Fix bug when using Lagrangian particles on Flat topologies

[Jamie-Hilditch]

Fixes #3545

• Add truncate_fractional_indices to handle the truncation of nothing indices returned by fractional_indices for Flat dimensions. nothing indices are "truncated" to 1.
• truncate_fractional_indices used in advect_particle and bounce_immersed_particle.
• Add method of enforce_boundary_conditions for Flat boundaries. This method does not modify the particle position.

[glwagner]

Looks great!

Should we add a test to make sure this doesn't break in the future? The low bar is just to test that interpolation doesn't error with a Flat direction (we don't have to test correctness, though we could do that too)

[Jamie-Hilditch]

Should we add a test to make sure this doesn't break in the future? The low bar is just to test that interpolation doesn't error with a Flat direction (we don't have to test correctness, though we could do that too)

Yes, probably should. The current tests have a hard coded topology

function run_simple_particle_tracking_tests(arch, timestepper; vertically_stretched=false)
    topo = (Periodic, Periodic, Bounded)


    Nx = Ny = Nz = 5
...

We could add tests to check that simulations can run on different topologies without error. Alternatively, I don't see anything in the existing tests that would not be supported if one or more of the Periodic dimensions was Flat. So we could extent the existing tests to allow for different topologies.

[navidcy]

We can do something like this?

topologies = ((Periodic, Periodic, Bounded), (Periodic, Flat, Bounded))

for topo in topologies
    run_simple_particle_tracking_tests(arch, timestepper; vertically_stretched=false), topo = topo .... )

[Jamie-Hilditch]

I think so. The only thing we'd have to add is a little bit of logic when constructing the grid.

[navidcy]

E.g.,

topologies = ((Periodic, Bounded, Bounded), (Periodic, Flat, Bounded))
sizes = ((5, 5, 5), (5, 5))

for (topo, size) in zip(topologies, sizes)
    @show topo
    @show size
end

topo = (Periodic, Bounded, Bounded)
size = (5, 5, 5)
topo = (Periodic, Flat, Bounded)
size = (5, 5)

[Jamie-Hilditch]

Yeah that would work. It's a bit weird to pass y=(-1,1) to the grid constructor when y is Flat but it seems to be ignored. An alternative would be to pass the grid as an input to run_simple_particle_tracking_tests and move all the grid construction logic (including the stretched vertical grid) to separate functions.

[glwagner]

Yeah that would work. It's a bit weird to pass y=(-1,1) to the grid constructor when y is Flat but it seems to be ignored. An alternative would be to pass the grid as an input to run_simple_particle_tracking_tests and move all the grid construction logic (including the stretched vertical grid) to separate functions.

Maybe better is to create a list of grids:

three_dim_grid = RectilinearGrid(arch, ...)
x_flat_grid = RectilinearGrid(arch, topology=(Flat, Periodic, Bounded), ...)

etc, then pass the grid into run_simple_particle_tracking_tests

[navidcy]

@glwagner's suggestion is cleaner!

[Jamie-Hilditch]

How about something like this

archs = (CPU(),) # just to make this code run

timesteppers = (:QuasiAdamsBashforth2, :RungeKutta3)
y_topologies = (Periodic(), Flat())
vertical_grids = (uniform=(-1,1), stretched=[-1, -0.5, 0.0, 0.4, 0.7, 1])

lagrangian_particle_test_grid(arch, ::Periodic, z) = RectilinearGrid(arch; topology=(Periodic, Periodic, Bounded), size=(5, 5, 5), x=(-1, 1), y=(-1, 1), z)
lagrangian_particle_test_grid(arch, ::Flat, z) = RectilinearGrid(arch; topology=(Periodic, Flat, Bounded), size=(5, 5), x=(-1, 1), z)

for arch in archs, timestepper in timesteppers, y_topo in y_topologies, (z_grid_type, z) in pairs(vertical_grids)
    @info "  Testing Lagrangian particle tracking [$(typeof(arch)), $timestepper] with y $(typeof(y_topo)) on vertically $z_grid_type grid ..."
    grid = lagrangian_particle_test_grid(arch, y_topo, z)
    # run_simple_particle_tracking_tests(arch, grid, timestepper)
    @show grid
end

