  Two-fluid Braginskii codes have simulated open-field line turbulence for over
a decade, and only recently has it become possible to study these systems with
continuum gyrokinetic codes. This work presents a first-of-its-kind comparison
between fluid and (long-wavelength) gyrokinetic models in open field-lines,
using the GDB and Gkeyll codes to simulate interchange turbulence in the
Helimak device at the University of Texas (T. N. Bernard, et. al., Phys. of
Plasmas 26, 042301 (2019)). Partial agreement is attained in a number of
diagnostic channels when the GDB sources and sheath boundary conditions (BCs)
are selected carefully, especially the heat-flux BCs which can drastically
alter the temperature. The radial profile of the fluctuation levels is
qualitatively similar and quantitatively comparable on the low-field side,
although statistics such as moments of the probability density function and the
high-frequency spectrum show greater differences. This comparison indicates
areas for future improvement in both simulations, such as sheath BCs, as well
as improvements in GDB like particle conservation and spatially varying thermal
conductivity, in order to achieve better fluid-gyrokinetic agreement and
increase fidelity when simulating experiments.
