Add Minimal Multi-Material hydro scheme (untested WIP)

configure.ac

@@ -779,7 +779,7 @@ fi
 # Hydro scheme.
 AC_ARG_WITH([hydro],
    [AS_HELP_STRING([--with-hydro=<scheme>],
-      [Hydro dynamics to use @<:@gadget2, minimal, hopkins, default, gizmo, shadowfax debug default: gadget2@:>@]
+      [Hydro dynamics to use @<:@gadget2, minimal, hopkins, default, gizmo, shadowfax, minimal-multi-mat debug default: gadget2@:>@]
    )],
    [with_hydro="$withval"],
    [with_hydro="gadget2"]
@@ -803,6 +803,9 @@ case "$with_hydro" in
    shadowfax)
       AC_DEFINE([SHADOWFAX_SPH], [1], [Shadowfax SPH])
    ;;
+   minimal-multi-mat)
+      AC_DEFINE([MINIMAL_MULTI_MAT_SPH], [1], [Minimal Multiple Material SPH])
+   ;;
 
    *)
       AC_MSG_ERROR([Unknown hydrodynamics scheme: $with_hydro])
@@ -890,7 +893,7 @@ esac
 #  Equation of state
 AC_ARG_WITH([equation-of-state],
    [AS_HELP_STRING([--with-equation-of-state=<EoS>],
-      [equation of state @<:@ideal-gas, isothermal-gas, tillotson-iron default: ideal-gas@:>@]
+      [equation of state @<:@ideal-gas, isothermal-gas, tillotson default: ideal-gas@:>@]
    )],
    [with_eos="$withval"],
    [with_eos="ideal-gas"]
@@ -902,8 +905,8 @@ case "$with_eos" in
    isothermal-gas)
       AC_DEFINE([EOS_ISOTHERMAL_GAS], [1], [Isothermal gas equation of state])
    ;;
-   tillotson-iron)
-      AC_DEFINE([EOS_TILLOTSON_IRON], [1], [Tillotson iron equation of state])
+   tillotson)
+      AC_DEFINE([EOS_TILLOTSON], [1], [Tillotson equation of state])
    ;;
    *)
       AC_MSG_ERROR([Unknown equation of state: $with_eos])

src/debug.c

@@ -52,6 +52,8 @@
 #include "./hydro/Gizmo/hydro_debug.h"
 #elif defined(SHADOWFAX_SPH)
 #include "./hydro/Shadowswift/hydro_debug.h"
+#elif defined(MINIMAL_MULTI_MAT_SPH)
+#include "./hydro/MinimalMultiMat/hydro_debug.h"
 #else
 #error "Invalid choice of SPH variant"
 #endif

src/hydro.h

@@ -52,6 +52,10 @@
 #include "./hydro/Shadowswift/hydro_iact.h"
 #define SPH_IMPLEMENTATION \
   "Shadowfax moving mesh (Vandenbroucke and De Rijcke 2016)"
+#elif defined(MINIMAL_MULTI_MAT_SPH)
+#include "./hydro/MinimalMultiMat/hydro.h"
+#include "./hydro/MinimalMultiMat/hydro_iact.h"
+#define SPH_IMPLEMENTATION "Minimal version of SPH with multiple materials"
 #else
 #error "Invalid choice of SPH variant"
 #endif

