  This paper illustrates the mechanism of U-shaped vortex formation which is
found both by experiment and DNS. The main goal of this paper is to explain how
the U-shaped vortex is formed and further develops. According to the results
obtained by our direct numerical simulation with high order accuracy, the
U-shaped vortex is part of the coherent vortex structure and is actually the
tertiary streamwise vortices induced by the secondary vortices. The new finding
is quite different from existing theories which describe that the U-shaped
vortex is newly formed as the head of young turbulence spot and finally break
down to small pieces. In addition, we find that the U-shaped vortex has the
same vorticity sign as the original {\lambda}-shaped vortex tube legs and
serves as a second neck to supply vorticity to the ringlike vortex when the
original vortex tube is stretched and multiple rings are generated.
