  We study the dynamics of gyrotactic swimmers in turbulence, whose orientation
is governed by gravitational torque and local fluid velocity gradient. The
gyrotaxis strength is measured by the ratio of the Kolmogorov time scale to the
reorientation time scale due to gravity, and a large value of this ratio means
the gyrotaxis is strong. By means of direct numerical simulations, we
investigate the effects of swimming velocity and gyrotactic stability on
spatial accumulation and alignment. Three-dimensional Vorono{\"\i} analysis is
used to study the spatial distribution and time evolution of the particle
concentration. We study spatial distribution by examing the overall
preferential sampling and where clusters and voids (subsets of particles that
have small and large Vorono{\"\i} volumes respectively) form. Compared with the
ensemble particles, the preferential sampling of clusters and voids is found to
be more pronounced. The clustering of fast swimmers lasts much longer than
slower swimmers when the gyrotaxis is strong and intermediate, but an opposite
trend emerges when the gyrotaxis is weak. In addition, we study the
preferential alignment with the Lagrangian stretching direction, with which
passive slender rods have been known to align. We show that the Lagrangian
alignment is reduced by the swimming velocity when the gyrotaxis is weak, while
the Lagrangian alignment is enhanced for the regime in which gyrotaxis is
strong.