src/hydro/MinimalMultiMat/hydro.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_MINIMAL_MULTI_MAT_HYDRO_H
+#define SWIFT_MINIMAL_MULTI_MAT_HYDRO_H
+
+/**
+ * @file MinimalMultiMat/hydro.h
+ * @brief Minimal conservative implementation of SPH (Non-neighbour loop
+ * equations) with multiple materials.
+ *
+ * The thermal variable is the internal energy (u). Simple constant
+ * viscosity term without switches is implemented. No thermal conduction
+ * term is implemented.
+ *
+ * This corresponds to equations (43), (44), (45), (101), (103)  and (104) with
+ * \f$\beta=3\f$ and \f$\alpha_u=0\f$ of Price, D., Journal of Computational
+ * Physics, 2012, Volume 231, Issue 3, pp. 759-794.
+ */
+
+#include "adiabatic_index.h"
+#include "approx_math.h"
+#include "cosmology.h"
+#include "dimension.h"
+#include "equation_of_state.h"
+#include "hydro_properties.h"
+#include "hydro_space.h"
+#include "kernel_hydro.h"
+#include "minmax.h"
+
+/**
+ * @brief Returns the comoving internal energy of a particle
+ *
+ * For implementations where the main thermodynamic variable
+ * is not internal energy, this function computes the internal
+ * energy from the thermodynamic variable.
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float
+hydro_get_comoving_internal_energy(const struct part *restrict p) {
+
+  return p->u;
+}
+
+/**
+ * @brief Returns the physical internal energy of a particle
+ *
+ * For implementations where the main thermodynamic variable
+ * is not internal energy, this function computes the internal
+ * energy from the thermodynamic variable and converts it to
+ * physical coordinates.
+ *
+ * @param p The particle of interest.
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float
+hydro_get_physical_internal_energy(const struct part *restrict p,
+                                   const struct cosmology *cosmo) {
+
+  return p->u * cosmo->a_factor_internal_energy;
+}
+
+/**
+ * @brief Returns the comoving pressure of a particle
+ *
+ * Computes the pressure based on the particle's properties.
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_comoving_pressure(
+    const struct part *restrict p) {
+
+  return gas_pressure_from_internal_energy(p->rho, p->u, p->mat_id);
+}
+
+/**
+ * @brief Returns the physical pressure of a particle
+ *
+ * Computes the pressure based on the particle's properties and
+ * convert it to physical coordinates.
+ *
+ * @param p The particle of interest
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_physical_pressure(
+    const struct part *restrict p, const struct cosmology *cosmo) {
+
+  return cosmo->a_factor_pressure *
+         gas_pressure_from_internal_energy(p->rho, p->u, p->mat_id);
+}
+
+/**
+ * @brief Returns the comoving entropy of a particle
+ *
+ * For implementations where the main thermodynamic variable
+ * is not entropy, this function computes the entropy from
+ * the thermodynamic variable.
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_comoving_entropy(
+    const struct part *restrict p) {
+
+  return gas_entropy_from_internal_energy(p->rho, p->u, p->mat_id);
+}
+
+/**
+ * @brief Returns the physical entropy of a particle
+ *
+ * For implementations where the main thermodynamic variable
+ * is not entropy, this function computes the entropy from
+ * the thermodynamic variable and converts it to
+ * physical coordinates.
+ *
+ * @param p The particle of interest
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_physical_entropy(
+    const struct part *restrict p, const struct cosmology *cosmo) {
+
+  /* Note: no cosmological conversion required here with our choice of
+   * coordinates. */
+  return gas_entropy_from_internal_energy(p->rho, p->u, p->mat_id);
+}
+
+/**
+ * @brief Returns the comoving sound speed of a particle
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float
+hydro_get_comoving_soundspeed(const struct part *restrict p) {
+
+  return p->force.soundspeed;
+}
+
+/**
+ * @brief Returns the physical sound speed of a particle
+ *
+ * @param p The particle of interest
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float
+hydro_get_physical_soundspeed(const struct part *restrict p,
+                              const struct cosmology *cosmo) {
+
+  return cosmo->a_factor_sound_speed * p->force.soundspeed;
+}
+
+/**
+ * @brief Returns the comoving density of a particle
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_comoving_density(
+    const struct part *restrict p) {
+
+  return p->rho;
+}
+
+/**
+ * @brief Returns the comoving density of a particle.
+ *
+ * @param p The particle of interest
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_physical_density(
+    const struct part *restrict p, const struct cosmology *cosmo) {
+
+  return cosmo->a3_inv * p->rho;
+}
+
+/**
+ * @brief Returns the mass of a particle
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_mass(
+    const struct part *restrict p) {
+
+  return p->mass;
+}
+
+/**
+ * @brief Sets the mass of a particle
+ *
+ * @param p The particle of interest
+ * @param m The mass to set.
+ */
+__attribute__((always_inline)) INLINE static void hydro_set_mass(
+    struct part *restrict p, float m) {
+
+  p->mass = m;
+}
+
+/**
+ * @brief Returns the velocities drifted to the current time of a particle.
+ *
+ * @param p The particle of interest
+ * @param xp The extended data of the particle.
+ * @param dt_kick_hydro The time (for hydro accelerations) since the last kick.
+ * @param dt_kick_grav The time (for gravity accelerations) since the last kick.
+ * @param v (return) The velocities at the current time.
+ */
+__attribute__((always_inline)) INLINE static void hydro_get_drifted_velocities(
+    const struct part *restrict p, const struct xpart *xp, float dt_kick_hydro,
+    float dt_kick_grav, float v[3]) {
+
+  v[0] = xp->v_full[0] + p->a_hydro[0] * dt_kick_hydro +
+         xp->a_grav[0] * dt_kick_grav;
+  v[1] = xp->v_full[1] + p->a_hydro[1] * dt_kick_hydro +
+         xp->a_grav[1] * dt_kick_grav;
+  v[2] = xp->v_full[2] + p->a_hydro[2] * dt_kick_hydro +
+         xp->a_grav[2] * dt_kick_grav;
+}
+
+/**
+ * @brief Returns the time derivative of internal energy of a particle
+ *
+ * We assume a constant density.
+ *
+ * @param p The particle of interest
+ */
+__attribute__((always_inline)) INLINE static float hydro_get_internal_energy_dt(
+    const struct part *restrict p) {
+
+  return p->u_dt;
+}
+
+/**
+ * @brief Returns the time derivative of internal energy of a particle
+ *
+ * We assume a constant density.
+ *
+ * @param p The particle of interest.
+ * @param du_dt The new time derivative of the internal energy.
+ */
+__attribute__((always_inline)) INLINE static void hydro_set_internal_energy_dt(
+    struct part *restrict p, float du_dt) {
+
+  p->u_dt = du_dt;
+}
+/**
+ * @brief Computes the hydro time-step of a given particle
+ *
+ * This function returns the time-step of a particle given its hydro-dynamical
+ * state. A typical time-step calculation would be the use of the CFL condition.
+ *
+ * @param p Pointer to the particle data
+ * @param xp Pointer to the extended particle data
+ * @param hydro_properties The SPH parameters
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static float hydro_compute_timestep(
+    const struct part *restrict p, const struct xpart *restrict xp,
+    const struct hydro_props *restrict hydro_properties,
+    const struct cosmology *restrict cosmo) {
+
+  const float CFL_condition = hydro_properties->CFL_condition;
+
+  /* CFL condition */
+  const float dt_cfl = 2.f * kernel_gamma * CFL_condition * cosmo->a * p->h /
+                       (cosmo->a_factor_sound_speed * p->force.v_sig);
+
+  return dt_cfl;
+}
+
+/**
+ * @brief Does some extra hydro operations once the actual physical time step
+ * for the particle is known.
+ *
+ * @param p The particle to act upon.
+ * @param dt Physical time step of the particle during the next step.
+ */
+__attribute__((always_inline)) INLINE static void hydro_timestep_extra(
+    struct part *p, float dt) {}
+
+/**
+ * @brief Prepares a particle for the density calculation.
+ *
+ * Zeroes all the relevant arrays in preparation for the sums taking place in
+ * the various density loop over neighbours. Typically, all fields of the
+ * density sub-structure of a particle get zeroed in here.
+ *
+ * @param p The particle to act upon
+ * @param hs #hydro_space containing hydro specific space information.
+ */
+__attribute__((always_inline)) INLINE static void hydro_init_part(
+    struct part *restrict p, const struct hydro_space *hs) {
+
+  p->density.wcount = 0.f;
+  p->density.wcount_dh = 0.f;
+  p->rho = 0.f;
+  p->density.rho_dh = 0.f;
+}
+
+/**
+ * @brief Finishes the density calculation.
+ *
+ * Multiplies the density and number of neighbours by the appropiate constants
+ * and add the self-contribution term.
+ * Additional quantities such as velocity gradients will also get the final
+ * terms added to them here.
+ *
+ * Also adds/multiplies the cosmological terms if need be.
+ *
+ * @param p The particle to act upon
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_end_density(
+    struct part *restrict p, const struct cosmology *cosmo) {
+
+  /* Some smoothing length multiples. */
+  const float h = p->h;
+  const float h_inv = 1.0f / h;                       /* 1/h */
