Cleaned up 1D Voronoi algorithm some more. Added SHADOWFAX_TOTAL_ENERGY flag...

Cleaned up 1D Voronoi algorithm some more. Added SHADOWFAX_TOTAL_ENERGY flag and support for an isothermal EoS.

src/const.h

@@ -60,6 +60,8 @@
 /* Options to control SHADOWFAX_SPH */
 /* This option disables cell movement */
 //#define SHADOWFAX_FIX_CELLS
+/* This option evolves the total energy instead of the thermal energy */
+//#define SHADOWFAX_TOTAL_ENERGY
 
 /* Source terms */
 #define SOURCETERMS_NONE

src/equation_of_state.h

@@ -69,6 +69,21 @@ __attribute__((always_inline)) INLINE static float gas_pressure_from_entropy(
   return entropy * pow_gamma(density);
 }
 
+/**
+ * @brief Returns the entropy given density and pressure.
+ *
+ * Computes \f$A = \frac{P}{\rho^\gamma}\f$.
+ *
+ * @param density The density \f$\rho\f$.
+ * @param pressure The pressure \f$P\f$.
+ * @return The entropy \f$A\f$.
+ */
+__attribute__((always_inline)) INLINE static float gas_entropy_from_pressure(
+    float density, float pressure) {
+
+  return pressure * pow_minus_gamma(density);
+}
+
 /**
  * @brief Returns the sound speed given density and entropy
  *
@@ -111,6 +126,20 @@ gas_pressure_from_internal_energy(float density, float u) {
   return hydro_gamma_minus_one * u * density;
 }
 
+/**
+ * @brief Returns the internal energy given density and pressure.
+ *
+ * Computes \f$u = \frac{1}{\gamma - 1}\frac{P}{\rho}\f$.
+ *
+ * @param density The density \f$\rho\f$.
+ * @param pressure The pressure \f$P\f$.
+ * @return The internal energy \f$u\f$.
+ */
+__attribute__((always_inline)) INLINE static float
+gas_internal_energy_from_pressure(float density, float pressure) {
+  return hydro_one_over_gamma_minus_one * pressure / density;
+}
+
 /**
  * @brief Returns the sound speed given density and internal energy
  *
@@ -172,6 +201,22 @@ __attribute__((always_inline)) INLINE static float gas_pressure_from_entropy(
   return hydro_gamma_minus_one * const_isothermal_internal_energy * density;
 }
 
+/**
+ * @brief Returns the entropy given density and pressure.
+ *
+ * Computes \f$A = \frac{P}{\rho^\gamma}\f$.
+ *
+ * @param density The density \f$\rho\f$.
+ * @param pressure The pressure \f$P\f$ (ignored).
+ * @return The entropy \f$A\f$.
+ */
+__attribute__((always_inline)) INLINE static float gas_entropy_from_pressure(
+    float density, float pressure) {
+
+  return hydro_gamma_minus_one * const_isothermal_internal_energy *
+         pow_minus_gamma_minus_one(density);
+}
+
 /**
  * @brief Returns the sound speed given density and entropy
  *
@@ -219,6 +264,20 @@ gas_pressure_from_internal_energy(float density, float u) {
   return hydro_gamma_minus_one * const_isothermal_internal_energy * density;
 }
 
+/**
+ * @brief Returns the internal energy given density and pressure.
+ *
+ * Just returns the constant internal energy.
+ *
+ * @param density The density \f$\rho\f$ (ignored).
+ * @param pressure The pressure \f$P\f$ (ignored).
+ * @return The internal energy \f$u\f$ (which is constant).
+ */
+__attribute__((always_inline)) INLINE static float
+gas_internal_energy_from_pressure(float density, float pressure) {
+  return const_isothermal_energy;
+}
+
 /**
  * @brief Returns the sound speed given density and internal energy
  *

src/hydro/Shadowswift/hydro.h

@@ -39,7 +39,13 @@ __attribute__((always_inline)) INLINE static float hydro_compute_timestep(
 
   float R = get_radius_dimension_sphere(p->cell.volume);
 
-  return CFL_condition * R / fabsf(p->timestepvars.vmax);
+  if (p->timestepvars.vmax == 0.) {
+    /* vmax can be zero in vacuum. We force the time step to become the maximal
+       time step in this case */
+    return FLT_MAX;
+  } else {
+    return CFL_condition * R / fabsf(p->timestepvars.vmax);
+  }
 }
 
