### 3.3.5. Using an Initial Values File that Contains Particles

If an Initial Values File contains the results of a Lagrangian particle tracking simulation, then three types of data can be made available to the new run:

- particle database containing particle positions at the end of the run, which will be used to continue particle tracking calculations in the new run if appropriate

- particle tracks (the tracks that the particles have already followed), which can be used for postprocessing

- particle source terms in the fluid(s) equations.

If the Continue History From is cleared (turned off), or the new run is steady-state, then only the particle source terms in the fluid(s) equations will be made available for the new run. The particle database and existing particle tracks are not used in the new run. The only particles present in the new run will be those injected during the new run.

If Continue History From is set to an Initial Values File which contains particle data, and the new run is transient, then all three types of data may be made available in the new run. However, there are a number of limitations that determine the type of data made available for any given run. Three scenarios are possible, with the final scenario being the default:

- Only the particle sources are made available for the new run. In this case, the only particles present in the new run will be those injected during the new run. When postprocessing, the only particle tracks available will be those calculated in the new run.

- The particle sources and the particle database are made available for the new run, but the particle tracks are not. In this case, if the new run defines particles of the same type as those contained in the Initial Values File, then the active particles from the Initial Values File will continue to be tracked from their positions and attributes at the end of the previous run (and new particles will be injected according to the settings in the new run). However, the tracks from the previous run will not be available in CFD-Post, so the history of how the particles from the Initial Values File arrived at those positions and attributes will be lost.

- All of the particle sources, particle database, and particle tracks are made available. In this case, particles from the Initial Values File continue to be tracked as described in the point above, and additionally the particle tracks postprocessed using the results from the new run will include the tracks from the Initial Values File.

Limitations associated with continuing particle calculations from an Initial Values File, and in making the particle data from the Initial Values file available for a new run, are detailed below:

- Only the particle database and particle tracks from the single Initial Values File specified by the "Continue History From" setting will be made available to the new run. Particle databases and tracks in other Initial Values Files will be ignored. It is not currently possible to combine the particle databases from two or more Initial Values Files for use in the new run.

- If the new run contains a different mesh to the Initial Values File, then the CFX-Interpolator is used to "relocalize" the particles from the Initial Values File onto the mesh in the CFX-Solver Input file. This means that for each particle in the Initial Values file, the CFX-Interpolator has to locate the element in the CFX-Solver Input file that now contains the particle, given its coordinates in the particle database from the Initial Values File. If the mesh has changed, then some particles in the particle database of the Initial Values File may appear to be outside the new run's mesh even if the geometry has not changed, if the underlying geometry has curved faces and the particles were very close to the edge of the mesh in the Initial Values File. In this case, the CFX-Interpolator will attempt to remap these initially-unmapped particles to the closest element, but will only succeed if the particles are close enough to that element, to within a tolerance. You can check whether particles were mapped successfully or not by referring to the section in the CFX-Solver Output file entitled "Particle Relocalization Information". If the geometry is highly curved and the mesh is coarse, then this tolerance might need to be adjusted to ensure that all the particles in the Initial Values File are successfully relocalized. See Adjusting the Bounding Box Tolerance for details on how to do this. If the geometry is significantly different between the Initial Values File and the CFX-Solver Input file, then any particles that cannot be relocalized onto the CFX-Solver Input file mesh will be lost from the particle database.

- By default, particle tracks are stored according to the domain that contains them. The particle tracks in this format from an Initial Values File can only be made available in the new run if the CFX-Interpolator is able to match the domain that contains the particles tracks to an equivalent domain in the CFX-Solver Input file. The tracks will only be made available if the geometry between the two equivalent domains is nominally the same; that is, 90% or greater of the nodes of the domain in the CFX-Solver Input file can be mapped to the equivalent domain in the Initial Values File and the bounding box volumes overlap by at least 90% between the two domains.

  - If the geometry is the same but due to numerical error or mesh faceting (due to geometrically curved surfaces) the 90% criteria is not met, then you can control the tolerance that the CFX-Interpolator uses to decide if the nodes are mapped or not. This is described in Adjusting the Bounding Box Tolerance.

  - If each domain in the Initial Values File does not have the same geometry as a domain in the CFX-Solver Input file (because the geometry has changed or because the domains have been defined differently (for example, by combining two domains into one)), then you can set the expert parameter `pt zone specific tracks = f`. This stores the particle tracks independent of the domain. This is the recommended setting for multi-configuration runs.

- Particle tracks in the new run can only be associated with a Boundary Condition if the Boundary Condition names remain the same in the CFX-Solver Input file as they were in the Initial Values File.

- Time-integrated particle boundary vertex fields (such as `Time Integrated Mass Flow Density`) are not accumulated from the Initial Values File into the new run unless the mesh in the Initial Values File is the same as the mesh in the CFX-Solver Input file.

- Wall impact variables on boundary patches are copied into the CFX-Solver input file if they are found in the initial values file. If the target mesh is different from the source mesh, the CFX-Interpolator maps each source boundary patch to its target boundary patch, and copies the wall impact variables to the CFX-Solver input file accordingly, in order to ensure a smooth restart.

- Time-integrated mass, momentum, and energy flows on particle injection regions are copied into the CFX-Solver input file if they are found in the initial values file. The CFX-Interpolator assumes that the particle injection regions are the same in the initial values file and CFX-Solver input file. If the particle injection regions are different, the CFX-Interpolator might not copy the time-integrated mass, momentum, or energy flows to the CFX-Solver input file correctly.

Release 18.2 - © ANSYS, Inc. All rights reserved.