  The spherical collapse scenario has great importance in cosmology since it
captures several crucial aspects of structure formation. The presence of
self-similar solutions in the Einstein-de Sitter (EdS) model greatly simplifies
its analysis, making it a powerful tool to gain valuable insights into the real
and more complicated physical processes involved in galaxy formation. While
there has been a large body of research to incorporate various additional
physical processes into spherical collapse, the effect of modified gravity (MG)
models, which are popular alternatives to the $\Lambda CDM$ paradigm to explain
the cosmic acceleration, is still not well understood in this scenario. In this
paper, we study the spherical accretion of collisional gas in a particular MG
model, which is a rare case that also admits self-similar solutions. The model
displays interesting behaviours caused by the enhanced gravity and a screening
mechanism. Despite the strong effects of MG, we find that its self-similar
solution agrees well with that of the EdS model. These results are used to
assess a new cosmological hydrodynamical code for spherical collapse
simulations introduced here, which is based on the hyperbolic partial
differential equation engine ExaHyPE 2. Its good agreement with the theoretical
predictions confirms the reliability of this code in modelling astrophysical
processes in spherical collapse. We will use this code to study the evolution
of gas in more realistic MG models in future work.