+  const float h_inv_dim = pow_dimension(h_inv);       /* 1/h^d */
+  const float h_inv_dim_plus_one = h_inv_dim * h_inv; /* 1/h^(d+1) */
+
+  /* Final operation on the density (add self-contribution). */
+  p->rho += p->mass * kernel_root;
+  p->density.rho_dh -= hydro_dimension * p->mass * kernel_root;
+  p->density.wcount += kernel_root;
+  p->density.wcount_dh -= hydro_dimension * kernel_root;
+
+  /* Finish the calculation by inserting the missing h-factors */
+  p->rho *= h_inv_dim;
+  p->density.rho_dh *= h_inv_dim_plus_one;
+  p->density.wcount *= h_inv_dim;
+  p->density.wcount_dh *= h_inv_dim_plus_one;
+}
+
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * In the desperate case where a particle has no neighbours (likely because
+ * of the h_max ceiling), set the particle fields to something sensible to avoid
+ * NaNs in the next calculations.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part *restrict p, struct xpart *restrict xp,
+    const struct cosmology *cosmo) {
+
+  /* Some smoothing length multiples. */
+  const float h = p->h;
+  const float h_inv = 1.0f / h;                 /* 1/h */
+  const float h_inv_dim = pow_dimension(h_inv); /* 1/h^d */
+
+  /* Re-set problematic values */
+  p->rho = p->mass * kernel_root * h_inv_dim;
+  p->density.wcount = kernel_root * kernel_norm * h_inv_dim;
+  p->density.rho_dh = 0.f;
+  p->density.wcount_dh = 0.f;
+}
+
+/**
+ * @brief Prepare a particle for the force calculation.
+ *
+ * This function is called in the ghost task to convert some quantities coming
+ * from the density loop over neighbours into quantities ready to be used in the
+ * force loop over neighbours. Quantities are typically read from the density
+ * sub-structure and written to the force sub-structure.
+ * Examples of calculations done here include the calculation of viscosity term
+ * constants, thermal conduction terms, hydro conversions, etc.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_prepare_force(
+    struct part *restrict p, struct xpart *restrict xp,
+    const struct cosmology *cosmo) {
+
+  /* Compute the pressure */
+  const float pressure = gas_pressure_from_internal_energy(p->rho, p->u);
+
+  /* Compute the sound speed */
+  const float soundspeed = gas_soundspeed_from_pressure(p->rho, pressure);
+
+  /* Compute the "grad h" term */
+  const float rho_inv = 1.f / p->rho;
+  const float grad_h_term =
+      1.f / (1.f + hydro_dimension_inv * p->h * p->density.rho_dh * rho_inv);
+
+  /* Update variables. */
+  p->force.f = grad_h_term;
+  p->force.pressure = pressure;
+  p->force.soundspeed = soundspeed;
+}
+
+/**
+ * @brief Reset acceleration fields of a particle
+ *
+ * Resets all hydro acceleration and time derivative fields in preparation
+ * for the sums taking  place in the various force tasks.
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_reset_acceleration(
+    struct part *restrict p) {
+
+  /* Reset the acceleration. */
+  p->a_hydro[0] = 0.0f;
+  p->a_hydro[1] = 0.0f;
+  p->a_hydro[2] = 0.0f;
+
+  /* Reset the time derivatives. */
+  p->u_dt = 0.0f;
+  p->force.h_dt = 0.0f;
+  p->force.v_sig = 0.0f;
+}
+
+/**
+ * @brief Sets the values to be predicted in the drifts to their values at a
+ * kick time
+ *
+ * @param p The particle.
+ * @param xp The extended data of this particle.
+ */
+__attribute__((always_inline)) INLINE static void hydro_reset_predicted_values(
+    struct part *restrict p, const struct xpart *restrict xp) {
+
+  /* Re-set the predicted velocities */
+  p->v[0] = xp->v_full[0];
+  p->v[1] = xp->v_full[1];
+  p->v[2] = xp->v_full[2];
+
+  /* Re-set the entropy */
+  p->u = xp->u_full;
+}
+
+/**
+ * @brief Predict additional particle fields forward in time when drifting
+ *
+ * Additional hydrodynamic quantites are drifted forward in time here. These
+ * include thermal quantities (thermal energy or total energy or entropy, ...).
+ *
+ * Note the different time-step sizes used for the different quantities as they
+ * include cosmological factors.
+ *
+ * @param p The particle.
+ * @param xp The extended data of the particle.
+ * @param dt_drift The drift time-step for positions.
+ * @param dt_therm The drift time-step for thermal quantities.
+ */
+__attribute__((always_inline)) INLINE static void hydro_predict_extra(
+    struct part *restrict p, const struct xpart *restrict xp, float dt_drift,
+    float dt_therm) {
+
+  const float h_inv = 1.f / p->h;
+
+  /* Predict smoothing length */
+  const float w1 = p->force.h_dt * h_inv * dt_drift;
+  if (fabsf(w1) < 0.2f)
+    p->h *= approx_expf(w1); /* 4th order expansion of exp(w) */
+  else
+    p->h *= expf(w1);
+
+  /* Predict density */
+  const float w2 = -hydro_dimension * w1;
+  if (fabsf(w2) < 0.2f)
+    p->rho *= approx_expf(w2); /* 4th order expansion of exp(w) */
+  else
+    p->rho *= expf(w2);
+
+  /* Predict the internal energy */
+  p->u += p->u_dt * dt_therm;
+
+  /* Compute the new pressure */
+  const float pressure = gas_pressure_from_internal_energy(p->rho, p->u);
+
+  /* Compute the new sound speed */
+  const float soundspeed = gas_soundspeed_from_pressure(p->rho, pressure);
+
+  p->force.pressure = pressure;
+  p->force.soundspeed = soundspeed;
+}
+
+/**
+ * @brief Finishes the force calculation.
+ *
+ * Multiplies the force and accelerations by the appropiate constants
+ * and add the self-contribution term. In most cases, there is little
+ * to do here.
+ *
+ * Cosmological terms are also added/multiplied here.
+ *
+ * @param p The particle to act upon
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_end_force(
+    struct part *restrict p, const struct cosmology *cosmo) {
+
+  p->force.h_dt *= p->h * hydro_dimension_inv;
+}
+
+/**
+ * @brief Kick the additional variables
+ *
+ * Additional hydrodynamic quantites are kicked forward in time here. These
+ * include thermal quantities (thermal energy or total energy or entropy, ...).
+ *
+ * @param p The particle to act upon.
+ * @param xp The particle extended data to act upon.
+ * @param dt_therm The time-step for this kick (for thermodynamic quantities).
+ * @param cosmo The cosmological model.
+ * @param hydro_props The constants used in the scheme
+ */
+__attribute__((always_inline)) INLINE static void hydro_kick_extra(
+    struct part *restrict p, struct xpart *restrict xp, float dt_therm,
+    const struct cosmology *cosmo, const struct hydro_props *hydro_props) {
+
+  /* Do not decrease the energy by more than a factor of 2*/
+  if (dt_therm > 0. && p->u_dt * dt_therm < -0.5f * xp->u_full) {
+    p->u_dt = -0.5f * xp->u_full / dt_therm;
+  }
+  xp->u_full += p->u_dt * dt_therm;
+
+  /* Apply the minimal energy limit */
+  const float min_energy =
+      hydro_props->minimal_internal_energy * cosmo->a_factor_internal_energy;
+  if (xp->u_full < min_energy) {
+    xp->u_full = min_energy;
+    p->u_dt = 0.f;
+  }
+
+  /* Compute the pressure */
+  const float pressure = gas_pressure_from_internal_energy(p->rho, xp->u_full, p->mat_id);
+
+  /* Compute the sound speed */
+  const float soundspeed = gas_soundspeed_from_internal_energy(p->rho, p->u, p->mat_id);
+
+  p->force.pressure = pressure;
+  p->force.soundspeed = soundspeed;
+}
+
+/**
+ * @brief Converts hydro quantity of a particle at the start of a run
+ *
+ * This function is called once at the end of the engine_init_particle()
+ * routine (at the start of a calculation) after the densities of
+ * particles have been computed.
+ * This can be used to convert internal energy into entropy for instance.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle to act upon
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_convert_quantities(
+    struct part *restrict p, struct xpart *restrict xp,
+    const struct cosmology *cosmo) {
+
+  /* Compute the pressure */
+  const float pressure = gas_pressure_from_internal_energy(p->rho, p->u, p->mat_id);
+
+  /* Compute the sound speed */
+  const float soundspeed = gas_soundspeed_from_internal_energy(p->rho, p->u, p->mat_id);
+
+  p->force.pressure = pressure;
+  p->force.soundspeed = soundspeed;
+}
+
+/**
+ * @brief Initialises the particles for the first time
+ *
+ * This function is called only once just after the ICs have been
+ * read in to do some conversions or assignments between the particle
+ * and extended particle fields.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_first_init_part(
+    struct part *restrict p, struct xpart *restrict xp) {
+
+  p->time_bin = 0;
+  xp->v_full[0] = p->v[0];
+  xp->v_full[1] = p->v[1];
+  xp->v_full[2] = p->v[2];
+  xp->a_grav[0] = 0.f;
+  xp->a_grav[1] = 0.f;
+  xp->a_grav[2] = 0.f;
+  xp->u_full = p->u;
+
+  hydro_reset_acceleration(p);
+  hydro_init_part(p, NULL);
+}
+
+/**
+ * @brief Overwrite the initial internal energy of a particle.
+ *
+ * Note that in the cases where the thermodynamic variable is not
+ * internal energy but gets converted later, we must overwrite that
+ * field. The conversion to the actual variable happens later after
+ * the initial fake time-step.
+ *
+ * @param p The #part to write to.
+ * @param u_init The new initial internal energy.
+ */
+__attribute__((always_inline)) INLINE static void
+hydro_set_init_internal_energy(struct part *p, float u_init) {
+
+  p->u = u_init;
+}
+
+#endif /* SWIFT_MINIMAL_MULTI_MAT_HYDRO_H */

