Choosing Between Steady and Unsteady

In most situations, the physics being simulated indicates the choice between the Steady and Unsteady (Implicit or Explicit) models.

Simulations that are time-dependent, and hence require the Unsteady model include:

• Time-varying boundary conditions
• Sliding or deforming mesh problems
• Free surface (VOF) problems
• Transient heat transfer

  Other simulations are separate cases, namely those using harmonic balance.

  In some situations, however, unsteadiness in the physical problem is not readily apparent. It can manifest itself in a failure to converge when running with the Steady model. One example is the solution of flow past a cylinder. At certain Reynolds numbers, the flow is unsteady because of vortex shedding, and it is incorrect to use a steady solution approach.

  When physical instabilities do exist, the correct way to model them is to perform a time-dependent simulation. Choose an appropriate time-step that is based on a representative Strouhal number. Accumulate time-averaged statistics only after the transient simulation has achieved a periodic state.

  Using the Steady model for unsteady physical phenomena is not recommended. Running an unsteady problem with a Steady model is equivalent to using an inaccurate time-step in an unsteady simulation.