  We present experimental and computational results for the orientation
distributions of slender fibers and ramified particles settling in an isotropic
turbulent flow. The rotational dynamics of the particles is modeled using a
slender-body theory that includes the inertial torque due to sedimentation that
tends to rotate the particles toward a broadside orientation. The particles are
assumed to rotate due to viscous forces associated with the turbulent velocity
gradients occurring on the particle length scale. In the simulations, the
turbulence is obtained from a stochastic model of the velocity gradient in a
Lagrangian reference frame. In the experiments, the turbulence is generated by
active jets in a vertical water tunnel. It is well known that axisymmetric
particles rotate according to Jeffery's solution for the rotation of a
spheroidal particle if one adopts an appropriate effective aspect ratio. We
show that the same result applies to a ramified particle consisting of three
coplanar fibers connected with equal angles at a central point which rotates
like a thin oblate spheroid. The orientation statistics can be quantified with
a single non-dimensional parameter, the settling factor $S_F$, defined as the
ratio of rotations due to sedimentation and turbulent shear. For low values of
$S_F$, we observe nearly isotropically oriented particles, whereas particles
become strongly aligned near the horizontal plane for high values of $S_F$. The
variance of the angle away from horizontal scales as $S_F^{-2}$ for $S_F \gg
1$, but the orientation distribution is non-Gaussian due to turbulent
intermittency in this limit.
