the wake vorticity comes mainly from surface friction instead of baroclinicity.