Gravity

Static Gravity

Static gravity is activated using the STATIC_GRAV macro, and is used in several of the example problems provided with Cholla. Static gravity is a simple prescription that does not require any other gravity flags, but does require the input file parameter ``custom_grav" to specify the analytic function that will be applied (dev branch only). Static gravity is applied as momentum and energy source terms in src/hydro/hydro_cuda.cu and the analytic functions are defined in src/gravity/static_grav.h. As of 10-27-2023 on the main branch, the static gravitational field is hard-coded to provide a Milky Way-like model or an M82-like model. On the dev branch, the input parameter flags correspond to:

1D:

• 1: a MW-like Miyamoto-Nagai disk + NFW halo potential (assumed z = 0)

2D:

• 1: Gresho vortex
• 2: Rayleigh-Taylor instability
• 3: A 2D Keplerin disk
• 4: A MW-like Kuzmin disk + NFW halo potential (assumed z = 0)

3D:

• 1: A MW-like Miyamoto-Nagai disk + NFW halo potential
• 2: An M82-like Miyamoto-Nagai disk + NFW halo potential

Self Gravity: FFT-based

In addition to static gravity, Cholla has an FFT-based self gravity solver. Only one or the other may be used. The self-gravity solver is turned on with the GRAVITY macro in the makefile. The default behavior in the make.type.gravity build (and builds that depend on it) is also to turn on the GRAVITY_GPU macro, which ensures that gravity fields reside on the GPU (required for gpu-based MPI communications), and the PARIS macro, which specifies that the Poisson solve will be carried out on the GPU by the cuFFT or rocFFT libraries. Cholla does also have CPU-based gravity solvers, although they are not currently maintained. Definitions of other macros options associated with the gravity solver are given below.

Macro flags associated with self-gravity:

GRAVITY: Turns on self-gravity. Necessary for particle-only simulations.

GRAVITY_GPU: Specifies that fields required by gravity are allocated on the GPU.

PARIS: Use the Paris 3D GPU-based Poisson solver to calculate the gravitational potential on a periodic domain.

GRAVITY_5_POINTS_GRADIENT: Use a 5-point stencil to calculate the gradient of the potential for gravity source terms (default behavior is a 3-point stencil)

GRAVITY_ANALYTIC_COMP: Add an analytic component to the gravitational potential. As of 10-27-2023, this is hard-coded to a Milky Way galaxy model in the function Setup_Analytic_Potential from gravity_functions.cpp.

PARIS_3PT: Use a 3-point gradient for the divergence operator approximation in Paris (default behavior is to use a spectral method)

PARIS_5PT: Use a 5-point gradient for the divergence operator approximation in Paris

PARIS_GALACTIC: Use the Paris Poisson solver on a domain with analytic boundaries set to match the selected model in the DiskGalaxy class. As of 10-27-2023, this is hard-coded to a Milky Way galaxy model in the function Compute_Gravitational_Potential from gravity_functions.cpp and in Compute_Potential_Isolated_Boundary from gravity_boundaries.cpp.

PARIS_GALACTIC_3PT: Same as above but for the analytic boundary version

PARIS_GALACTIC_5PT: Same as above but for the analytic boundary version

PARIS_GALACTIC_TEST: Turn on to test whether Paris returns the same gravitational potential as the SOR solver. Doesn't work with GRAVITY_GPU, should probably be deprecated.

Self Gravity: SOR based

To-do: Describe the SOR solver