  This study performs a large-eddy simulation of turbulent separated and
reattached flow in an enlarged annular pipe. A vortex ring is periodically shed
from the sudden expansion part. A longitudinal vortex occurs around the vortex
ring, making the flow three-dimensional. As a result, the vortex ring becomes
unstable downstream and splits into small vortices. A tubular longitudinal
vortex structure occurs downstream of the reattachment point near the wall
surface on the inner pipe side. A low-frequency fluctuation occurs at each pipe
diameter ratio. The smaller the pipe diameter ratio is, the more downstream the
influences of small-scale vortices and low-frequency fluctuation on the flow
field appear. The smaller the pipe diameter ratio, the slower the pressure
recovery downstream from the reattachment point. The pressure recovery on the
inner pipe side is delayed compared to the outer pipe side. Turbulence is
maximum upstream of the reattachment point due to the small-scale vortices
generated by the collapse of the vortex ring. This maximum value decreases as
the pipe diameter ratio decreases. The smaller the pipe diameter ratio, the
higher the turbulence downstream from the reattachment point.