 /**
@@ -91,7 +97,17 @@ __attribute__((always_inline)) INLINE static void hydro_first_init_part(
   p->conserved.momentum[1] = mass * p->primitives.v[1];
   p->conserved.momentum[2] = mass * p->primitives.v[2];
 
+#ifdef EOS_ISOTHERMAL_GAS
+  p->conserved.energy = mass * const_isothermal_internal_energy;
+#else
   p->conserved.energy *= mass;
+#endif
+
+#ifdef SHADOWFAX_TOTAL_ENERGY
+  p->conserved.energy += 0.5f * (p->conserved.momentum[0] * p->primitives.v[0] +
+                                 p->conserved.momentum[1] * p->primitives.v[1] +
+                                 p->conserved.momentum[2] * p->primitives.v[2]);
+#endif
 
 #if defined(SHADOWFAX_FIX_CELLS)
   p->v[0] = 0.;
@@ -99,6 +115,7 @@ __attribute__((always_inline)) INLINE static void hydro_first_init_part(
   p->v[2] = 0.;
 #endif
 
+  /* set the initial velocity of the cells */
   xp->v_full[0] = p->v[0];
   xp->v_full[1] = p->v[1];
   xp->v_full[2] = p->v[2];
@@ -117,7 +134,6 @@ __attribute__((always_inline)) INLINE static void hydro_init_part(
 
   p->density.wcount = 0.0f;
   p->density.wcount_dh = 0.0f;
-  p->rho = 0.0f;
 
   voronoi_cell_init(&p->cell, p->x);
 
@@ -165,13 +181,22 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
     /* Iteration not succesful: we force h to become 1.1*hnew */
     p->density.wcount = 0.0f;
     p->density.wcount_dh = 1.0f / (1.1f * hnew - p->h);
+    return;
   }
   volume = p->cell.volume;
 
+#ifdef SWIFT_DEBUG_CHECKS
+  /* the last condition checks for NaN: a NaN value always evaluates to false,
+     even when checked against itself */
+  if (volume == 0. || volume == INFINITY || volume != volume) {
+    error("Invalid value for cell volume (%g)!", volume);
+  }
+#endif
+
   /* compute primitive variables */
   /* eqns (3)-(5) */
   m = p->conserved.mass;
-  if (m) {
+  if (m > 0.) {
     momentum[0] = p->conserved.momentum[0];
     momentum[1] = p->conserved.momentum[1];
     momentum[2] = p->conserved.momentum[2];
@@ -179,9 +204,36 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
     p->primitives.v[0] = momentum[0] / m;
     p->primitives.v[1] = momentum[1] / m;
     p->primitives.v[2] = momentum[2] / m;
+
     energy = p->conserved.energy;
-    p->primitives.P = hydro_gamma_minus_one * energy / volume;
+
+#ifdef SHADOWFAX_TOTAL_ENERGY
+    energy -= 0.5f * (momentum[0] * p->primitives.v[0] +
+                      momentum[1] * p->primitives.v[1] +
+                      momentum[2] * p->primitives.v[2]);
+#endif
+
+    energy /= m;
+
+    p->primitives.P =
+        gas_pressure_from_internal_energy(p->primitives.rho, energy);
+  } else {
+    p->primitives.rho = 0.;
+    p->primitives.v[0] = 0.;
+    p->primitives.v[1] = 0.;
+    p->primitives.v[2] = 0.;
+    p->primitives.P = 0.;
   }
+
+#ifdef SWIFT_DEBUG_CHECKS
+  if (p->primitives.rho < 0.) {
+    error("Negative density!");
+  }
+
+  if (p->primitives.P < 0.) {
+    error("Negative pressure!");
+  }
+#endif
 }
 
 /**
@@ -197,15 +249,10 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
  *
  * @param p The particle to act upon.
  * @param xp The extended particle data to act upon.
- * @param ti_current Current integer time.
- * @param timeBase Conversion factor between integer time and physical time.
  */
 __attribute__((always_inline)) INLINE static void hydro_prepare_force(
     struct part* restrict p, struct xpart* restrict xp) {
 
