Table of Contents
Initial conditions are used to define the initial state of each cell. A hierarchy is used to determine the cell state; if defined at the cell, those values are used; if defined at the region, those are used; finally, the default model initial conditions will be used.
The specific initial conditions are different for each EOS. For any specific EOS, there are at least single and two phase initial conditions, as well as options for different components. The user is referred to Chapter 3, Tough Concepts for a discussion of components and an example of setting single and two phase initial conditions.
Only for the simplest models will the initial conditions be uniform over the model. In most realistic analyses, a steady state simulation will be used to reach an equilibrium solution. For example, this will be used to reach gravity-capillary equilibrium in a vadose zone analysis or heat and fluid flow equilibrium in a geothermal reservoir analysis. The steady state results will then be used as initial conditions for the transient analysis. When this approach is used, two separate folders should be used to store the steady state analysis and the transient analysis.
Because of the complex physics represented in TOUGH, setting of the initial conditions can be challenging. The user is directed to the many examples available at the PetraSim web site for guidance on how others specify initial conditions.
Default initial conditions are always defined for the model. To set these conditions, select Properties->Initial Conditions... or 
to open the Default Initial Conditions dialog, Figure 9.1.
You will then be provided all the available initial condition options that are valid for the EOS and selected components. Based on your knowledge of the problem, define these appropriately.
To define initial conditions by region of the model, select a region then select Model->Edit Selection.... On the Edit Region Data dialog, select the Initial Conditions tab. After selecting, Specify by Region, you will be able to define initial conditions for a region, Figure 9.2. If you do not want region initial conditions to be used, unselect Specify by Region.
To define initial conditions by cell, open the Grid Editor. Then right-click on a cell and select Initial Conditions in the context menu (or select a cell and Edit->Properties.... On the Initial Conditions tab of the Edit Cell Data dialog, select Specify Initial Conditions by Cell, Figure 9.3. You will then be able to define initial conditions for a cell. If you do not want cell initial conditions to be used, unselect Specify Initial Conditions by Cell.
To read initial conditions from a previous analysis, select File->Load Initial Conditions... and read a previous SAVE file. The model used to write the SAVE file must have the same geometry as the model for which you are reading data. To avoid over-writing of files by TOUGH, each analysis should be run in a separate directory.
The TOUGH2 example problem 8 "Contamination of an Aquifer from VOC Vapors in the Vadose Zone" provides an excellent tutorial in the setting of two-phase capillary equilibrium initial conditions and the definition of an atmospheric boundary. A steady state analysis is used to define the initial conditions for the transient analysis.
The T2VOC Problem 2 example also demonstrates the use of a steady state analysis to define initial conditions, followed by several consecutive transient analyses representing a spill, spreading of the spill, and clean-up.