(In)stationary flow

A stationary state is always given when the descriptive state variables do not depend on time. A stationary flow therefore exists when the velocity, pressure and fluid properties remain constant at any point in the flow field. Strictly speaking, this only applies to the undisturbed or fully developed laminar flow that rarely occurs in technical applications. In this case, a steady-state solver can be used to solve the problem.

By previous definition, a stationary flow can only occur in steady state; turbulent flow, starting or stopping processes are not included. The same also applies to sound or compression impacts. As soon as a transient flow state is present, the local acceleration or deceleration of the fluid generates additional inertial forces which cause corresponding pressure changes. For numerical simulation this means the use of a transient solver, which also includes the corresponding time integration. The required time step size depends on the speed at which the expected change in the state variables takes place. As a result, the numerical effort increases sharply.

If the change of the state variables of a transient flow is periodic, the temporal averaging of the flow field results in a constant state. For sufficiently small frequencies of the fluctuations, the transient flow can then be regarded as a quasi-steady flow. If the conditions are right, this leads to significantly shorter calculation times.