src/hydro/MinimalMultiMat/hydro_debug.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Coypright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_MINIMAL_MULTI_MAT_HYDRO_DEBUG_H
+#define SWIFT_MINIMAL_MULTI_MAT_HYDRO_DEBUG_H
+
+/**
+ * @file MinimalMultiMat/hydro_debug.h
+ * @brief MinimalMultiMat conservative implementation of SPH (Debugging routines)
+ *
+ * The thermal variable is the internal energy (u). Simple constant
+ * viscosity term without switches is implemented. No thermal conduction
+ * term is implemented.
+ *
+ * This corresponds to equations (43), (44), (45), (101), (103)  and (104) with
+ * \f$\beta=3\f$ and \f$\alpha_u=0\f$ of
+ * Price, D., Journal of Computational Physics, 2012, Volume 231, Issue 3,
+ * pp. 759-794.
+ */
+
+__attribute__((always_inline)) INLINE static void hydro_debug_particle(
+    const struct part* p, const struct xpart* xp) {
+  printf(
+      "x=[%.3e,%.3e,%.3e], "
+      "v=[%.3e,%.3e,%.3e],v_full=[%.3e,%.3e,%.3e] \n a=[%.3e,%.3e,%.3e], "
+      "u=%.3e, du/dt=%.3e v_sig=%.3e, P=%.3e\n"
+      "h=%.3e, dh/dt=%.3e wcount=%d, m=%.3e, dh_drho=%.3e, rho=%.3e, "
+      "time_bin=%d, mat_id=%d\n",
+      p->x[0], p->x[1], p->x[2], p->v[0], p->v[1], p->v[2], xp->v_full[0],
+      xp->v_full[1], xp->v_full[2], p->a_hydro[0], p->a_hydro[1], p->a_hydro[2],
+      p->u, p->u_dt, p->force.v_sig, hydro_get_comoving_pressure(p), p->h,
+      p->force.h_dt, (int)p->density.wcount, p->mass, p->density.rho_dh, p->rho,
+      p->time_bin, p->mat_id);
+}
+
+#endif /* SWIFT_MINIMAL_MULTI_MAT_HYDRO_DEBUG_H */

