  We use the boundary element method to study the low-Reynolds number
locomotion of a spherical model microorganism in a circular tube. The swimmer
propels itself by tangen- tial or normal surface motion in a tube whose radius
is on the order of the swimmer size. Hydrodynamic interactions with the tube
walls significantly affect the average swimming speed and power consumption of
the model microorganism. In the case of swimming parallel to the tube axis, the
locomotion speed is always reduced (resp. increased) for swimmers with
tangential (resp. normal) deformation. In all cases, the rate of work nec-
essary for swimming is increased by confinement. Swimmers with no force-dipoles
in the far field generally follow helical trajectories, solely induced by
hydrodynamic interactions with the tube walls, and in qualitative agreement
with recent experimental observations for Paramecium. Swimmers of the puller
type always display stable locomotion at a lo- cation which depends on the
strength of their force dipoles: swimmers with weak dipoles (small {\alpha})
swim in the centre of the tube while those with strong dipoles (large {\alpha})
swim near the walls. In contrast, pusher swimmers and those employing normal
deformation are unstable and end up crashing into the walls of the tube.
Similar dynamics is observed for swimming into a curved tube. These results
could be relevant for the future design of artificial microswimmers in confined
geometries.
