# How to perform a transient simulation (SIRENE)

In order to perform a transient calculation, it is first necessary at least to have been able to carry out a steady-state calculation or another calculation in transient mode which will serve as an initial condition for the calculation. In addition, the calculation used as an initial condition must have been carried out with the parameter "Write in all calculation sections" checked. This parameter is checked by default for the permanent calculation parameters and is unchecked by default for transient calculation parameters.

### Creating a new scenario or variant

The calculation of this new simulation will be carried out either in a new scenario or in a variant of the scenario serving as an initial condition. The first operation to be performed in the project explorer is therefore to choose:

- Duplicate the initial condition scenario
- Create a new variant in the initial condition scenario

Since transient computation requires an initial condition, it is important that the boundary conditions and the parameters of the start of the transient simulation are compatible with the initial waterline originating from a previous calculation. To obtain this guarantee, it is advisable to create a variant in the scenario where the calculation is used as the initial condition for the transient calculation. |

### Import of initial condition

In the Project Explorer, go to the line of the scenario or variant where the transient calculation is to be done and click on the " Import C.I." button (C.I. = Initial Condition). Choose from the list of results present the calculation results which will serve as initial condition.

Once this is done, the " Transient Calculation" button becomes active.

### Edit Transitional Computing Events

The user will then enter the changes that will take place on the system over time (Variation of supplied or retrieved flow rates, operation of devices ...). This is done by the seizure of laws as a function of time.

If the types of boundary conditions do not change between the permanent scenario and the transient calculation, it is advisable to enter the time-dependent laws directly in the permanent scenario. The permanent calculation will be carried out on the first time step only and the transient calculation in a variant (which does not contain any modification with respect to the scenario) will calculate from the second time step on the data of the scenario. |

### Calculation Parameters

The user will have to define the time parameters of the simulation as presented on the page Time Settings. It may also possibly modify the parameters of the transient computation and the writing parameters of the results.

### Start simulation

Follow the instructions on page Performing a Transient Calculation (SIRENE)