which would give us these 8 (per architecture) test cases

[ Info:   Testing Lagrangian particle tracking [CPU, QuasiAdamsBashforth2] with y Periodic on vertically uniform grid ...
grid = 5×5×5 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── Periodic x ∈ [-1.0, 1.0) regularly spaced with Δx=0.4
├── Periodic y ∈ [-1.0, 1.0) regularly spaced with Δy=0.4
└── Bounded  z ∈ [-1.0, 1.0] regularly spaced with Δz=0.4
[ Info:   Testing Lagrangian particle tracking [CPU, QuasiAdamsBashforth2] with y Periodic on vertically stretched grid ...
grid = 5×5×5 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── Periodic x ∈ [-1.0, 1.0) regularly spaced with Δx=0.4
├── Periodic y ∈ [-1.0, 1.0) regularly spaced with Δy=0.4
└── Bounded  z ∈ [-1.0, 1.0] variably spaced with min(Δz)=0.3, max(Δz)=0.5
[ Info:   Testing Lagrangian particle tracking [CPU, QuasiAdamsBashforth2] with y Flat on vertically uniform grid ...
grid = 5×1×5 RectilinearGrid{Float64, Periodic, Flat, Bounded} on CPU with 3×0×3 halo 
├── Periodic x ∈ [-1.0, 1.0)      regularly spaced with Δx=0.4
├── Flat y
└── Bounded  z ∈ [-1.0, 1.0]      regularly spaced with Δz=0.4
[ Info:   Testing Lagrangian particle tracking [CPU, QuasiAdamsBashforth2] with y Flat on vertically stretched grid ...
grid = 5×1×5 RectilinearGrid{Float64, Periodic, Flat, Bounded} on CPU with 3×0×3 halo
├── Periodic x ∈ [-1.0, 1.0)      regularly spaced with Δx=0.4
├── Flat y
└── Bounded  z ∈ [-1.0, 1.0]      variably spaced with min(Δz)=0.3, max(Δz)=0.5       
[ Info:   Testing Lagrangian particle tracking [CPU, RungeKutta3] with y Periodic on vertically uniform grid ...
grid = 5×5×5 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── Periodic x ∈ [-1.0, 1.0) regularly spaced with Δx=0.4
├── Periodic y ∈ [-1.0, 1.0) regularly spaced with Δy=0.4
└── Bounded  z ∈ [-1.0, 1.0] regularly spaced with Δz=0.4
[ Info:   Testing Lagrangian particle tracking [CPU, RungeKutta3] with y Periodic on vertically stretched grid ...
grid = 5×5×5 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── Periodic x ∈ [-1.0, 1.0) regularly spaced with Δx=0.4
├── Periodic y ∈ [-1.0, 1.0) regularly spaced with Δy=0.4
└── Bounded  z ∈ [-1.0, 1.0] variably spaced with min(Δz)=0.3, max(Δz)=0.5
[ Info:   Testing Lagrangian particle tracking [CPU, RungeKutta3] with y Flat on vertically uniform grid ...
grid = 5×1×5 RectilinearGrid{Float64, Periodic, Flat, Bounded} on CPU with 3×0×3 halo 
├── Periodic x ∈ [-1.0, 1.0)      regularly spaced with Δx=0.4
├── Flat y
└── Bounded  z ∈ [-1.0, 1.0]      regularly spaced with Δz=0.4
[ Info:   Testing Lagrangian particle tracking [CPU, RungeKutta3] with y Flat on vertically stretched grid ...
grid = 5×1×5 RectilinearGrid{Float64, Periodic, Flat, Bounded} on CPU with 3×0×3 halo 
├── Periodic x ∈ [-1.0, 1.0)      regularly spaced with Δx=0.4
├── Flat y
└── Bounded  z ∈ [-1.0, 1.0]      variably spaced with min(Δz)=0.3, max(Δz)=0.5 

[glwagner]

that looks good, just make sure to add the missing space after a comma and also we prefer arch = tuple(CPU()) to arch = (CPU(),) (slightly easier to read)

[navidcy]

lgtm