Sensitivity of non-radiative cloud-wind interactions to the
  hydrodynamics solver
  Cloud-wind interactions are common in the interstellar and circumgalactic
media. Many studies have used simulations of such interactions to investigate
the effect of particular physical processes, but the impact of the choice of
hydrodynamics solver has largely been overlooked. Here we study the cloud-wind
interaction, also known as the "blob test", using seven different hydrodynamics
solvers: Three flavours of SPH, a moving mesh, adaptive mesh refinement and two
meshless schemes. The evolution of masses in dense gas and
intermediate-temperature gas, as well as the covering fraction of
intermediate-temperature gas, are systematically compared for initial density
contrasts of 10 and 100, and four numerical resolutions. To isolate the
differences due to the hydrodynamics solvers, we use non-radiative simulations
without physical conduction. We find large differences between these methods.
SPH methods show slower dispersal of the cloud, particularly for the higher
density contrast, but faster convergence, especially for the lower density
contrast. Predictions for the intermediate-temperature gas differ particularly
strongly, also between non-SPH codes, and converge most slowly. We conclude
that the hydrodynamical interaction between a dense cloud and a supersonic wind
remains an unsolved problem. Studies aiming to understand the physics or
observational signatures of cloud-wind interactions should test the robustness
of their results by comparing different hydrodynamics solvers.
