  The present study performs a numerical simulation of the interference and
heat transfer between hairpin vortices formed in wakes behind staggered hills
in a laminar boundary layer. Hairpin vortices are periodically shed in the wake
of a row of hills, causing interference between the hairpin vortices. As the
spanwise distance between the hills decreases, interference increases and the
hairpin vortices become strong. At that time, because the interference between
the legs of the hairpin vortex and the Q2 ejection becomes strong, the head of
each hairpin vortex rises sharply. When the hill spacing decreases, the
turbulence caused by the head and both legs of the hairpin vortex generated
from a hill in the second row increases remarkably. In addition, the secondary
vortex also generates turbulence. The hairpin vortex and the secondary vortex
are attracted to adjacent hairpin vortices, causing widespread high turbulence
in the spanwise direction near the wall surface. Regardless of the hill
spacing, Q2 ejection and Q4 sweep due to the hairpin vortex occur, and the
secondary vortex forms around the hairpin vortex, activating heat transport and
increasing the heat transfer coefficient in the wake. When the hill spacing
becomes narrower, the interference between the hairpin vortices strengthens the
legs of each hairpin vortex and secondary vortex, and heat transport near the
wall surface becomes very active. The heat transfer increases over a wide range
of the wake because the legs of hairpin vortices flowing downstream are spread
in the spanwise direction.
