Cosmological expansion missing in estimate of DivVel?

From: Leonard Romano <>
Date: Wed, 14 Jul 2021 12:25:04 +0200

Dear gadget-list members,

I am wondering if in cosmological simulations we are failing to take
into account the geometry of space-time into the computed estimates of
velocity gradients.
I would expect that whenever we compute differences of velocities of
neighboring SPH particles the velocity should be corrected by the
isotropic Hubble flow, similar to what is done for artificial viscosity.
In the estimate of the velocity-gradient tensor this would result in an
isotropic contribution -a²H x *1*,**where *1***is the 3x3 identity
matrix. This does not affect the symmetric trace-free shear tensor or
the curl, but it does add to the divergence (by an amount of -3a²H).

Maybe I am mistaken, so then I would be very interested in why here the
Hubble-flow does not contribute.

Best regards,
Leonard Romano

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 2021-07-14 12:25:10

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