  We perform a numerical simulation of the dynamics of quantized vortices
produced by coflow in a square channel using the vortex filament model. Unlike
the situation in thermal counterflow, where the superfluid velocity $v_{\rm s}$
and normal-fluid velocity $v_{\rm n}$ flow in opposite directions, in coflow,
$v_{\rm s}$ and $v_{\rm n}$ flow in the same direction. Quantum turbulence in
thermal counterflow has been long studied theoretically and experimentally, and
its various features have been revealed.In recent years, an experiment on
quantum turbulence in coflow has been performed to observe different features
of thermal counterflow. By supposing that $v_{\rm s}$ is uniform and $v_{\rm
n}$ takes the Hagen-Poiseuille profile, which is different from the experiment
where $v_{\rm n}$ is thought to be turbulent, we calculate the coflow
turbulence. Vortices preferentially accumulate on the surface of a cylinder for
$v_{\rm s} \simeq v_{\rm n}$ by mutual friction; namely, the coflow turbulence
has an attractor. How strongly the vortices are attracted depends on the
temperature and velocity. The length of the vortices increases as the vortices
protruding from the cylindrical attractor continue to wrap around it. As the
vortices become dense on the attractor, they spread toward its interior by
their repulsive interaction. Then, the superfluid velocity profile induced by
the vortices gradually mimics the normal-fluid velocity profile. This is an
indication of velocity matching, which is an important feature of coflow
turbulence.
