The transport block is not really a block but a sub-system. It is used to automatically generate a chain of compartments for one dimensional transport.

There are three components to a transport block:

- Begin represents the first compartment of the chain.
- End represents the last compartment of the chain
- N is an expression that gives the total number of compartments, or discretizations, in the transport.

When a transport sub-system is created the begin, end and N blocks are added automatically. The transport chain is defined by connecting the begin and end blocks with transfers. The begin block will be duplicated (N-1) times, where N is the number of discretizations.

From the Projects window

- Expand the Project to which you wish to add the expression by clicking the + symbol next to it. Right-click on ‘Blocks’, or a sub-system in ‘Blocks’, and select
**Transport**from the pop-up menu.

From the Model window?

- Right click an empty diagonal cell of the matrix and select
**Other | Transport**from the the pop-up menu.

From the Blocks window

- Place the mouse cursor over the table and right-click on it. Select
**New | Transport**from the pop-up menu.

The transport sub-system is edited the same way as a normal sub-system. You can add expressions, parameters and other blocks to it the same way as you would in an ordinary sub-system.

Consider a vertical transport from a source, via a layered media, to a sink. The model is implemented with a transport sub-system according to the matrix below.

Source | Input | |||

Begin | TC | |||

End | Out | |||

N | ||||

Sink |

Where

**Source**is a Source/Sink block**Input**is a Transfer block**Begin**is a Transport begin block**End**is a Transport end block**TC**is a Transfer block**N**is a Transport N block with the value N=5**Sink**is a Source/Sink block

The corresponding matrix, during simulation, looks like this:

Source | Input | |||||

Begin | TC | |||||

Begin | TC | |||||

Begin | TC | |||||

Begin | TC | |||||

End | Out | |||||

Sink |

A common use for the transport sub-system is to model vertical transport through a media with advection and diffusion. This example shows transport from a top soil layer (implemented as a compartment), through several layers of soil (implemented as a transport sub-system) to a ground water compartment.

Top soil | Advection Diffusion |
|||

Diffusion | Begin | Advection Diffusion |
||

Diffusion | End | Advection Diffusion |
||

N | ||||

Ground water |

Where

**Top soil**is a Compartment block**Advection**are all Transfer blocks with the expression**Diffusion**are all Transfer blocks with the expression**Begin**is a Transport begin block**End**is a Transport end block**N**is a Transport N block with the expression N=”peckle’s number”**Ground water**is a Compartment block

The corresponding matrix, during simulation, looks like this:

Top soil | Advection Diffusion |
|||||

Diffusion | Begin | Advection Diffusion |
||||

Diffusion | Begin | Advection Diffusion |
||||

Diffusion | Begin | Advection Diffusion |
||||

Diffusion | Begin | Advection Diffusion |
||||

Diffusion | End | Advection Diffusion |
||||

Diffusion | Ground water |

Within the transport, the compartments must be homogenous. This means that each “layer” must have the same size and that all the transfers expressions betweeen layers must be identical.

Another limitation is that only the begin and end compartment are available as outputs or when connecting the transport to other objects of a model.

To calculate the mean or total inventory of each transport layer you can use a Transport operation block.

- Transport begin
- Transport end
- Transport N
- Transport operation
- Blocks
- The Blocks window
- The Model window?