Even though the wall film is assumed to be not moving due to external forces, there may be situations, where the wall film is either moving with the wall (for example, piston top) or moving relative to the grid (for example, cylinder wall). The second situation occurs due to the moving mesh approach used for simulations with changing geometries.
Both situations are handled with the current approach without user interference. The distinction whether wall particles move with a wall or move relative to the underlying mesh is based on the value of the so called Wall Velocity, which is given as:
For the moving wall:
Velocity of mesh (Vmesh)
Velocity of wall (Vwall)
Velocity of wall-particle (Vwall-particle)
For the stationary wall:
Velocity of mesh (Vmesh) 
0,
Velocity of wall (Vwall)
Velocity of wall-particle (Vwall-particle) 
0
The situation is identical to the case, where wall particles are located at a non-moving wall.
When a particle has hit a wall face, the topological data of the impact is known and is stored in the particle database. Because the particle does not move, there is no need to update this data during the simulation.
This case requires the update of the wall particle topology data during the simulation, as wall particles may cross elements due to the specified mesh movement. The relocalization of wall particles is currently done after all particles have been tracked. Due to performance reasons the update is not done while tracking wall particles. Relocalizing wall particles at the end of a fluid time step therefore puts an upper limit onto the maximum allowed fluid time step, because for accurate simulations wall particle should not cross more than one element per fluid time step. If a too large time step is used, wall particles will put their heat and mass sources into the last known element.