GPGI

A Generic Particle + Grid data Interface

This small Python library implements fundamental grid deposition algorithms to analyse (rectilinear) grid + particle datasets, with an emphasize on performance. Core algorithms are implemented as Cython extensions.

Installation

python -m pip install --upgrade pip
python -m pip install gpgi

Supported applications

A rectilinear grid is defined as 1D arrays representing cell left edges in each directions. Note that the last point of such an array is interpreted as the right edge of the rightmost cell, so for instance, a 1D grid containing 100 cells is defined by 101 edges.

Particles are defined as points that live on the grid.

Deposition is the action of going from particule description to a grid description of a field. It is useful to analyze, compare and combine simulation data that exists in a combination of the two formalisms. This process is not reversible as it degrades information.

For instance, here's a simple overlay of a particle set (red dots) against a background that represents the deposited particle count.

This example illustrates the simplest possible deposition method "Particle in Cell", in which each particle contributes only to the cell that contains it.

More refined methods are also available.

Supported deposition methods

Supported geometries

Time complexity

An important step in perfoming deposition is to associate particle indices to cell indices. This step is called "particle indexing". In directions where the grid is uniformly stepped (if any), indexing a particle is an O(1) operation. In the more general case, indexing is performed by bisection, which is a O(log(nx))) operation (where nx represents the number of cells in the direction of interest).

Usage

The API consists in a load function, which returns a Dataset object.

Load data

import numpy as np
import gpgi

nx = ny = 64
nparticles = 600_000

prng = np.random.RandomState(0)
ds = gpgi.load(
    geometry="cartesian",
    grid={
        "cell_edges": {
            "x": np.linspace(-1, 1, nx),
            "y": np.linspace(-1, 1, ny),
        },
    },
    particles={
        "coordinates": {
            "x": 2 * (prng.normal(0.5, 0.25, nparticles) % 1 - 0.5),
            "y": 2 * (prng.normal(0.5, 0.25, nparticles) % 1 - 0.5),
        },
        "fields": {
            "mass": np.ones(nparticles),
        },
    },
)

The Dataset object holds a grid and a particles attribute, which both hold a fields attribute for accessing their data. But more importantly, the Dataset has a deposit method to translate particle fields to the grid formalism.

Deposit Particle fields on the grid

particle_mass = ds.deposit("mass", method="nearest_grid_point")  # or "ngp" for shorts

Visualize In this example we'll use matplotlib for rendering, but note that matplotlib is not a dependency to gpgi

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set(aspect=1, xlabel="x", ylabel="y")

im = ax.pcolormesh(
    "x",
    "y",
    particle_mass.T,
    data=ds.grid.cell_edges,
    cmap="viridis",
)
fig.colorbar(im, ax=ax)

The example script given here takes about a second (top to bottom).

Supplying arbitrary metadata

new in gpgi 0.4.0

Dataset objects have a special attribute metadata which is a dictionary with string keys. This attribute is meant to hold any special metadata that may be relevant for labelling or processing (e.g. simulation time, author, ...). Metadata can be supplied at load time as

ds = gpgi.load(
    geometry="cartesian",
    grid=...,
    particles=...,
    metadata={"simulation_time": 12.5, "author": "Clément Robert"}
)

Boundary conditions

new in gpgi 0.5.0

With CIC and TSC deposition, particles contribute to cells neighbouring the one that contains them. For particles that live in the outermost layer of the domain, this means some of their contribution is lost. This behaviour corresponds to the default 'open' boundary condition, but gpgi has builtin support for more conservative boundary conditions.

Boundary conditions can selected per field, per axis and per side. Builtin recipes all perform linear combinations of ghost layers (same-side and opposite side) and active domain layers (same-side and opposite side), and replace the same-side active layer with the result.

User-selected boundary conditions take the form of an optional argument to Dataset.deposit, as dictionnary with keys being axes names, and values being 2-tuples of boundary conditions names (for left and right side respectively). For instance, here's how one would require periodic boundary conditions on all axes:

ds.deposit(
    "mass",
    method="cic",
    boundaries={
        "x": ("periodic", "periodic"),
        "y": ("periodic", "periodic"),
    }
)

Unspecified axes will use the default 'open' boundary.

Builtin recipes

Define custom recipes

gpgi's boundary recipes can be customized. Let's illustrate this feature with a simple example. Say we want to fix the value of the deposited field in some outer layer. This is done by defining a new function on the user side:

def ones(
    same_side_active_layer,
    same_side_ghost_layer,
    opposite_side_active_layer,
    opposite_side_ghost_layer,
    weight_same_side_active_layer,
    weight_same_side_ghost_layer,
    weight_opposite_side_active_layer,
    weight_opposite_side_ghost_layer,
    side,
    metadata,
):
   return 1.0

where all first eight arguments are numpy.ndarray objects with the same shape (which includes ghost padding !), to which the return value must be broadcastable, side can only be either "left" or "right", and metadata is the special Dataset.metadata attribute. Not all arguments need be used in the body of the function, but this signature is required.

The method must then be registered as a boundary condition recipe as

ds.boundary_recipes.register("ones", ones)

where the associated key (here "ones") is arbitrary. The recipe can now be used exactly as builtin ones, and all of them can be mixed arbitrarily.

ds.deposit(
    "mass",
    method="cic",
    boundaries={
        "x": ("ones", "wall"),
        "y": ("periodic", "periodic"),
    }
)

Note that all first eight arguments in a boundary recipe function should represent an extensive physical quantity (as opposed to intensive). When depositing an intensive quantity u, a weight field w should be supplied (see next section), in which case, the first four arguments represent u*w and the following four represent w, so that u can still be obtained within the function as a ratio if needed.

Weight fields

new in gpgi 0.7.0

Intrisically, deposition algorithms assume that the deposited field u represents an extensive physical quantity (like mass or momentum). In order to deposit a intentive quantity (like velocity or temperature), one must provide an appropriate weight field w.

Let u' and w' be the equivalent on-grid descriptions to u and w. They are obtained as

w'(x) = Σ w(i) c(i,x)
u'(x) = (1/w'(x)) Σ u(i) w(i) c(i,x)

where x is the spatial position, i is a particle index, and c(i,x) are geometric coefficients associated with the deposition method.

Boundary recipes may be associated to the weight field with the weight_field_boundaries argument.

Call help(ds.deposit) for more detail.