src/hydro/MinimalMultiMat/hydro_iact.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_MINIMAL_MULTI_MAT_HYDRO_IACT_H
+#define SWIFT_MINIMAL_MULTI_MAT_HYDRO_IACT_H
+
+/**
+ * @file MinimalMultiMat/hydro_iact.h
+ * @brief MinimalMultiMat conservative implementation of SPH (Neighbour loop equations)
+ *
+ * The thermal variable is the internal energy (u). Simple constant
+ * viscosity term without switches is implemented. No thermal conduction
+ * term is implemented.
+ *
+ * This corresponds to equations (43), (44), (45), (101), (103)  and (104) with
+ * \f$\beta=3\f$ and \f$\alpha_u=0\f$ of Price, D., Journal of Computational
+ * Physics, 2012, Volume 231, Issue 3, pp. 759-794.
+ */
+
+#include "adiabatic_index.h"
+#include "minmax.h"
+
+/**
+ * @brief Density interaction between two particles.
+ *
+ * @param r2 Comoving square distance between the two particles.
+ * @param dx Comoving vector separating both particles (pi - pj).
+ * @param hi Comoving smoothing-length of particle i.
+ * @param hj Comoving smoothing-length of particle j.
+ * @param pi First particle.
+ * @param pj Second particle.
+ * @param a Current scale factor.
+ * @param H Current Hubble parameter.
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_density(
+    float r2, const float *dx, float hi, float hj, struct part *restrict pi,
+    struct part *restrict pj, float a, float H) {
+
+  float wi, wj, wi_dx, wj_dx;
+
+  /* Get r. */
+  const float r_inv = 1.0f / sqrtf(r2);
+  const float r = r2 * r_inv;
+
+  /* Get the masses. */
+  const float mi = pi->mass;
+  const float mj = pj->mass;
+
+  /* Compute density of pi. */
+  const float hi_inv = 1.f / hi;
+  const float ui = r * hi_inv;
+  kernel_deval(ui, &wi, &wi_dx);
+
+  pi->rho += mj * wi;
+  pi->density.rho_dh -= mj * (hydro_dimension * wi + ui * wi_dx);
+  pi->density.wcount += wi;
+  pi->density.wcount_dh -= (hydro_dimension * wi + ui * wi_dx);
+
+  /* Compute density of pj. */
+  const float hj_inv = 1.f / hj;
+  const float uj = r * hj_inv;
+  kernel_deval(uj, &wj, &wj_dx);
+
+  pj->rho += mi * wj;
+  pj->density.rho_dh -= mi * (hydro_dimension * wj + uj * wj_dx);
+  pj->density.wcount += wj;
+  pj->density.wcount_dh -= (hydro_dimension * wj + uj * wj_dx);
+}
+
+/**
+ * @brief Density interaction between two particles (non-symmetric).
+ *
+ * @param r2 Comoving square distance between the two particles.
+ * @param dx Comoving vector separating both particles (pi - pj).
+ * @param hi Comoving smoothing-length of particle i.
+ * @param hj Comoving smoothing-length of particle j.
+ * @param pi First particle.
+ * @param pj Second particle (not updated).
+ * @param a Current scale factor.
+ * @param H Current Hubble parameter.
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_nonsym_density(
+    float r2, const float *dx, float hi, float hj, struct part *restrict pi,
+    const struct part *restrict pj, float a, float H) {
+
+  float wi, wi_dx;
+
+  /* Get the masses. */
+  const float mj = pj->mass;
+
+  /* Get r. */
+  const float r_inv = 1.0f / sqrtf(r2);
+  const float r = r2 * r_inv;
+
+  const float h_inv = 1.f / hi;
+  const float ui = r * h_inv;
+  kernel_deval(ui, &wi, &wi_dx);
+
+  pi->rho += mj * wi;
+  pi->density.rho_dh -= mj * (hydro_dimension * wi + ui * wi_dx);
+  pi->density.wcount += wi;
+  pi->density.wcount_dh -= (hydro_dimension * wi + ui * wi_dx);
+}
+
+/**
+ * @brief Force interaction between two particles.
+ *
+ * @param r2 Comoving square distance between the two particles.
+ * @param dx Comoving vector separating both particles (pi - pj).
+ * @param hi Comoving smoothing-length of particle i.
+ * @param hj Comoving smoothing-length of particle j.
+ * @param pi First particle.
+ * @param pj Second particle.
+ * @param a Current scale factor.
+ * @param H Current Hubble parameter.
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_force(
+    float r2, const float *dx, float hi, float hj, struct part *restrict pi,
+    struct part *restrict pj, float a, float H) {
+
+  /* Cosmological factors entering the EoMs */
+  const float fac_mu = pow_three_gamma_minus_five_over_two(a);
+  const float a2_Hubble = a * a * H;
+
+  /* Get r and r inverse. */
+  const float r_inv = 1.0f / sqrtf(r2);
+  const float r = r2 * r_inv;
+
+  /* Recover some data */
+  const float mi = pi->mass;
+  const float mj = pj->mass;
+  const float rhoi = pi->rho;
+  const float rhoj = pj->rho;
+  const float pressurei = pi->force.pressure;
+  const float pressurej = pj->force.pressure;
+
+  /* Get the kernel for hi. */
+  const float hi_inv = 1.0f / hi;
+  const float hid_inv = pow_dimension_plus_one(hi_inv); /* 1/h^(d+1) */
+  const float xi = r * hi_inv;
+  float wi, wi_dx;
+  kernel_deval(xi, &wi, &wi_dx);
+  const float wi_dr = hid_inv * wi_dx;
+
+  /* Get the kernel for hj. */
+  const float hj_inv = 1.0f / hj;
+  const float hjd_inv = pow_dimension_plus_one(hj_inv); /* 1/h^(d+1) */
+  const float xj = r * hj_inv;
+  float wj, wj_dx;
+  kernel_deval(xj, &wj, &wj_dx);
+  const float wj_dr = hjd_inv * wj_dx;
+
+  /* Compute gradient terms */
+  const float P_over_rho2_i = pressurei / (rhoi * rhoi) * pi->force.f;
+  const float P_over_rho2_j = pressurej / (rhoj * rhoj) * pj->force.f;
+
+  /* Compute dv dot r. */
+  const float dvdr = (pi->v[0] - pj->v[0]) * dx[0] +
+                     (pi->v[1] - pj->v[1]) * dx[1] +
+                     (pi->v[2] - pj->v[2]) * dx[2] + a2_Hubble * r2;
+
+  /* Are the particles moving towards each others ? */
