Setting Initial Conditions

Initial conditions assign values to the dependent variables at the centroids of all mesh cells before the start of the simulation. For a fluid flow simulation, the dependent variables are velocity and pressure.

The implication of initial conditions depends on the type of problem being considered:

  • For steady simulations, the initial values represent an initial guess of the flow field. This initial guess has no influence on the final result, but it can determine the convergence behavior.
  • For unsteady simulations, the initial values have physical significance and influence the time-dependent results. You provide the exact values that prevail at time t = 0s.
Poor initial field specifications can put severe demands on the numerical algorithm when substituted into the finite-volume equations. As a consequence, the following special start-up measures may be necessary to ensure numerical stability:
  • Use of strong under-relaxation in steady simulations.
  • Use of small time steps in unsteady simulations.
Specific recommendations concerning these practices are given in Running the Simulation. In either case, increased computing times can be an undesirable side effect.

For a fluid flow simulation, you can specify the initial values for velocity and pressure manually or use an automatic initialization procedure.