Modify the time-step multiplier in the Lambda-cooling model, if the user wishes to.

examples/AgoraDisk/agora_disk.yml

@@ -39,7 +39,7 @@ Gravity:
 SPH:
   resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   10      # (internal units)
+  minimal_temperature:   10.      # Kelvin
 
 # Parameters related to the initial conditions
 InitialConditions:
@@ -50,9 +50,6 @@ InitialConditions:
 # Dimensionless pre-factor for the time-step condition
 LambdaCooling:
   lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
-  minimum_temperature:         1.0e2      # Minimal temperature (Kelvin)
-  mean_molecular_weight:       0.59       # Mean molecular weight
-  hydrogen_mass_abundance:     0.75       # Hydrogen mass abundance (dimensionless)
   cooling_tstep_mult:          1.0        # Dimensionless pre-factor for the time-step condition
 
 # Cooling with Grackle 2.0

examples/CoolingBox/coolingBox.yml

@@ -27,6 +27,7 @@ Statistics:
 SPH:
   resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature: 100.       # Kelvin
   
 # Parameters related to the initial conditions
 InitialConditions:
@@ -35,9 +36,6 @@ InitialConditions:
 # Dimensionless pre-factor for the time-step condition
 LambdaCooling:
   lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
-  minimum_temperature:         1.0e4      # Minimal temperature (Kelvin)
-  mean_molecular_weight:       0.59       # Mean molecular weight
-  hydrogen_mass_abundance:     0.75       # Hydrogen mass abundance (dimensionless)
   cooling_tstep_mult:          1.0        # Dimensionless pre-factor for the time-step condition
 
 # Cooling with Grackle 2.0

examples/CoolingHalo/cooling_halo.yml

@@ -27,7 +27,8 @@ Snapshots:
 SPH:
   resolution_eta:        1.2349   # Target smoothing length in units of the mean inter-particle separation (1.2349 == 48Ngbs with the cubic spline kernel).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-
+  minimal_temperature:   1e4      # Kelvin
+  
 # Parameters related to the initial conditions
 InitialConditions:
   file_name:  CoolingHalo.hdf5       # The file to read
@@ -41,7 +42,4 @@ IsothermalPotential:
 # Cooling parameters
 LambdaCooling:
   lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
-  minimum_temperature:         1.0e4  # Minimal temperature (Kelvin)
-  mean_molecular_weight:       0.59   # Mean molecular weight
-  hydrogen_mass_abundance:     0.75   # Hydrogen mass abundance (dimensionless)
   cooling_tstep_mult:          1.0    # Dimensionless pre-factor for the time-step condition

examples/CoolingHaloWithSpin/cooling_halo.yml

@@ -27,6 +27,7 @@ Snapshots:
 SPH:
   resolution_eta:        1.2349   # Target smoothing length in units of the mean inter-particle separation (1.2349 == 48Ngbs with the cubic spline kernel).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   1e4      # Kelvin
 
 # Parameters related to the initial conditions
 InitialConditions:
@@ -41,7 +42,4 @@ IsothermalPotential:
 # Cooling parameters
 LambdaCooling:
   lambda_cgs:                  1.0e-22   # Cooling rate (in cgs units)
-  minimum_temperature:         1.0e4  # Minimal temperature (Kelvin)
-  mean_molecular_weight:       0.59   # Mean molecular weight
-  hydrogen_mass_abundance:     0.75   # Hydrogen mass abundance (dimensionless)
   cooling_tstep_mult:          0.1    # Dimensionless pre-factor for the time-step condition

examples/SmallCosmoVolume_cooling/README

@@ -7,6 +7,11 @@ on the options to cancel the h-factors and a-factors at reading time.
 We generate gas from the ICs using SWIFT's internal mechanism and set the
 temperature to the expected gas temperature at this redshift.
 
