Fix advection velocity for packing (#835)

Co-authored-by: LasNikas <niklas.nehe@web.de>

Author: LasNikas

src/preprocessing/particle_packing/system.jl

@@ -387,7 +387,7 @@ end
     v = wrap_v(v_ode, system, semi)
     @threaded semi for particle in each_moving_particle(system)
         for i in 1:ndims(system)
-            system.initial_condition.velocity[i, particle] = v[i, particle]
+            system.advection_velocity[i, particle] = v[i, particle]
 
             # The particle velocity is set to zero at the beginning of each time step to
             # achieve a fully stationary state.
@@ -404,7 +404,7 @@ end
 # Add advection velocity.
 @inline function add_velocity!(du, v, particle, system::ParticlePackingSystem)
     for i in 1:ndims(system)
-        du[i, particle] = system.initial_condition.velocity[i, particle]
+        du[i, particle] = system.advection_velocity[i, particle]
     end
 
     return du