  In the past few years, the Event Horizon Telescope (EHT) has provided the
first-ever event horizon-scale images of the supermassive black holes (BHs)
(M87*) and Sagittarius A$^*$ (Sgr A*). The next-generation EHT project is an
extension of the EHT array that promises larger angular resolution and higher
sensitivity to the dim, extended flux around the central ring-like structure,
possibly connecting the accretion flow and the jet. The ngEHT Analysis
Challenges aim to understand the science extractability from synthetic images
and movies to inform the ngEHT array design and analysis algorithm development.
In this work, we compare the accretion flow structure and dynamics in numerical
fluid simulations that specifically target M87* and Sgr A*, and were used to
construct the source models in the challenge set. We consider (1) a
steady-state axisymmetric radiatively inefficient accretion flow model with a
time-dependent shearing hotspot, (2) two time-dependent single fluid general
relativistic magnetohydrodynamic (GRMHD) simulations from the H-AMR code, (3) a
two-temperature GRMHD simulation from the BHAC code, and (4) a two-temperature
radiative GRMHD simulation from the KORAL code. We find that the different
models exhibit remarkably similar temporal and spatial properties, except for
the electron temperature, since radiative losses substantially cool down
electrons near the BH and the jet sheath, signaling the importance of radiative
cooling even for slowly accreting BHs such as M87*. We restrict ourselves to
standard torus accretion flows, and leave larger explorations of alternate
accretion models to future work.
