### 1.2.1. Steady State and Transient Flows

The time dependence of the flow characteristics can be specified as either steady-state or transient. Steady-state simulations, by definition, are those whose characteristics do not change with time and whose steady conditions are assumed to have been reached after a relatively long time interval. They therefore require no real time information to describe them. Many practical flows can be assumed to be steady after initial unsteady flow development, for example, after the start up of a rotating machine.

Transient simulations require real time information to determine the time intervals at which the CFX-Solver calculates the flow field. Transient behavior can be caused by the initially changing boundary conditions of the flow, as in start up, or it can be inherently related to the flow characteristics, so that a steady-state condition is never reached, even when all other aspects of the flow conditions are unchanging. Many flows, particularly those driven by buoyancy, do not have a steady-state solution, and may exhibit cyclic behavior.

Sometimes simulations that are run in steady-state mode will have difficulty converging, and no matter what action you take regarding mesh quality and time step size, the solution does not converge. This could be an indication of transient behavior. If you have run a steady-state calculation and you see oscillatory behavior of the residual plots, you can test to see if you are observing a transient effect by reducing/increasing the time step size by known factors:

- If the period of oscillation of the residual plot changes by changing the time step size, then the phenomenon is most likely a numerical effect.

- If the period stays the same, then it is probably a transient effect.

In transient mode, you must set both physical time steps and the maximum number of coefficient iterations per time step.

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