Revise custom quantities (trixi-framework#457)

* Start revising custom quantities

* Revise remaining custom quantity functions

* Update src/general/custom_quantities.jl

Co-authored-by: Niklas Neher <[email protected]>

---------

Co-authored-by: Niklas Neher <[email protected]>

src/general/custom_quantities.jl

@@ -4,11 +4,8 @@
 Returns the total kinetic energy of all particles in a system.
 """
 function kinetic_energy(v, u, t, system)
-    if n_moving_particles(system) == 0
-        return 0.0
-    end
-
-    return sum(each_moving_particle(system)) do particle
+    # If `each_moving_particle` is empty (no moving particles), return zero
+    return sum(each_moving_particle(system), init=0.0) do particle
         velocity = current_velocity(v, system, particle)
         return 0.5 * system.mass[particle] * dot(velocity, velocity)
     end
@@ -20,77 +17,107 @@ end
 Returns the total mass of all particles in a system.
 """
 function total_mass(v, u, t, system)
-    return sum(each_moving_particle(system)) do particle
+    return sum(eachparticle(system)) do particle
         return system.mass[particle]
     end
 end
 
+function total_mass(v, u, t, system::BoundarySystem)
+    # It does not make sense to return a mass for boundary systems.
+    # The material density and therefore the physical mass of the boundary is not relevant
+    # when simulating a solid, stationary wall. The boundary always behaves as if it had
+    # infinite mass. There is no momentum transferred to the boundary on impact.
+    #
+    # When the dummy particles model is used, i.e., boundary particles behave like fluid
+    # particles when interacting with actual fluid particles, the boundary particles do have
+    # a "hydrodynamic mass", which corresponds to the fluid density, but this is only
+    # relevant for the fluid interaction, and it has no connection to the physical mass
+    # of the boundary. Returning the "hydrodynamic mass" here would thus be misleading.
+    return NaN
+end
+
 """
     max_pressure
 
 Returns the maximum pressure over all particles in a system.
 """
-function max_pressure(v, u, t, system)
+function max_pressure(v, u, t, system::FluidSystem)
     return maximum(particle -> particle_pressure(v, system, particle),
-                   each_moving_particle(system))
+                   eachparticle(system))
+end
+
+function max_pressure(v, u, t, system)
+    return NaN
 end
 
 """
     min_pressure
 
 Returns the minimum pressure over all particles in a system.
 """
-function min_pressure(v, u, t, system)
+function min_pressure(v, u, t, system::FluidSystem)
     return minimum(particle -> particle_pressure(v, system, particle),
-                   each_moving_particle(system))
+                   eachparticle(system))
+end
+
+function min_pressure(v, u, t, system)
+    return NaN
 end
 
 """
     avg_pressure
 
 Returns the average pressure over all particles in a system.
 """
-function avg_pressure(v, u, t, system)
-    if n_moving_particles(system) == 0
-        return 0.0
-    end
-
+function avg_pressure(v, u, t, system::FluidSystem)
     sum_ = sum(particle -> particle_pressure(v, system, particle),
-               each_moving_particle(system))
-    return sum_ / n_moving_particles(system)
+               eachparticle(system))
+    return sum_ / nparticles(system)
+end
+
+function avg_pressure(v, u, t, system)
+    return NaN
 end
 
 """
     max_density
 
 Returns the maximum density over all particles in a system.
 """
-function max_density(v, u, t, system)
+function max_density(v, u, t, system::FluidSystem)
     return maximum(particle -> particle_density(v, system, particle),
-                   each_moving_particle(system))
+                   eachparticle(system))
+end
+
+function max_density(v, u, t, system)
+    return NaN
 end
 
 """
     min_density
 
 Returns the minimum density over all particles in a system.
 """
-function min_density(v, u, t, system)
+function min_density(v, u, t, system::FluidSystem)
     return minimum(particle -> particle_density(v, system, particle),
-                   each_moving_particle(system))
+                   eachparticle(system))
+end
+
+function min_density(v, u, t, system)
+    return NaN
 end
 
 """
     avg_density
 
 Returns the average_density over all particles in a system.
 """
-function avg_density(v, u, t, system)
-    if n_moving_particles(system) == 0
-        return 0.0
-    end
-
+function avg_density(v, u, t, system::FluidSystem)
     sum_ = sum(particle -> particle_density(v, system, particle),
-               each_moving_particle(system))
-    return sum_ / n_moving_particles(system)
+               eachparticle(system))
+    return sum_ / nparticles(system)
+end
+
+function avg_density(v, u, t, system)
+    return NaN
 end