Particle Tracking Control


Particle Tracking Control
Conditions:	if the Include Advection Dispersion (AD) Water Quality option selected in the Simulation Specification dialogue and the Random Walk Particle Tracking sub-option is selected.

The particle tracking simulation is completely decoupled from the water movement simulation and like the water movement itself, the water quality time steps can be different in each of the overland flow, unsaturated flow and saturated flow.

Apply vertical correction when moving between cells with variable thicknesses - This correction adjusts the vertical location of the particle, if the particle moves into a neighbouring cell with a different cell thickness. This prevents particles from being discharged to the ground surface when they move into an adjacent cell with a lower topography. By default, this option is on.

Ignore vertical courant criteria - With this option on, the calculation of the maximum time step length based on the courant criteria ignores the vertical velocities. In some cases, very small time steps may be required if the layers are thin and the vertical velocities are high. However, such conditions often occur near particle sinks (e.g. wells and seepag faces) where detailed particle pathlines are not important.

Time step control

Maximum Simulation Time Step - This is the maximum user-specified time step allowed. The default value is very high so that the simulation runs by default with the highest possible time step. You might want to set this value to a short time interval, if you want the WQ time step to be uniform during the WQ simulation.

The courant number is a measure of the ratio of flow rate to grid size. For numerical stability, it is important that a dissolved solute does not travel too far in one time step. A courant number greater than 1.0 implies that a particle (solute) would move completely across or through a cell in a time step. The time step is reduced until all the time step criteria below are met.

Max. Advective Courant Number - The advective courant number represents the ratio of cell size to the time it would take a particle to move across a cell. This criteria is likely to be controlling if your flow velocities are high, or your dispersivity values are very low or zero. The default value is 0.8. If your actual time step is being controlled by this criteria, then you could increase it to make the simulation run faster. However, you will need to check to make sure the simulation has converged properly and that the mass balance is reasonable.

Max. Dispersive Courant Number - The dispersive courant number represents the ratio of the cell size to time it would take a particle to move across a cell due to dispersion. This criteria is likely to be controlling when the velocities are very slow and the dispersivity is non-zero. The default value is 0.5. If your actual time step is being controlled by this criteria, then you could increase it to make the simulation run faster. However, you will need to check to make sure the simulation has converged properly and that the mass balance is reasonable.