-  /* Set the physical time step */
-  //  p->force.dt = (p->ti_end - p->ti_begin) * timeBase;
-
   /* Initialize time step criterion variables */
   p->timestepvars.vmax = 0.0f;
 
@@ -292,15 +339,7 @@ __attribute__((always_inline)) INLINE static void hydro_predict_extra(
     struct part* p, struct xpart* xp, float dt) {}
 
 /**
- * @brief Set the particle acceleration after the flux loop
- *
- * We use the new conserved variables to calculate the new velocity of the
- * particle, and use that to derive the change of the velocity over the particle
- * time step. We also add a correction to the velocity which steers the
- * generator towards the centroid of the cell.
- *
- * If the particle time step is zero, we set the accelerations to zero. This
- * should only happen at the start of the simulation.
+ * @brief Set the particle acceleration after the flux loop.
  *
  * @param p Particle to act upon.
  */
@@ -318,6 +357,7 @@ __attribute__((always_inline)) INLINE static void hydro_end_force(
  */
 __attribute__((always_inline)) INLINE static void hydro_kick_extra(
     struct part* p, struct xpart* xp, float dt) {
+
   float vcell[3];
 
   /* Update the conserved variables. We do this here and not in the kick,
@@ -328,6 +368,18 @@ __attribute__((always_inline)) INLINE static void hydro_kick_extra(
   p->conserved.momentum[2] += p->conserved.flux.momentum[2];
   p->conserved.energy += p->conserved.flux.energy;
 
+#ifdef EOS_ISOTHERMAL_GAS
+  /* reset the thermal energy */
+  p->conserved.energy = p->conserved.mass * const_isothermal_internal_energy;
+
+#ifdef SHADOWFAX_TOTAL_ENERGY
+  p->conserved.energy += 0.5f * (p->conserved.momentum[0] * p->primitives.v[0] +
+                                 p->conserved.momentum[1] * p->primitives.v[1] +
+                                 p->conserved.momentum[2] * p->primitives.v[2]);
+#endif
+
+#endif
+
   /* reset fluxes */
   /* we can only do this here, since we need to keep the fluxes for inactive
      particles */
@@ -337,11 +389,17 @@ __attribute__((always_inline)) INLINE static void hydro_kick_extra(
   p->conserved.flux.momentum[2] = 0.0f;
   p->conserved.flux.energy = 0.0f;
 
-  /* We want the cell velocity to be as close as possible to the fluid
-     velocity */
-  vcell[0] = p->conserved.momentum[0] / p->conserved.mass;
-  vcell[1] = p->conserved.momentum[1] / p->conserved.mass;
-  vcell[2] = p->conserved.momentum[2] / p->conserved.mass;
+  if (p->conserved.mass > 0.) {
+    /* We want the cell velocity to be as close as possible to the fluid
+       velocity */
+    vcell[0] = p->conserved.momentum[0] / p->conserved.mass;
+    vcell[1] = p->conserved.momentum[1] / p->conserved.mass;
+    vcell[2] = p->conserved.momentum[2] / p->conserved.mass;
+  } else {
+    vcell[0] = 0.;
+    vcell[1] = 0.;
+    vcell[2] = 0.;
+  }
 
   /* To prevent stupid things like cell crossovers or generators that move
      outside their cell, we steer the motion of the cell somewhat */
@@ -399,7 +457,12 @@ __attribute__((always_inline)) INLINE static void hydro_kick_extra(
 __attribute__((always_inline)) INLINE static float hydro_get_internal_energy(
     const struct part* restrict p) {
 
-  return p->primitives.P / hydro_gamma_minus_one / p->primitives.rho;
+  if (p->primitives.rho > 0.) {
+    return gas_internal_energy_from_pressure(p->primitives.rho,
+                                             p->primitives.P);
+  } else {
+    return 0.;
+  }
 }
 
 /**
@@ -411,7 +474,11 @@ __attribute__((always_inline)) INLINE static float hydro_get_internal_energy(
 __attribute__((always_inline)) INLINE static float hydro_get_entropy(
     const struct part* restrict p) {
 
-  return p->primitives.P / pow_gamma(p->primitives.rho);
+  if (p->primitives.rho > 0.) {
+    return gas_entropy_from_pressure(p->primitives.rho, p->primitives.P);
+  } else {
+    return 0.;
+  }
 }
 
