Should timesteps for hydrodynamics be assigned after the force is computed?

From: Leonard Romano <>
Date: Sat, 31 Jul 2021 13:22:36 +0200

Dear gadget-list members,

Recently Julianne Goddard raised an issue with crashes in cosmological
simulations with subgrid physics (Cooling, star formation and feedback)
due to exceedingly small timesteps
I am running similar simulations with a custom feedback model, and often
times run into similar problems. Now I noticed that a major difference
between previous versions of Gadget and Gadget-4 is, that in Gadget-4
the time steps for hydrodynamics are assigned before the forces are
computed. Without sub-grid physics this is probably fine as changes in
quantities like Entropy and velocities are small, but with sub-grid
physics there can be sudden changes by orders of magnitude and so the
time steps before the force calculation might be heavily overestimated
possibly leading to dramatic violations of the Courant-condition.
This is why suggest to move the assignment of time steps in between the
first force calculation and the first kick, in order to protect against
such rapid changes in these quantities.


Leonard Romano, B.Sc.(レオナルド・ロマノ)
Physics Department
Technical University of Munich (TUM), Germany
Theoretical Astrophysics Group
Department of Earth and Space Science
Graduate School of Science, Osaka University, Japan
he / him / his
Received on 2021-07-31 13:22:39

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