  This paper presents simulations of isolated 3D filaments in a slab geometry
obtained using a 3D reduced fluid code. First, systematic scans were performed
to investigate how the dynamics of a filament are affected by its amplitude,
perpendicular size and parallel extent. The perpendicular size of the filament
was found to have a strong influence on its motions, as it determined the
relative importance of parallel currents to polarisation and viscous currents,
whilst drift-wave instabilities were observed if the initial amplitude of the
blob was increased sufficiently.
  Next, the 3D simulations were compared to 2D simulations using different
parallel closures; namely, the sheath dissipation closure, which neglects
parallel gradients, and the vorticity advection closure, which neglects the
influence of parallel currents. The vorticity advection closure was found to
not replicate the 3D perpendicular dynamics and overestimated the initial
radial acceleration of all the filaments studied. In contrast, a more
satisfactory comparison with the sheath dissipation closure was obtained, even
in the presence of significant parallel gradients, where the closure is no
longer valid. Specifically it captured the contrasting dynamics of filaments
with different perpen- dicular sizes that were observed in the 3D simulations
which the vorticity advection closure failed to replicate. However, neither
closure successfully replicated the Boltzmann spinning effects and associated
poloidal drift of the blob that was observed in the 3D simulations.
  Although the sheath dissipation closure was concluded to be more successful
in replicating the 3D dynamics, it is emphasised that the vorticity closure may
still be relevant for situations where the parallel current is inhibited from
closing through the sheath due to effects such as increased resistivity near
the targets.