 /**
@@ -423,7 +490,11 @@ __attribute__((always_inline)) INLINE static float hydro_get_entropy(
 __attribute__((always_inline)) INLINE static float hydro_get_soundspeed(
     const struct part* restrict p) {
 
-  return sqrtf(hydro_gamma * p->primitives.P / p->primitives.rho);
+  if (p->primitives.rho > 0.) {
+    return gas_soundspeed_from_pressure(p->primitives.rho, p->primitives.P);
+  } else {
+    return 0.;
+  }
 }
 
 /**
@@ -473,7 +544,18 @@ __attribute__((always_inline)) INLINE static float hydro_get_density(
 __attribute__((always_inline)) INLINE static void hydro_set_internal_energy(
     struct part* restrict p, float u) {
 
-  p->conserved.energy = u * p->conserved.mass;
+  if (p->primitives.rho > 0.) {
+    p->conserved.energy = u * p->conserved.mass;
+
+#ifdef SHADOWFAX_TOTAL_ENERGY
+    p->conserved.energy +=
+        0.5f * (p->conserved.momentum[0] * p->primitives.v[0] +
+                p->conserved.momentum[1] * p->primitives.v[1] +
+                p->conserved.momentum[2] * p->primitives.v[2]);
+#endif
+
+    p->primitives.P = gas_pressure_from_internal_energy(p->primitives.rho, u);
+  }
 }
 
 /**
@@ -488,6 +570,18 @@ __attribute__((always_inline)) INLINE static void hydro_set_internal_energy(
 __attribute__((always_inline)) INLINE static void hydro_set_entropy(
     struct part* restrict p, float S) {
 
-  p->conserved.energy = gas_internal_energy_from_entropy(p->primitives.rho, S) *
-                        p->conserved.mass;
+  if (p->primitives.rho > 0.) {
+    p->conserved.energy =
+        gas_internal_energy_from_entropy(p->primitives.rho, S) *
+        p->conserved.mass;
+
+#ifdef SHADOWFAX_TOTAL_ENERGY
+    p->conserved.energy +=
+        0.5f * (p->conserved.momentum[0] * p->primitives.v[0] +
+                p->conserved.momentum[1] * p->primitives.v[1] +
+                p->conserved.momentum[2] * p->primitives.v[2]);
+#endif
+
+    p->primitives.P = gas_pressure_from_entropy(p->primitives.rho, S);
+  }
 }

src/hydro/Shadowswift/hydro_debug.h

@@ -45,8 +45,7 @@ __attribute__((always_inline)) INLINE static void hydro_debug_particle(
       "vmax=%.3e}, "
       "density={"
       "wcount_dh=%.3e, "
-      "wcount=%.3e}, "
-      "mass=%.3e\n",
+      "wcount=%.3e}",
       p->x[0], p->x[1], p->x[2], p->v[0], p->v[1], p->v[2], p->a_hydro[0],
       p->a_hydro[1], p->a_hydro[2], p->h, p->primitives.v[0],
       p->primitives.v[1], p->primitives.v[2], p->primitives.rho,
@@ -66,5 +65,5 @@ __attribute__((always_inline)) INLINE static void hydro_debug_particle(
       p->primitives.limiter.maxr, p->conserved.momentum[0],
       p->conserved.momentum[1], p->conserved.momentum[2], p->conserved.mass,
       p->conserved.energy, p->timestepvars.vmax, p->density.wcount_dh,
-      p->density.wcount, p->mass);
+      p->density.wcount);
 }

src/hydro/Shadowswift/hydro_iact.h

@@ -168,17 +168,21 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
   dtj = pj->force.dt;
 