+  const float omega_ij = min(dvdr, 0.f);
+  const float mu_ij = fac_mu * r_inv * omega_ij; /* This is 0 or negative */
+
+  /* Compute sound speeds and signal velocity */
+  const float ci = pi->force.soundspeed;
+  const float cj = pj->force.soundspeed;
+  const float v_sig = ci + cj - 3.f * mu_ij;
+
+  /* Construct the full viscosity term */
+  const float rho_ij = 0.5f * (rhoi + rhoj);
+  const float visc = -0.5f * const_viscosity_alpha * v_sig * mu_ij / rho_ij;
+
+  /* Convolve with the kernel */
+  const float visc_acc_term = 0.5f * visc * (wi_dr + wj_dr) * r_inv;
+
+  /* SPH acceleration term */
+  const float sph_acc_term =
+      (P_over_rho2_i * wi_dr + P_over_rho2_j * wj_dr) * r_inv;
+
+  /* Assemble the acceleration */
+  const float acc = sph_acc_term + visc_acc_term;
+
+  /* Use the force Luke ! */
+  pi->a_hydro[0] -= mj * acc * dx[0];
+  pi->a_hydro[1] -= mj * acc * dx[1];
+  pi->a_hydro[2] -= mj * acc * dx[2];
+
+  pj->a_hydro[0] += mi * acc * dx[0];
+  pj->a_hydro[1] += mi * acc * dx[1];
+  pj->a_hydro[2] += mi * acc * dx[2];
+
+  /* Get the time derivative for u. */
+  const float sph_du_term_i = P_over_rho2_i * dvdr * r_inv * wi_dr;
+  const float sph_du_term_j = P_over_rho2_j * dvdr * r_inv * wj_dr;
+
+  /* Viscosity term */
+  const float visc_du_term = 0.5f * visc_acc_term * dvdr;
+
+  /* Assemble the energy equation term */
+  const float du_dt_i = sph_du_term_i + visc_du_term;
+  const float du_dt_j = sph_du_term_j + visc_du_term;
+
+  /* Internal energy time derivatibe */
+  pi->u_dt += du_dt_i * mj;
+  pj->u_dt += du_dt_j * mi;
+
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
+  pj->force.h_dt -= mi * dvdr * r_inv / rhoi * wj_dr;
+
+  /* Update the signal velocity. */
+  pi->force.v_sig = max(pi->force.v_sig, v_sig);
+  pj->force.v_sig = max(pj->force.v_sig, v_sig);
+}
+
+/**
+ * @brief Force interaction between two particles (non-symmetric).
+ *
+ * @param r2 Comoving square distance between the two particles.
+ * @param dx Comoving vector separating both particles (pi - pj).
+ * @param hi Comoving smoothing-length of particle i.
+ * @param hj Comoving smoothing-length of particle j.
+ * @param pi First particle.
+ * @param pj Second particle (not updated).
+ * @param a Current scale factor.
+ * @param H Current Hubble parameter.
+ */
+__attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
+    float r2, const float *dx, float hi, float hj, struct part *restrict pi,
+    const struct part *restrict pj, float a, float H) {
+
+  /* Cosmological factors entering the EoMs */
+  const float fac_mu = pow_three_gamma_minus_five_over_two(a);
+  const float a2_Hubble = a * a * H;
+
+  /* Get r and r inverse. */
+  const float r_inv = 1.0f / sqrtf(r2);
+  const float r = r2 * r_inv;
+
+  /* Recover some data */
+  // const float mi = pi->mass;
+  const float mj = pj->mass;
+  const float rhoi = pi->rho;
+  const float rhoj = pj->rho;
+  const float pressurei = pi->force.pressure;
+  const float pressurej = pj->force.pressure;
+
+  /* Get the kernel for hi. */
+  const float hi_inv = 1.0f / hi;
+  const float hid_inv = pow_dimension_plus_one(hi_inv); /* 1/h^(d+1) */
+  const float xi = r * hi_inv;
+  float wi, wi_dx;
+  kernel_deval(xi, &wi, &wi_dx);
+  const float wi_dr = hid_inv * wi_dx;
+
+  /* Get the kernel for hj. */
+  const float hj_inv = 1.0f / hj;
+  const float hjd_inv = pow_dimension_plus_one(hj_inv); /* 1/h^(d+1) */
+  const float xj = r * hj_inv;
+  float wj, wj_dx;
+  kernel_deval(xj, &wj, &wj_dx);
+  const float wj_dr = hjd_inv * wj_dx;
+
+  /* Compute gradient terms */
+  const float P_over_rho2_i = pressurei / (rhoi * rhoi) * pi->force.f;
+  const float P_over_rho2_j = pressurej / (rhoj * rhoj) * pj->force.f;
+
+  /* Compute dv dot r. */
+  const float dvdr = (pi->v[0] - pj->v[0]) * dx[0] +
+                     (pi->v[1] - pj->v[1]) * dx[1] +
+                     (pi->v[2] - pj->v[2]) * dx[2] + a2_Hubble * r2;
+
+  /* Are the particles moving towards each others ? */
+  const float omega_ij = min(dvdr, 0.f);
+  const float mu_ij = fac_mu * r_inv * omega_ij; /* This is 0 or negative */
+
+  /* Compute sound speeds and signal velocity */
+  const float ci = pi->force.soundspeed;
+  const float cj = pj->force.soundspeed;
+  const float v_sig = ci + cj - 3.f * mu_ij;
+
+  /* Construct the full viscosity term */
+  const float rho_ij = 0.5f * (rhoi + rhoj);
+  const float visc = -0.5f * const_viscosity_alpha * v_sig * mu_ij / rho_ij;
+
+  /* Convolve with the kernel */
+  const float visc_acc_term = 0.5f * visc * (wi_dr + wj_dr) * r_inv;
+
+  /* SPH acceleration term */
+  const float sph_acc_term =
+      (P_over_rho2_i * wi_dr + P_over_rho2_j * wj_dr) * r_inv;
+
+  /* Assemble the acceleration */
+  const float acc = sph_acc_term + visc_acc_term;
+
+  /* Use the force Luke ! */
+  pi->a_hydro[0] -= mj * acc * dx[0];
+  pi->a_hydro[1] -= mj * acc * dx[1];
+  pi->a_hydro[2] -= mj * acc * dx[2];
+
+  /* Get the time derivative for u. */
+  const float sph_du_term_i = P_over_rho2_i * dvdr * r_inv * wi_dr;
+
+  /* Viscosity term */
+  const float visc_du_term = 0.5f * visc_acc_term * dvdr;
+
+  /* Assemble the energy equation term */
+  const float du_dt_i = sph_du_term_i + visc_du_term;
+
+  /* Internal energy time derivatibe */
+  pi->u_dt += du_dt_i * mj;
+
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
+
+  /* Update the signal velocity. */
+  pi->force.v_sig = max(pi->force.v_sig, v_sig);
+}
+
+#endif /* SWIFT_MINIMAL_MULTI_MAT_HYDRO_IACT_H */

