Implement the time integration method used in DualSPHysics

Project.toml

@@ -31,6 +31,12 @@ TimerOutputs = "a759f4b9-e2f1-59dc-863e-4aeb61b1ea8f"
 TrixiBase = "9a0f1c46-06d5-4909-a5a3-ce25d3fa3284"
 WriteVTK = "64499a7a-5c06-52f2-abe2-ccb03c286192"
 
+[weakdeps]
+OrdinaryDiffEqCore = "bbf590c4-e513-4bbe-9b18-05decba2e5d8"
+
+[extensions]
+TrixiParticlesOrdinaryDiffEqExt = "OrdinaryDiffEqCore"
+
 [compat]
 Adapt = "3, 4"
 CSV = "0.10"
@@ -44,6 +50,7 @@ GPUArraysCore = "0.1, 0.2"
 JSON = "0.21"
 KernelAbstractions = "0.9"
 MuladdMacro = "0.2"
+OrdinaryDiffEqCore = "1.14.1"
 PointNeighbors = "0.4.2"
 Polyester = "0.7.10"
 RecipesBase = "1"

ext/TrixiParticlesOrdinaryDiffEqExt.jl

@@ -0,0 +1,175 @@
+module TrixiParticlesOrdinaryDiffEqExt
+
+using TrixiParticles
+
+# This is needed because `TrixiParticles.@threaded` translates
+# to `PointNeighbors.parallel_foreach`, so `PointNeighbors` must be available.
+const PointNeighbors = TrixiParticles.PointNeighbors
+
+using OrdinaryDiffEqCore: @.., @muladd, @cache, OrdinaryDiffEqCore,
+                          OrdinaryDiffEqPartitionedAlgorithm, OrdinaryDiffEqMutableCache
+
+struct SymplecticPositionVerlet <: OrdinaryDiffEqPartitionedAlgorithm end
+
+TrixiParticles.SymplecticPositionVerlet() = SymplecticPositionVerlet()
+
+OrdinaryDiffEqCore.default_linear_interpolation(alg::SymplecticPositionVerlet, prob) = true
+
+@cache struct SymplecticPositionVerletCache{uType, rateType, uEltypeNoUnits} <:
+              OrdinaryDiffEqMutableCache
+    u::uType
+    uprev::uType
+    tmp::uType
+    k::rateType
+    fsalfirst::rateType
+    half::uEltypeNoUnits
+end
+
+function OrdinaryDiffEqCore.get_fsalfirstlast(cache::SymplecticPositionVerletCache, u)
+    return (cache.fsalfirst, cache.k)
+end
+
+# Copied from OrdinaryDiffEqSymplecticRK
+#
+# provide the mutable uninitialized objects to keep state and derivative in case of mutable caches
+# no such objects are required for constant caches
+function alloc_symp_state(integrator)
+    (integrator.u.x..., integrator.cache.tmp.x...)
+end
+
+# load state and derivatives at begin of symplectic iteration steps
+function load_symp_state(integrator)
+    (integrator.uprev.x..., integrator.fsallast.x...)
+end
+
+# store state and derivatives at the end of symplectic iteration steps
+function store_symp_state!(integrator, cache, kdu, ku)
+    copyto!(integrator.k[1].x[1], integrator.k[2].x[1])
+    copyto!(integrator.k[1].x[2], integrator.k[2].x[2])
+    copyto!(integrator.k[2].x[2], ku)
+    copyto!(integrator.k[2].x[1], kdu)
+    nothing
+end
+
+function OrdinaryDiffEqCore.alg_cache(alg::SymplecticPositionVerlet, u, rate_prototype,
+                                      ::Type{uEltypeNoUnits},
+                                      ::Type{uBottomEltypeNoUnits}, ::Type{tTypeNoUnits},
+                                      uprev,
+                                      uprev2, f, t,
+                                      dt, reltol, p, calck,
+                                      ::Val{true}) where {uEltypeNoUnits,
+                                                          uBottomEltypeNoUnits,
+                                                          tTypeNoUnits}
+    tmp = zero(u)
+    k = zero(rate_prototype)
+    fsalfirst = zero(rate_prototype)
+    half = uEltypeNoUnits(1 // 2)
+    SymplecticPositionVerletCache(u, uprev, k, tmp, fsalfirst, half)
+end
+
+function OrdinaryDiffEqCore.alg_cache(alg::SymplecticPositionVerlet, u, rate_prototype,
+                                      ::Type{uEltypeNoUnits},
+                                      ::Type{uBottomEltypeNoUnits}, ::Type{tTypeNoUnits},
+                                      uprev,
+                                      uprev2, f, t,
+                                      dt, reltol, p, calck,
+                                      ::Val{false}) where {uEltypeNoUnits,
+                                                           uBottomEltypeNoUnits,
+                                                           tTypeNoUnits}
+    error("`SymplecticPositionVerlet` only supports inplace functions")
+end
+
+function OrdinaryDiffEqCore.initialize!(integrator,
+                                        cache::C) where {C <: SymplecticPositionVerletCache}
+    integrator.kshortsize = 2
+    resize!(integrator.k, integrator.kshortsize)
+    integrator.k[1] = integrator.fsalfirst
+    integrator.k[2] = integrator.fsallast
+
+    duprev, uprev = integrator.uprev.x
+    integrator.f.f1(integrator.k[2].x[1], duprev, uprev, integrator.p, integrator.t)
+    # verify_f2(integrator.f.f2, integrator.k[2].x[2], duprev, uprev, integrator.p,
+    #     integrator.t, integrator, cache)
+    OrdinaryDiffEqCore.increment_nf!(integrator.stats, 1)
+    integrator.stats.nf2 += 1
+end
+
+@muladd function OrdinaryDiffEqCore.perform_step!(integrator,
+                                                  cache::SymplecticPositionVerletCache,
+                                                  repeat_step=false)
+    (; t, dt, f, p) = integrator
+    duprev, uprev, _, _ = load_symp_state(integrator)
+    du, u, kdu, ku = alloc_symp_state(integrator)
+
+    # update position half step
+    half = cache.half
+    f.f2(ku, duprev, uprev, p, t)
+    @.. broadcast=false u=uprev + dt * half * ku
+
+    # update velocity half step
+    f.f1(kdu, duprev, uprev, p, t)
+    @.. broadcast=false du=duprev + dt * half * kdu
+
+    # update velocity (add to previous full step velocity)
+    f.f1(kdu, du, u, p, t + half * dt)
+    # The following is equivalent to `du = duprev + dt * kdu` for the velocity, but when
+    # the density is integrated, a different update is used for the density.
+    semi = p
+    TrixiParticles.foreach_system(semi) do system
+        kdu_system = TrixiParticles.wrap_v(kdu, system, semi)
+        du_system = TrixiParticles.wrap_v(du, system, semi)
+        duprev_system = TrixiParticles.wrap_v(duprev, system, semi)
+
+        update_velocity!(du_system, kdu_system, duprev_system, system, dt)
+        update_density!(du_system, kdu_system, duprev_system, system, dt)
+    end
+
+    # update position (add to half step position)
+    f.f2(ku, du, u, p, t + dt)
+    @.. broadcast=false u=u + dt * half * ku
+
+    OrdinaryDiffEqCore.increment_nf!(integrator.stats, 2)
+    integrator.stats.nf2 += 2
+    store_symp_state!(integrator, cache, kdu, ku)
+end
+
+@muladd function update_velocity!(du_system, kdu_system, duprev_system, system, dt)
+    @.. broadcast=false du_system=duprev_system + dt * kdu_system
+end
+
+@inline function update_density!(du_system, kdu_system, duprev_system, system, dt)
+    return du_system
+end
+
+@muladd function update_velocity!(du_system, kdu_system, duprev_system,
+                                  system::WeaklyCompressibleSPHSystem, dt)
+    TrixiParticles.@threaded system for particle in TrixiParticles.each_moving_particle(system)
+        for i in 1:ndims(system)
+            du_system[i, particle] = duprev_system[i, particle] +
+                                     dt * kdu_system[i, particle]
+        end
+    end
+end
+
+@inline function update_density!(du_system, kdu_system, duprev_system,
+                                 system::WeaklyCompressibleSPHSystem, dt)
+    update_density!(du_system, kdu_system, duprev_system,
+                    system.density_calculator, system, dt)
+end
+
+@inline function update_density!(du_system, kdu_system, duprev_system,
+                                 density_calculator, system, dt)
+    return du_system
+end
+
+@muladd function update_density!(du_system, kdu_system, duprev_system,
+                                 ::ContinuityDensity, system, dt)
+    TrixiParticles.@threaded system for particle in TrixiParticles.each_moving_particle(system)
+        density_prev = duprev_system[end, particle]
+        density_half = du_system[end, particle]
+        epsilon = -kdu_system[end, particle] / density_half * dt
+        du_system[end, particle] = density_prev * (2 - epsilon) / (2 + epsilon)
+    end
+end
+
+end # module