+This example runs with cooling switch on. Depending on the cooling
+model chosen at the time SWIFT was configured, the answer will be
+different. Interesting cases to compare to the no-cooling case are
+a constant cooling rate or Compton cooling.
+
 The 'plotTempEvolution.py' plots the temperature evolution of the gas
 in the simulated volume.
 

examples/SmallCosmoVolume_cooling/small_cosmo_volume.yml

@@ -59,5 +59,4 @@ InitialConditions:
 
 # Constant lambda cooling function
 LambdaCooling:
-  lambda_cgs:                  1e-30 # Cooling rate (in cgs units)
-  cooling_tstep_mult:          1.0   # Dimensionless pre-factor for the time-step condition
+  lambda_cgs:                  1e-26 # Cooling rate (in cgs units)

examples/parameter_example.yml

@@ -199,10 +199,7 @@ ConstCooling:
 # Constant lambda cooling function
 LambdaCooling:
   lambda_cgs:                  2.0   # Cooling rate (in cgs units)
-  minimum_temperature:         1.0e4 # Minimal temperature (Kelvin)
-  mean_molecular_weight:       0.59  # Mean molecular weight
-  hydrogen_mass_abundance:     0.75  # Hydrogen mass abundance (dimensionless)
-  cooling_tstep_mult:          1.0   # Dimensionless pre-factor for the time-step condition
+  cooling_tstep_mult:          1.0   # (Optional) Dimensionless pre-factor for the time-step condition.
 
 # Cooling with Grackle 3.0
 GrackleCooling:

src/cooling/const_lambda/cooling.h

@@ -159,6 +159,11 @@ __attribute__((always_inline)) INLINE static float cooling_timestep(
     const struct unit_system* restrict us,
     const struct hydro_props* hydro_props, const struct part* restrict p) {
 
+  /* Start with the case where there is no limit */
+  if (cooling->cooling_tstep_mult == FLT_MAX) {
+    return FLT_MAX;
+  }
+
   /* Get current internal energy and cooling rate */
   const float u = hydro_get_physical_internal_energy(p, cosmo);
   const double cooling_du_dt_cgs =
@@ -225,8 +230,8 @@ static INLINE void cooling_init_backend(struct swift_params* parameter_file,
   /* Read in the cooling parameters */
   cooling->lambda_cgs =
       parser_get_param_double(parameter_file, "LambdaCooling:lambda_cgs");
-  cooling->cooling_tstep_mult = parser_get_param_double(
-      parameter_file, "LambdaCooling:cooling_tstep_mult");
+  cooling->cooling_tstep_mult = parser_get_opt_param_float(
+      parameter_file, "LambdaCooling:cooling_tstep_mult", FLT_MAX);
 
   /* Some useful conversion values */
   cooling->conv_factor_density_to_cgs =
@@ -251,6 +256,12 @@ static INLINE void cooling_print_backend(
 
   message("Cooling function is 'Constant lambda' with Lambda=%g [cgs]",
           cooling->lambda_cgs);
+
+  if (cooling->cooling_tstep_mult == FLT_MAX)
+    message("Cooling function time-step size is unlimited");
+  else
+    message("Cooling function time-step size limited to %f of u/(du/dt)",
+            cooling->cooling_tstep_mult);
 }
 
 #endif /* SWIFT_COOLING_CONST_LAMBDA_H */

src/runner.c

@@ -461,11 +461,11 @@ void runner_do_cooling(struct runner *r, struct cell *c, int timer) {
       if (part_is_active(p, e)) {
 
         double dt_cool, dt_therm;
-        ;
         if (with_cosmology) {
           const integertime_t ti_step = get_integer_timestep(p->time_bin);
           const integertime_t ti_begin =
-              get_integer_time_begin(ti_current + 1, p->time_bin);
+              get_integer_time_begin(ti_current - 1, p->time_bin);
+
           dt_cool =
               cosmology_get_delta_time(cosmo, ti_begin, ti_begin + ti_step);
           dt_therm = cosmology_get_therm_kick_factor(e->cosmology, ti_begin,