src/hydro/MinimalMultiMat/hydro_io.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Coypright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_MINIMAL_MULTI_MAT_HYDRO_IO_H
+#define SWIFT_MINIMAL_MULTI_MAT_HYDRO_IO_H
+
+/**
+ * @file MinimalMultiMat/hydro_io.h
+ * @brief MinimalMultiMat conservative implementation of SPH (i/o routines)
+ *
+ * The thermal variable is the internal energy (u). Simple constant
+ * viscosity term without switches is implemented. No thermal conduction
+ * term is implemented.
+ *
+ * This corresponds to equations (43), (44), (45), (101), (103)  and (104) with
+ * \f$\beta=3\f$ and \f$\alpha_u=0\f$ of
+ * Price, D., Journal of Computational Physics, 2012, Volume 231, Issue 3,
+ * pp. 759-794.
+ */
+
+#include "adiabatic_index.h"
+#include "hydro.h"
+#include "io_properties.h"
+#include "kernel_hydro.h"
+
+/**
+ * @brief Specifies which particle fields to read from a dataset
+ *
+ * @param parts The particle array.
+ * @param list The list of i/o properties to read.
+ * @param num_fields The number of i/o fields to read.
+ */
+void hydro_read_particles(struct part* parts, struct io_props* list,
+                          int* num_fields) {
+
+  *num_fields = 8;
+
+  /* List what we want to read */
+  list[0] = io_make_input_field("Coordinates", DOUBLE, 3, COMPULSORY,
+                                UNIT_CONV_LENGTH, parts, x);
+  list[1] = io_make_input_field("Velocities", FLOAT, 3, COMPULSORY,
+                                UNIT_CONV_SPEED, parts, v);
+  list[2] = io_make_input_field("Masses", FLOAT, 1, COMPULSORY, UNIT_CONV_MASS,
+                                parts, mass);
+  list[3] = io_make_input_field("SmoothingLength", FLOAT, 1, COMPULSORY,
+                                UNIT_CONV_LENGTH, parts, h);
+  list[4] = io_make_input_field("InternalEnergy", FLOAT, 1, COMPULSORY,
+                                UNIT_CONV_ENERGY_PER_UNIT_MASS, parts, u);
+  list[5] = io_make_input_field("ParticleIDs", ULONGLONG, 1, COMPULSORY,
+                                UNIT_CONV_NO_UNITS, parts, id);
+  list[6] = io_make_input_field("Accelerations", FLOAT, 3, OPTIONAL,
+                                UNIT_CONV_ACCELERATION, parts, a_hydro);
+  list[7] = io_make_input_field("Density", FLOAT, 1, OPTIONAL,
+                                UNIT_CONV_DENSITY, parts, rho);
+  list[8] = io_make_input_field("MaterialID", INT, 1, OPTIONAL, 1, parts,
+                                mat_id);
+}
+
+void convert_S(const struct engine* e, const struct part* p,
+               const struct xpart* xp, float* ret) {
+
+  ret[0] = hydro_get_comoving_entropy(p);
+}
+
+void convert_P(const struct engine* e, const struct part* p,
+               const struct xpart* xp, float* ret) {
+
+  ret[0] = hydro_get_comoving_pressure(p);
+}
+
+void convert_part_pos(const struct engine* e, const struct part* p,
+                      const struct xpart* xp, double* ret) {
+
+  if (e->s->periodic) {
+    ret[0] = box_wrap(p->x[0], 0.0, e->s->dim[0]);
+    ret[1] = box_wrap(p->x[1], 0.0, e->s->dim[1]);
+    ret[2] = box_wrap(p->x[2], 0.0, e->s->dim[2]);
+  } else {
+    ret[0] = p->x[0];
+    ret[1] = p->x[1];
+    ret[2] = p->x[2];
+  }
+}
+
+void convert_part_vel(const struct engine* e, const struct part* p,
+                      const struct xpart* xp, float* ret) {
+
+  const int with_cosmology = (e->policy & engine_policy_cosmology);
+  const struct cosmology* cosmo = e->cosmology;
+  const integertime_t ti_current = e->ti_current;
+  const double time_base = e->time_base;
+
+  const integertime_t ti_beg = get_integer_time_begin(ti_current, p->time_bin);
+  const integertime_t ti_end = get_integer_time_end(ti_current, p->time_bin);
+
+  /* Get time-step since the last kick */
+  float dt_kick_grav, dt_kick_hydro;
+  if (with_cosmology) {
+    dt_kick_grav = cosmology_get_grav_kick_factor(cosmo, ti_beg, ti_current);
+    dt_kick_grav -=
+        cosmology_get_grav_kick_factor(cosmo, ti_beg, (ti_beg + ti_end) / 2);
+    dt_kick_hydro = cosmology_get_hydro_kick_factor(cosmo, ti_beg, ti_current);
+    dt_kick_hydro -=
+        cosmology_get_hydro_kick_factor(cosmo, ti_beg, (ti_beg + ti_end) / 2);
+  } else {
+    dt_kick_grav = (ti_current - ((ti_beg + ti_end) / 2)) * time_base;
+    dt_kick_hydro = (ti_current - ((ti_beg + ti_end) / 2)) * time_base;
+  }
+
+  /* Extrapolate the velocites to the current time */
+  hydro_get_drifted_velocities(p, xp, dt_kick_hydro, dt_kick_grav, ret);
+
+  /* Conversion from internal units to peculiar velocities */
+  ret[0] *= cosmo->a2_inv;
+  ret[1] *= cosmo->a2_inv;
+  ret[2] *= cosmo->a2_inv;
+}
+
+void convert_part_potential(const struct engine* e, const struct part* p,
+                            const struct xpart* xp, float* ret) {
+
+  if (p->gpart != NULL)
+    ret[0] = gravity_get_comoving_potential(p->gpart);
+  else
+    ret[0] = 0.f;
+}
+
+/**
+ * @brief Specifies which particle fields to write to a dataset
+ *
+ * @param parts The particle array.
+ * @param xparts The extended particle array.
+ * @param list The list of i/o properties to write.
+ * @param num_fields The number of i/o fields to write.
+ */
+void hydro_write_particles(const struct part* parts, const struct xpart* xparts,
+                           struct io_props* list, int* num_fields) {
+
+  *num_fields = 10;
+
+  /* List what we want to write */
+  list[0] = io_make_output_field_convert_part("Coordinates", DOUBLE, 3,
+                                              UNIT_CONV_LENGTH, parts, xparts,
+                                              convert_part_pos);
+  list[1] = io_make_output_field_convert_part(
+      "Velocities", FLOAT, 3, UNIT_CONV_SPEED, parts, xparts, convert_part_vel);
+  list[2] =
+      io_make_output_field("Masses", FLOAT, 1, UNIT_CONV_MASS, parts, mass);
+  list[3] = io_make_output_field("SmoothingLength", FLOAT, 1, UNIT_CONV_LENGTH,
+                                 parts, h);
+  list[4] = io_make_output_field("InternalEnergy", FLOAT, 1,
+                                 UNIT_CONV_ENERGY_PER_UNIT_MASS, parts, u);
+  list[5] = io_make_output_field("ParticleIDs", ULONGLONG, 1,
+                                 UNIT_CONV_NO_UNITS, parts, id);
+  list[6] =
+      io_make_output_field("Density", FLOAT, 1, UNIT_CONV_DENSITY, parts, rho);
+  list[7] = io_make_output_field_convert_part("Entropy", FLOAT, 1,
+                                              UNIT_CONV_ENTROPY_PER_UNIT_MASS,
+                                              parts, xparts, convert_S);
+  list[8] = io_make_output_field_convert_part("MaterialID", INT, 1, 1, parts,
+                                              mat_id);
+  list[8] = io_make_output_field_convert_part(
+      "Pressure", FLOAT, 1, UNIT_CONV_PRESSURE, parts, xparts, convert_P);
+
+  list[9] = io_make_output_field_convert_part("Potential", FLOAT, 1,
+                                              UNIT_CONV_POTENTIAL, parts,
+                                              xparts, convert_part_potential);
+}
+
+/**
+ * @brief Writes the current model of SPH to the file
+ * @param h_grpsph The HDF5 group in which to write
+ */
+void hydro_write_flavour(hid_t h_grpsph) {
+
+  /* Viscosity and thermal conduction */
+  /* Nothing in this minimal model... */
+  io_write_attribute_s(h_grpsph, "Thermal Conductivity Model", "No treatment");
+  io_write_attribute_s(h_grpsph, "Viscosity Model",
+                       "Minimal treatment as in Monaghan (1992)");
+
+  /* Time integration properties */
+  io_write_attribute_f(h_grpsph, "Maximal Delta u change over dt",
+                       const_max_u_change);
+}
+
+/**
+ * @brief Are we writing entropy in the internal energy field ?
+ *
+ * @return 1 if entropy is in 'internal energy', 0 otherwise.
+ */
+int writeEntropyFlag() { return 0; }
+
+#endif /* SWIFT_MINIMAL_MULTI_MAT_HYDRO_IO_H */

