On the trailing coherent structures of a Delta wing in ground effect

Near-ground coherent structures are occasionally generated at the aircraft's trailing during takeoff, causing high risk to following aircraft because of hazardous rolling moments. This study numerically characterizes for the first time these three-dimensional vortex structures of the near-ground delta wing at Re=6000 (based on c-delta wing chord) using the lattice Boltzmann method with a block refinement framework. The simulation quantifies how the angle of attack (α) modulates vortex dynamics across four spatial regimes: pre-attachment, attachment, aloft, and post-attachment. In the pre-attachment regime, increasing α from 15° to 25° intensifies circulation and shifts the primary vortex ground-collision point upstream from 14c to 8c. Upon collision in the attachment regime, the primary vortices induce a strong adverse pressure gradient on the ground. This adverse pressure gradient generates secondary vortices that subsequently wraparound the primary vortex cores. This helical wrapping mechanism directly forces the primary pair into the aloft regime, where the vortices rebound vertically to a height of 0.45c and diverge transversely to spanwise length larger than c. Furthermore, the angle of attack dictates the structural breakdown in the farfield wake. At α=20° and 25°, the increased circulation accelerates mutual induction, which triggers the long-wavelength Crow instability and breaks the vortex pair into descending rings. In contrast to these high angles, α=15° reduces the circulation and completely suppresses the Crow instability.