# The initial conditions: overview

## Transient Magnetic application: reminder

A transient application is a physical application characterized by:

• properties that vary in function of time: variable sources, variable material characteristics…
• a time equation (differential equation of the 1 st order) : d/dt ≠ 0 (transient regime or variable regime).

The resolution consists of a sequence of resolutions in time. The solutions are not independent from each other: each solution is linked to the previous one by the temporal equation.

## Initialization « at zero »

In the most general case, state variables are initialized to zero (for t = 0). The 1st step of time (t = t1) corresponds to the first Flux solving step as presented in the table below.

Initial instant 1 st time step
Flux solver t = 0 t = t1
Result B, H… null everywhere * B, H… calculated
Note: * In 3D, if there are non-meshed sources (non meshed coils or imposed field), the values of B and H are those related to these sources (Hj field).

## Initialization by static computation

In the presence of magnets or sources of non null current/voltage at t = 0 s, the zero initialization in Flux is not carried out « in real accordance with the physical reality ». The starting is accompanied then by a “numeric transient”, which distorts the solving process.

To prevent from this problem, it is possible in Flux to take into account initials conditions of the static computation initialization type.

## Initialization by FE solution

It is possible to initiate a transient computation by using as initial state the result of a Magnetostatic, AC Steady State, or Transient Magnetic problem.

This permits for example :

• to study a variation around an average value without being obliged to carry out all the time steps permitting to attain the steady state regime
• to resume a computation after the modification of the value of a physical size (current...)