src/hydro/MinimalMultiMat/hydro_part.h

+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (c) 2016 Matthieu Schaller (matthieu.schaller@durham.ac.uk)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#ifndef SWIFT_MINIMAL_MULTI_MAT_HYDRO_PART_H
+#define SWIFT_MINIMAL_MULTI_MAT_HYDRO_PART_H
+
+/**
+ * @file MinimalMultiMat/hydro_part.h
+ * @brief MinimalMultiMat conservative implementation of SPH (Particle definition)
+ *
+ * The thermal variable is the internal energy (u). Simple constant
+ * viscosity term without switches is implemented. No thermal conduction
+ * term is implemented.
+ *
+ * This corresponds to equations (43), (44), (45), (101), (103)  and (104) with
+ * \f$\beta=3\f$ and \f$\alpha_u=0\f$ of Price, D., Journal of Computational
+ * Physics, 2012, Volume 231, Issue 3, pp. 759-794.
+ */
+
+#include "chemistry_struct.h"
+#include "cooling_struct.h"
+
+/**
+ * @brief Particle fields not needed during the SPH loops over neighbours.
+ *
+ * This structure contains the particle fields that are not used in the
+ * density or force loops. Quantities should be used in the kick, drift and
+ * potentially ghost tasks only.
+ */
+struct xpart {
+
+  /*! Offset between current position and position at last tree rebuild. */
+  float x_diff[3];
+
+  /*! Offset between the current position and position at the last sort. */
+  float x_diff_sort[3];
+
+  /*! Velocity at the last full step. */
+  float v_full[3];
+
+  /*! Gravitational acceleration at the last full step. */
+  float a_grav[3];
+
+  /*! Internal energy at the last full step. */
+  float u_full;
+
+  /*! Additional data used to record cooling information */
+  struct cooling_xpart_data cooling_data;
+
+} SWIFT_STRUCT_ALIGN;
+
+/**
+ * @brief Particle fields for the SPH particles
+ *
+ * The density and force substructures are used to contain variables only used
+ * within the density and force loops over neighbours. All more permanent
+ * variables should be declared in the main part of the part structure,
+ */
+struct part {
+
+  /*! Particle unique ID. */
+  long long id;
+
+  /*! Pointer to corresponding gravity part. */
+  struct gpart* gpart;
+
+  /*! Particle position. */
+  double x[3];
+
+  /*! Particle predicted velocity. */
+  float v[3];
+
+  /*! Particle acceleration. */
+  float a_hydro[3];
+
+  /*! Particle mass. */
+  float mass;
+
+  /*! Particle smoothing length. */
+  float h;
+
+  /*! Particle internal energy. */
+  float u;
+
+  /*! Time derivative of the internal energy. */
+  float u_dt;
+
+  /*! Particle density. */
+  float rho;
+
+  /*! Material identifier flag (integer) */
+  material_id mat_id;
+
+  /* Store density/force specific stuff. */
+  union {
+
+    /**
+     * @brief Structure for the variables only used in the density loop over
+     * neighbours.
+     *
+     * Quantities in this sub-structure should only be accessed in the density
+     * loop over neighbours and the ghost task.
+     */
+    struct {
+
+      /*! Neighbour number count. */
+      float wcount;
+
+      /*! Derivative of the neighbour number with respect to h. */
+      float wcount_dh;
+
+      /*! Derivative of density with respect to h */
+      float rho_dh;
+
+    } density;
+
+    /**
+     * @brief Structure for the variables only used in the force loop over
+     * neighbours.
+     *
+     * Quantities in this sub-structure should only be accessed in the force
+     * loop over neighbours and the ghost, drift and kick tasks.
+     */
+    struct {
+
+      /*! "Grad h" term */
+      float f;
+
+      /*! Particle pressure. */
+      float pressure;
+
+      /*! Particle soundspeed. */
+      float soundspeed;
+
+      /*! Particle signal velocity */
+      float v_sig;
+
+      /*! Time derivative of smoothing length  */
+      float h_dt;
+
+    } force;
+  };
+
+  /* Chemistry information */
+  struct chemistry_part_data chemistry_data;
+
+  /*! Time-step length */
+  timebin_t time_bin;
+
+#ifdef SWIFT_DEBUG_CHECKS
+
+  /* Time of the last drift */
+  integertime_t ti_drift;
+
+  /* Time of the last kick */
+  integertime_t ti_kick;
+
+#endif
+
+} SWIFT_STRUCT_ALIGN;
+
+#endif /* SWIFT_MINIMAL_MULTI_MAT_HYDRO_PART_H */

src/hydro_io.h

@@ -35,6 +35,8 @@
 #include "./hydro/Gizmo/hydro_io.h"
 #elif defined(SHADOWFAX_SPH)
 #include "./hydro/Shadowswift/hydro_io.h"
+#elif defined(MINIMAL_MULTI_MAT_SPH)
+#include "./hydro/MinimalMultiMat/hydro_io.h"
 #else
 #error "Invalid choice of SPH variant"
 #endif