src/TrixiParticles.jl

@@ -88,5 +88,6 @@ export kinetic_energy, total_mass, max_pressure, min_pressure, avg_pressure,
 export interpolate_line, interpolate_point, interpolate_plane_3d, interpolate_plane_2d,
        interpolate_plane_2d_vtk
 export SurfaceTensionAkinci, CohesionForceAkinci
+export SymplecticPositionVerlet
 
 end # module

src/general/general.jl

@@ -9,3 +9,4 @@ include("buffer.jl")
 include("interpolation.jl")
 include("custom_quantities.jl")
 include("neighborhood_search.jl")
+include("time_integration.jl")

src/general/time_integration.jl

@@ -0,0 +1,13 @@
+# Time integration is handles by the package OrdinaryDiffEq.jl.
+# See the docs for more details.
+# In this file, we define the structs for extra time integration schemes that
+# are implemented in the package extension TrixiParticlesOrdinaryDiffEqExt.jl.
+"""
+    SymplecticPositionVerlet()
+
+Modified leapfrog integration scheme for Weakly Compressible SPH (WCSPH) when integrating
+the density with [`ContinuityDensity`](@ref).
+This scheme is used by DualSPHysics:
+https://github.com/DualSPHysics/DualSPHysics/wiki/3.-SPH-formulation#372-symplectic-position-verlet-scheme
+"""
+SymplecticPositionVerlet(_) = nothing