Smoothing length time derivative computed correctly in the Gadget-2 case

src/hydro/Default/hydro.h

@@ -70,9 +70,10 @@ __attribute__((always_inline))
  * and add the self-contribution term.
  *
  * @param p The particle to act upon
+ * @param time The current time
  */
 __attribute__((always_inline))
-    INLINE static void hydro_end_density(struct part* p) {
+    INLINE static void hydro_end_density(struct part* p, float time) {
 
   /* Some smoothing length multiples. */
   const float h = p->h;
@@ -166,12 +167,15 @@ __attribute__((always_inline))
  *
  * @param p The particle
  * @param xp The extended data of the particle
- * @param dt The time-step over which to drift
+ * @param t0 The time at the start of the drift
+ * @param t1 The time at the end of the drift
  */
 __attribute__((always_inline)) INLINE static void hydro_predict_extra(
-    struct part* p, struct xpart* xp, float dt) {
+    struct part* p, struct xpart* xp, float t0, float t1) {
   float u, w;
 
+  const float dt = t1 - t0;
+
   /* Predict internal energy */
   w = p->force.u_dt / p->u * dt;
   if (fabsf(w) < 0.01f) /* 1st order expansion of exp(w) */
@@ -196,3 +200,22 @@ __attribute__((always_inline))
 
   p->force.h_dt *= p->h * 0.333333333f;
 }
+
+/**
+ * @brief Kick the additional variables
+ *
+ * @param p The particle to act upon
+ * @param dt The time-step for this kick
+ */
+__attribute__((always_inline))
+    INLINE static void hydro_kick_extra(struct part* p, float dt) {}
+
+/**
+ * @brief Converts hydro quantity of a particle
+ *
+ * Requires the density to be known
+ *
+ * @param p The particle to act upon
+ */
+__attribute__((always_inline))
+    INLINE static void hydro_convert_quantities(struct part* p) {}

src/hydro/Gadget2/hydro.h

@@ -203,7 +203,7 @@ __attribute__((always_inline)) INLINE static void hydro_predict_extra(
  */
 __attribute__((always_inline))
     INLINE static void hydro_end_force(struct part* p) {
-
+  
   p->entropy_dt *=
       (const_hydro_gamma - 1.f) * powf(p->rho, -(const_hydro_gamma - 1.f));
 }

src/hydro/Gadget2/hydro_iact.h

@@ -197,7 +197,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   const float r_inv = 1.0f / r;
 
   /* Get some values in local variables. */
-  // const float mi = pi->mass;
+  const float mi = pi->mass;
   const float mj = pj->mass;
   const float rhoi = pi->rho;
   const float rhoj = pj->rho;
@@ -280,6 +280,10 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   pj->a[1] += acc * dx[1];
   pj->a[2] += acc * dx[2];
 
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr / rhoj * wi_dr;
+  pj->force.h_dt -= mi * dvdr / rhoi * wj_dr;
+  
   /* Update the signal velocity. */
   pi->v_sig = fmaxf(pi->v_sig, v_sig);
   pj->v_sig = fmaxf(pj->v_sig, v_sig);
@@ -366,6 +370,9 @@ __attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
   pi->a[1] -= acc * dx[1];
   pi->a[2] -= acc * dx[2];
 
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr / rhoj * wi_dr;
+  
   /* Update the signal velocity. */
   pi->v_sig = fmaxf(pi->v_sig, v_sig);
 

src/runner.c

@@ -851,8 +851,13 @@ void runner_dokick(struct runner *r, struct cell *c, int timer) {
       if (is_fixdt || p->t_end <= t_current) {
 
         /* First, finish the force loop */
+        p->force.h_dt *= p->h * 0.333333333f;
+
+        /* And do the same of the extra variable */
         hydro_end_force(p);
 
+        /* Now we are ready to compute the next time-step size */
+
         if (is_fixdt) {
 
           /* Now we have a time step, proceed with the kick */