DYNAMIC PACKET DISTRIBUTION

Rather than placing many packets in every cell, MT3D can be directed to allocate packets dynamically so they are placed where they are needed to represent contaminant and not where the concentration is zero (or very low).

At first, if the system is full of clean, or background concentration, water little or no packets are needed.

As particles leave source cells more packets need to be added

As particles accumulate at sink cells packets need to be removed

At other cells, packets need to be added or deleted as the concentration gradient changes, particularly more packets are required at the front of the plume
The # of packets is set at a high or low level according to the relative cell concentration gradient DCCELL which indicates the "sharpness" of the plume front near each cell

CMAXi,j,k - maximum concentration near the cell
CMINi,j,k - minimum concentration near the cell
CMAX - maximum concentration in the entire grid
CMIN - minimum concentration in the entire grid



The modeler defines a HIGH (NPH, # packets high) and LOW (NPL, # packets low) number of packets to be used (the low could be zero)

The modeler also defines a critical value of DCCELL called DCEPS at which the high or low number of cells will be allocated.
for:
DCCELL < DCEPS fewer packets are used (NPL packets)
DCCELL > DCEPS more packets are used (NPH packets)
A dynamic insertion-deletion procedure is undertaken by defining the minimum NPMIN and maximum NPMAX number of packets allowed per cell

When the # of packets in a cell is less than NPMIN new packets equal to NPL or NPH are added

Existing packets are not changed

When the # of packets in a cell is greater than NPMAX all packets are removed from the cell and replaced by a new set of NPH packets , maintaining mass balance in the cell

The packet distribution ends up changing, for example as shown below:



A fixed or random distribution of packets in a cell can be selected. A fixed distribution works better in uniform flow fields and a random distribution works better in heterogeneous media

Packets are assigned the concentration of the cell in which they are added

FOR MMOC
sink cells receive multiple packets based on above criteria, because addition of one packet at a sink cell is likely to result in zero velocity due to symmetry of convergent flow on the sink

The concentration is then the weighted average of the packet concentrations

source cell packets will converge on the node so Cn* tends toward Cn
Recall that HMOC switching from MOC to MMOC is also dependent on DCCELL.