Particle Tracking Specification

In the Particle Tracking Specification dialogue, you can select the initial den­sity of particles, the initial location of the particles in the cells, and how the particles should be registered as the move through the saturated zone.

Initial particle density

The particle density relates the number of particles to a solute concentration. However, the initial number of particles can have a significant affect on simu­lation run time and output file size.

Define initial particle density as initial concentration - The initial concen­tration is defined for each SZ layer. See Initial (secondary) concentra­tion (V1 p. 323). The number of particles in each cell is calculated by dividing the mass of solute in the cell by the particle mass specified in the text box below.

Define initial particle density as number of particles - If this option is selected, then an Initial number of particles per cell (V1 p. 324) item is added to the data tree for each SZ layer.

Particle mass for concentration calculations - This mass is used for deter­mining the intial number of particles if the first option above is selected. It is also used for converting the number of particles in a cell into a con­centration during the simulation. A low mass per particle will yield more particles for the same concentration.

Initial particle location

The options for initial particle location determine the placement of the parti­cles in each cell. If only a few particles are involved, the initial particle location can influence the results - especially if the cells are large.

A random distribution means that the particles may not be evenly distributed in every cell, but on average will be.

If the particles are evenly disributed on a plane, the particles are uniformly distributed on a plane parallel to the bottom of the cell. In this case, the optional fraction defines the location of the plane in the cell as a fraction of the cell thickness.

If the particles are evenly distributed in the cell, then the algorithm tries to dis­tribute the particles evenly. This is relatively straight forward for regular num­bers of particles, such as 4 or 5 particles, but more difficult for irregular numbers of particles, such as 11 particles.

Avoid initial placement in fixed head and concentration cells - Avoiding the initial placement in fixed head and fixed concentration cells, pre­vents some particles from being immediately extracted. (Default off)

Avoid initial placement above the water table - Avoiding the initial place­ment above the water table prevents particles being stuck right from the beginning of the simulation. If a particle is above the water table, then it will be immobile until the water table re-wets the location of the particle. This can happen when water tables are rising and falling. But, an initial particle above the water table may never become active and you may have a situation were nearly all of the initial particles in the upper SZ layer remain immobile for the entire simulation. (Default on)

Particle registration

The random walk method can track thousands of particles during a simula­tion. Unlike pathline analysis, the random walk method does not usually store all the resultant pathlines. Instead it stores the starting point and the end loca­tion or exit location of the particle. If you want to know if the particle has passed through a particle zone or layer, then you can create a registration zone. If the particle passes through the registration zone, then this will be recorded.

You can define the registration zones by numerical layer, or you can define them be lenses similar to the way you define geologic lenses. In both cases additional items are added to the SZ data tree. Defining the registration codes by lense is slower, because the PT engine has to continually check to see if the cell is located in one or more registration zone.

Particle registration is by default automatically done when particles are removed at wells to make it easier to calcuate well capture zones.

Related Items:

·         PT Registration Codes/Lenses (V1 p. 324)

·         Particle Tracking-Reference (V1 p. 723)