   /* calculate the maximal signal velocity */
-  if (Wi[0] && Wj[0]) {
-    vmax =
-        sqrtf(hydro_gamma * Wi[4] / Wi[0]) + sqrtf(hydro_gamma * Wj[4] / Wj[0]);
-  } else {
-    vmax = 0.0f;
+  vmax = 0.0f;
+  if (Wi[0] > 0.) {
+    vmax += gas_soundspeed_from_pressure(Wi[0], Wi[4]);
   }
+
+  if (Wj[0] > 0.) {
+    vmax += gas_soundspeed_from_pressure(Wj[0], Wj[4]);
+  }
+
   dvdotdx = (Wi[1] - Wj[1]) * dx[0] + (Wi[2] - Wj[2]) * dx[1] +
             (Wi[3] - Wj[3]) * dx[2];
   if (dvdotdx > 0.) {
     vmax -= dvdotdx / r;
   }
+
   pi->timestepvars.vmax = fmaxf(pi->timestepvars.vmax, vmax);
   if (mode == 1) {
     pj->timestepvars.vmax = fmaxf(pj->timestepvars.vmax, vmax);
@@ -187,8 +191,6 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
   /* The flux will be exchanged using the smallest time step of the two
    * particles */
   mindt = fminf(dti, dtj);
-  dti = mindt;
-  dtj = mindt;
 
   /* compute the normal vector of the interface */
   for (k = 0; k < 3; ++k) {
@@ -260,19 +262,21 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
 
   /* Update conserved variables */
   /* eqn. (16) */
-  pi->conserved.flux.mass -= dti * A * totflux[0];
-  pi->conserved.flux.momentum[0] -= dti * A * totflux[1];
-  pi->conserved.flux.momentum[1] -= dti * A * totflux[2];
-  pi->conserved.flux.momentum[2] -= dti * A * totflux[3];
-  pi->conserved.flux.energy -= dti * A * totflux[4];
+  pi->conserved.flux.mass -= mindt * A * totflux[0];
+  pi->conserved.flux.momentum[0] -= mindt * A * totflux[1];
+  pi->conserved.flux.momentum[1] -= mindt * A * totflux[2];
+  pi->conserved.flux.momentum[2] -= mindt * A * totflux[3];
+  pi->conserved.flux.energy -= mindt * A * totflux[4];
 
+#ifndef SHADOWFAX_TOTAL_ENERGY
   float ekin = 0.5f * (pi->primitives.v[0] * pi->primitives.v[0] +
                        pi->primitives.v[1] * pi->primitives.v[1] +
                        pi->primitives.v[2] * pi->primitives.v[2]);
-  pi->conserved.flux.energy += dti * A * totflux[1] * pi->primitives.v[0];
-  pi->conserved.flux.energy += dti * A * totflux[2] * pi->primitives.v[1];
-  pi->conserved.flux.energy += dti * A * totflux[3] * pi->primitives.v[2];
-  pi->conserved.flux.energy -= dti * A * totflux[0] * ekin;
+  pi->conserved.flux.energy += mindt * A * totflux[1] * pi->primitives.v[0];
+  pi->conserved.flux.energy += mindt * A * totflux[2] * pi->primitives.v[1];
+  pi->conserved.flux.energy += mindt * A * totflux[3] * pi->primitives.v[2];
+  pi->conserved.flux.energy -= mindt * A * totflux[0] * ekin;
+#endif
 
   /* here is how it works:
      Mode will only be 1 if both particles are ACTIVE and they are in the same
@@ -287,19 +291,21 @@ __attribute__((always_inline)) INLINE static void runner_iact_fluxes_common(
      ==> we update particle j if (MODE IS 1) OR (j IS INACTIVE)
   */
   if (mode == 1 || pj->force.active == 0) {
-    pj->conserved.flux.mass += dtj * A * totflux[0];
-    pj->conserved.flux.momentum[0] += dtj * A * totflux[1];
-    pj->conserved.flux.momentum[1] += dtj * A * totflux[2];
-    pj->conserved.flux.momentum[2] += dtj * A * totflux[3];
-    pj->conserved.flux.energy += dtj * A * totflux[4];
+    pj->conserved.flux.mass += mindt * A * totflux[0];
+    pj->conserved.flux.momentum[0] += mindt * A * totflux[1];
+    pj->conserved.flux.momentum[1] += mindt * A * totflux[2];
+    pj->conserved.flux.momentum[2] += mindt * A * totflux[3];
+    pj->conserved.flux.energy += mindt * A * totflux[4];
 
+#ifndef SHADOWFAX_TOTAL_ENERGY
     ekin = 0.5f * (pj->primitives.v[0] * pj->primitives.v[0] +
                    pj->primitives.v[1] * pj->primitives.v[1] +
                    pj->primitives.v[2] * pj->primitives.v[2]);
-    pj->conserved.flux.energy -= dtj * A * totflux[1] * pj->primitives.v[0];
-    pj->conserved.flux.energy -= dtj * A * totflux[2] * pj->primitives.v[1];
-    pj->conserved.flux.energy -= dtj * A * totflux[3] * pj->primitives.v[2];
-    pj->conserved.flux.energy += dtj * A * totflux[0] * ekin;
+    pj->conserved.flux.energy -= mindt * A * totflux[1] * pj->primitives.v[0];
+    pj->conserved.flux.energy -= mindt * A * totflux[2] * pj->primitives.v[1];
+    pj->conserved.flux.energy -= mindt * A * totflux[3] * pj->primitives.v[2];
+    pj->conserved.flux.energy += mindt * A * totflux[0] * ekin;
+#endif
   }
 }
 

src/hydro/Shadowswift/hydro_io.h

@@ -18,6 +18,7 @@
  ******************************************************************************/
 
 #include "adiabatic_index.h"
+#include "equation_of_state.h"
 #include "hydro_gradients.h"
 #include "hydro_slope_limiters.h"
 #include "io_properties.h"
@@ -63,7 +64,12 @@ void hydro_read_particles(struct part* parts, struct io_props* list,
  * @return Internal energy of the particle
  */
 float convert_u(struct engine* e, struct part* p) {
-  return p->primitives.P / hydro_gamma_minus_one / p->primitives.rho;
+  if (p->primitives.rho > 0.) {
+    return gas_internal_energy_from_pressure(p->primitives.rho,
+                                             p->primitives.P);
+  } else {
+    return 0.;
+  }
 }
 
 /**
@@ -74,7 +80,11 @@ float convert_u(struct engine* e, struct part* p) {
  * @return Entropic function of the particle
  */
 float convert_A(struct engine* e, struct part* p) {
-  return p->primitives.P / pow_gamma(p->primitives.rho);
+  if (p->primitives.rho > 0.) {
+    return gas_entropy_from_pressure(p->primitives.rho, p->primitives.P);
+  } else {
+    return 0.;
+  }
 }
 
 /**
@@ -85,13 +95,21 @@ float convert_A(struct engine* e, struct part* p) {
  * @return Total energy of the particle
  */
 float convert_Etot(struct engine* e, struct part* p) {
-  float momentum2;
-
-  momentum2 = p->conserved.momentum[0] * p->conserved.momentum[0] +
-              p->conserved.momentum[1] * p->conserved.momentum[1] +
-              p->conserved.momentum[2] * p->conserved.momentum[2];
-
-  return p->conserved.energy + 0.5f * momentum2 / p->conserved.mass;
+#ifdef SHADOWFAX_TOTAL_ENERGY
+  return p->conserved.energy;
+#else
+  if (p->conserved.mass > 0.) {
+    float momentum2;
+
+    momentum2 = p->conserved.momentum[0] * p->conserved.momentum[0] +
+                p->conserved.momentum[1] * p->conserved.momentum[1] +
+                p->conserved.momentum[2] * p->conserved.momentum[2];
+
+    return p->conserved.energy + 0.5f * momentum2 / p->conserved.mass;
+  } else {
+    return 0.;
+  }
+#endif
 }
 
 /**

src/hydro/Shadowswift/hydro_part.h

@@ -31,9 +31,6 @@ struct xpart {
   /* Velocity at the last full step. */
   float v_full[3];
 
-  /* Old density. */
-  float omega;
-
   /* Additional data used to record cooling information */
   struct cooling_xpart_data cooling_data;
 
@@ -42,6 +39,12 @@ struct xpart {
 /* Data of a single particle. */
 struct part {
 
+  /* Particle ID. */
+  long long id;
+
+  /* Associated gravitas. */
+  struct gpart *gpart;
+
   /* Particle position. */
   double x[3];
 
@@ -163,18 +166,6 @@ struct part {
 
   } force;
 
-  /* Particle mass (this field is also part of the conserved quantities...). */
-  float mass;
-
-  /* Particle ID. */
-  long long id;
-
-  /* Associated gravitas. */
-  struct gpart *gpart;
-
-  /* Variables needed for the code to compile (should be removed/replaced). */
-  float rho;
-
   /* Time-step length */
   timebin_t time_bin;